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LTC2435/LTC2435-1 20-Bit No Latency TM ADCs with Differential Input and Differential Reference DESCRIPTIO
The LTC(R)2435/2435-1 are 2.7V to 5.5V micropower 20-bit differential analog to digital converters with integrated oscillator, 3ppm INL and 0.8ppm RMS noise. They use delta-sigma technology and provide single cycle settling time for multiplexed applications. Through a single pin, the LTC2435 can be configured for better than 110dB input differential mode rejection at 50Hz or 60Hz 2%, or it can be driven by an external oscillator for a user defined rejection frequency. The LTC2435-1 can be configured for better than 87dB input differential mode rejection over the range of 49Hz to 61.2Hz (50Hz and 60Hz 2% simultaneously). The internal oscillator requires no external frequency setting components. The converters accept any external differential reference voltage from 0.1V to VCC for flexible ratiometric and remote sensing measurement configurations. The fullscale differential input range is from - 0.5VREF to 0.5VREF. The reference common mode voltage, VREFCM, and the input common mode voltage, VINCM, may be independently set anywhere within the GND to VCC range of the LTC2435/LTC2435-1. The DC common mode input rejection is better than 120dB. The LTC2435/LTC2435-1 communicate through a flexible 3-wire digital interface which is compatible with SPI and MICROWIRETM protocols.
, LTC and LT are registered trademarks of Linear Technology Corporation. No Latency is a trademark of Linear Technology Corporation. MICROWIRE is a trademark of National Semiconductor Corporation. Protected by U.S. Patents including 6140950, 6169506.
FEATURES

2x Speed Up Version of the LTC2430: 15Hz Output x Rate, 60Hz Notch--LTC2435; 13.75Hz Output Rate, Simultaneous 50Hz/60Hz Notch--LTC2435-1 Differential Input and Differential Reference with GND to VCC Common Mode Range 3ppm INL, No Missing Codes 10ppm Gain Error 0.8ppm Noise Single Conversion Settling Time for Multiplexed Applications Internal Oscillator--No External Components Required Single Supply 2.7V to 5.5V Operation Low Supply Current (200A,4A in Auto Sleep) 20-Bit ADC in Narrow SSOP-16 Package (SO-8 Footprint)
APPLICATIO S

Direct Sensor Digitizer Weight Scales Direct Temperature Measurement Gas Analyzers Strain Gage Transducers Instrumentation Data Acquisition Industrial Process Control 6-Digit DVMs
TYPICAL APPLICATIO S
2.7V TO 5.5V 2 14
INL (ppm OF VREF)
10 8 6
VCC
1F VCC FO
LTC2435/ LTC2435-1 REFERENCE VOLTAGE 0.1V TO VCC ANALOG INPUT RANGE -0.5VREF TO 0.5VREF 3 4 5 6 REF + REF IN + IN - GND
-
= INTERNAL OSC/50Hz REJECTION (LTC2435) = EXTERNAL CLOCK SOURCE = INTERNAL OSC/60Hz REJECTION (LTC2435) = INTERNAL 50Hz/60Hz REJECTION (LTC2435-1)
SCK
13 3-WIRE SPI INTERFACE
-2 -4 -6 -8 FO = GND VCC = 5V VREF = 5V VINCM = VINCM = 2.5V -1.5
SDO CS
12 11
1, 7, 8, 9, 10, 15, 16
-10 -2.5
2435 TA01
U
4 2 0
U
U
Integral Nonlinearity vs Input
TA = 25C TA = 85C TA = -45C
-0.5 0.5 1.5 INPUT VOLTAGE (V)
2.5
2435 G04
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LTC2435/LTC2435-1
ABSOLUTE
(Notes 1, 2)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW GND VCC REF + REF - IN + IN - GND GND 1 2 3 4 5 6 7 8 16 GND 15 GND 14 FO 13 SCK 12 SDO 11 CS 10 GND 9 GND
Supply Voltage (VCC) to GND .......................- 0.3V to 7V Analog Input Pins Voltage to GND .................................... - 0.3V to (VCC + 0.3V) Reference Input Pins Voltage to GND .................................... - 0.3V to (VCC + 0.3V) Digital Input Voltage to GND ........ - 0.3V to (VCC + 0.3V) Digital Output Voltage to GND ..... - 0.3V to (VCC + 0.3V) Operating Temperature Range LTC2435C/LTC2435-1C ........................... 0C to 70C LTC2435I/LTC2435-1I ........................ - 40C to 85C Storage Temperature Range ................. - 65C to 150C Lead Temperature (Soldering, 10 sec).................. 300C
ORDER PART NUMBER LTC2435CGN LTC2435IGN LTC2435-1CGN LTC2435-1IGN GN PART MARKING 2435 2435I 24351 24351I
GN PACKAGE 16-LEAD PLASTIC SSOP TJMAX = 125C, JA = 95C/W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 3, 4)
PARAMETER Integral Nonlinearity CONDITIONS

ELECTRICAL CHARACTERISTICS
MIN 20
TYP 2 3 10 2 100
MAX
UNITS Bits ppm of VREF ppm of VREF ppm of VREF mV nV/C
Resolution (No Missing Codes) 0.1V VREF VCC, -0.5 * VREF VIN 0.5 * VREF, (Note 5) 5V VCC 5.5V, REF+ = 2.5V, REF- = GND, VINCM = 1.25V, (Note 6) 5V VCC 5.5V, REF+ = 5V, REF- = GND, VINCM = 2.5V, (Note 6) 2.7V VCC 5.5V, REF+ = 2.5V, REF- = GND, VINCM = 1.25V, (Note 6) 2.5V REF+ VCC, REF- = GND, GND IN+ = IN- VCC, (Note 14) 2.5V REF+ VCC, REF- = GND, GND IN+ = IN- VCC 2.5V REF+ VCC, REF- = GND, IN+ = 0.75REF+, IN- = 0.25 * REF+ 2.5V REF+ VCC, REF- = GND, IN+ = 0.75REF+, IN- = 0.25 * REF+ 2.5V REF+ VCC, REF- = GND, IN+ = 0.25 * REF+, IN- = 0.75 * REF+ 2.5V REF+ VCC, REF- = GND, IN+ = 0.25 * REF+, IN- = 0.75 * REF+ 5V VCC 5.5V, REF+ = 5V, REF - = GND, GND IN- = IN+ VCC, (Note 13)
20 5
Offset Error Offset Error Drift Positive Gain Error Positive Gain Error Drift Negative Gain Error Negative Gain Error Drift Output Noise
10 0.1
25
ppm of VREF ppm of VREF/C
10 0.1 4
25
ppm of VREF ppm of VREF/C VRMS
CO VERTER CHARACTERISTICS
PARAMETER Input Common Mode Rejection DC CONDITIONS 2.5V GND REF+
The denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 3, 4)
MIN
TYP 120
MAX
UNITS dB
VCC, REF- = GND, IN- = IN+ VCC (Note 5)
110
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U
W
U
U
WW
W
U
LTC2435/LTC2435-1
CO VERTER CHARACTERISTICS
PARAMETER Input Common Mode Rejection 60Hz 2% (LTC2435) Input Common Mode Rejection 50Hz 2% (LTC2435) Input Normal Mode Rejection 60Hz 2% (LTC2435) Input Normal Mode Rejection 50Hz 2% (LTC2435) Input Common Mode Rejection 49Hz to 61.2Hz (LTC2435-1) Input Normal Mode Rejection 49Hz to 61.2Hz (LTC2435-1) Input Normal Mode Rejection External Clock fEOSC/2560 14% Input Normal Mode Rejection External Clock fEOSC/2560 4% Reference Common Mode Rejection DC Power Supply Rejection, DC Power Supply Rejection, 50Hz 2% CONDITIONS
- 2.5V CC, REF = GND, - = IN+ V , (Notes 5, 7) GND IN CC 2.5V REF+ VCC, REF- = GND, GND IN - = IN+ VCC, (Notes 5, 8)
The denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Notes 3, 4)
MIN

Power Supply Rejection, 60Hz 2% REF+ = 2.5V, REF- = GND, IN- = IN+ = GND, (Note 7) REF+ = 2.5V, REF- = GND, IN- = IN+ = GND, (Note 8)
A ALOG I PUT A D REFERE CE The denotes specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25C. (Note 3)
SYMBOL IN+ IN- VIN REF+ REF- VREF CS (IN+) CS CS (IN-) (REF-) (IN+) (REF+) (REF-) CS (REF+) IDC_LEAK IDC_LEAK IDC_LEAK PARAMETER Absolute/Common Mode IN+ Voltage Absolute/Common Mode IN- Voltage Input Differential Voltage Range (IN+ - IN-) Absolute/Common Mode REF+ Voltage Absolute/Common Mode REF- Voltage Reference Differential Voltage Range (REF+ - REF-) IN+ Sampling Capacitance IN- Sampling Capacitance REF+ Sampling Capacitance REF- Sampling Capacitance IN+ DC Leakage Current IN- DC Leakage Current REF+ DC Leakage Current REF- DC Leakage Current CS = VCC, IN+ = GND CS = VCC, IN- = VCC CS = VCC, REF+ = VCC CS = VCC, REF- = GND

IDC_LEAK (IN-)
U
U
U
U
TYP
MAX
UNITS dB dB
REF+ V
140 140 110 110 120 87 87 110 130 120 140 100 120 120 120 120
(Notes 5, 7) (Notes 5, 8) 2.5V REF+ VCC, REF- = GND, GND IN- = IN+ VCC, (Notes 5, 7) FO = GND (Note 5) External Oscillator (Note 5) External Oscillator (Note 5) 2.5V REF+ VCC, GND REF- 2.5V, VREF = 2.5V, IN- = IN+ = GND (Note 5) REF+ = VCC, REF- = GND, IN- = IN+ = GND
dB dB dB dB dB dB dB dB dB dB
U
CONDITIONS

MIN GND - 0.3V GND - 0.3V -VREF/2 0.1 GND 0.1
TYP
MAX VCC + 0.3V VCC + 0.3V VREF/2 VCC VCC - 0.1V VCC
UNITS V V V V V V pF pF pF pF
1.5 1.5 1.5 1.5 -10 -10 -10 -10 1 1 1 1 10 10 10 10
nA nA nA nA
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LTC2435/LTC2435-1
DIGITAL I PUTS A D DIGITAL OUTPUTS
SYMBOL VIH VIL VIH VIL IIN IIN CIN CIN VOH VOL VOH VOL IOZ PARAMETER High Level Input Voltage CS, FO Low Level Input Voltage CS, FO High Level Input Voltage SCK Low Level Input Voltage SCK Digital Input Current CS, FO Digital Input Current SCK Digital Input Capacitance CS, FO Digital Input Capacitance SCK High Level Output Voltage SDO Low Level Output Voltage SDO High Level Output Voltage SCK Low Level Output Voltage SCK Hi-Z Output Leakage SDO (Note 9) IO = -800A IO = 1.6mA IO = -800A (Note 10) IO = 1.6mA (Note 10)

The denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3)
CONDITIONS 2.7V VCC 5.5V 2.7V VCC 3.3V 4.5V VCC 5.5V 2.7V VCC 5.5V 2.7V VCC 5.5V (Note 9) 2.7V VCC 3.3V (Note 9) 4.5V VCC 5.5V (Note 9) 2.7V VCC 5.5V (Note 9) 0V VIN VCC 0V VIN VCC (Note 9)

POWER REQUIRE E TS
SYMBOL VCC ICC PARAMETER Supply Voltage Supply Current Conversion Mode Sleep Mode Sleep Mode
The denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3)
CONDITIONS
4
UW
U
U
MIN 2.5 2.0
TYP
MAX
UNITS V V
0.8 0.6 2.5 2.0 0.8 0.6 -10 -10 10 10 VCC - 0.5 0.4 VCC - 0.5 0.4 -10 10 10 10
V V V V V V A A pF pF V V V V A
MIN 2.7
TYP
MAX 5.5
UNITS V A A A
CS = 0V (Note 12) CS = VCC (Note 12) CS = VCC, 2.7V VCC 3.3V (Note 12)

200 4 2
300 10
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LTC2435/LTC2435-1
TI I G CHARACTERISTICS
SYMBOL fEOSC tHEO tLEO tCONV PARAMETER External Oscillator Frequency Range External Oscillator High Period External Oscillator Low Period Conversion Time (LTC2435)
The denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 3)
CONDITIONS

fISCK
DISCK fESCK tLESCK tHESCK tDOUT_ISCK
tDOUT_ESCK t1 t2 t3 t4 tKQMAX tKQMIN t5 t6
Note 1: Absolute Maximum Ratings are those values beyond which the life of the device may be impaired. Note 2: All voltage values are with respect to GND. Note 3: VCC = 2.7 to 5.5V unless otherwise specified. VREF = REF + - REF -, VREFCM = (REF + + REF -)/2; VIN = IN + - IN -, VINCM = (IN + + IN -)/2. Note 4: FO pin tied to GND or to VCC or to external conversion clock source with fEOSC = 153600Hz unless otherwise specified. Note 5: Guaranteed by design, not subject to test. Note 6: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band. Note 7: FO = 0V (internal oscillator) or fEOSC = 153600Hz 2% (external oscillator) for the LTC2435 or fEOSC = 139800Hz 2% for the LTC2435-1.
UW
MIN 5 0.25 0.25 65.6 78.7 72
TYP
MAX 2000 200 200
UNITS kHz s s ms ms ms ms ms kHz kHz kHz
FO = 0V FO = VCC External Oscillator (Note 11) FO = 0V External Oscillator (Note 11) Internal Oscillator (Note 10), LTC2435 Internal Oscillator (Note 10), LTC2435-1 External Oscillator (Notes 10, 11) (Note 10) (Note 9) (Note 9) (Note 9)

68.3 81.9 10278/fEOSC (in kHz) 73.5 75 10278/fEOSC (in kHz) 19.2 17.5 fEOSC/8
66.9 80.3
Conversion Time (LTC2435-1) Internal SCK Frequency
Internal SCK Duty Cycle External SCK Frequency Range External SCK Low Period External SCK High Period Internal SCK 24-Bit Data Output Time

45 250 250 1.22 1.34
55 2000
% kHz ns ns
Internal Oscillator (Notes 10, 12), LTC2435 Internal Oscillator (Notes 10, 12), LTC2435-1 External Oscillator (Notes 10, 11)

1.25 1.28 1.37 1.40 192/fEOSC (in kHz) 24/fESCK (in kHz) 200 200 200 220
ms ms ms ms ns ns ns ns ns ns ns
External SCK 24-Bit Data Output Time (Note 9) CS to SDO Low Z CS to SDO High Z CS to SCK CS to SCK SCK to SDO Valid SDO Hold After SCK SCK Set-Up Before CS SCK Hold After CS (Note 5) (Note 10) (Note 9)
0 0 0 50 15 50

50
ns
Note 8: FO = VCC (internal oscillator) or fEOSC = 128000Hz 2% (external oscillator). Note 9: The converter is in external SCK mode of operation such that the SCK pin is used as digital input. The frequency of the clock signal driving SCK during the data output is fESCK and is expressed in kHz. Note 10: The converter is in internal SCK mode of operation such that the SCK pin is used as digital output. In this mode of operation the SCK pin has a total equivalent load capacitance CLOAD = 20pF. Note 11: The external oscillator is connected to the FO pin. The external oscillator frequency, fEOSC, is expressed in kHz. Note 12: The converter uses the internal oscillator. FO = 0V or FO = VCC. Note 13: The output noise includes the contribution of the internal calibration operations. Note 14: Refer to Offset Accuracy and Drift in the Applications Information section.
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LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS
Total Unadjusted Error (VCC = 5V, VREF = 5V)
-340
-680
FO = GND VCC = 5V VREF = 5V -345 VINCM = VINCM = 2.5V
TA = -45C
TUE (ppm OF VREF)
TUE (ppm OF VREF)
-690 TA = 25C -695 -700 -705 TA = 85C
TUE (ppm OF VREF)
-350
TA = 25C
-355
TA = 85C
-360 -2.5
-1.5
0.5 1.5 -0.5 INPUT VOLTAGE (V)
Integral Nonlinearity (VCC = 5V, VREF = 5V)
10 8 6 INL (ppm OF VREF)
INL (ppm OF VREF)
2 0 -2 -4 -6 -8 FO = GND VCC = 5V VREF = 5V VINCM = VINCM = 2.5V -1.5
TA = 25C TA = 85C TA = -45C
1 0 -1 TA = 85C
INL (ppm of VREF)
4
-10 -2.5
-0.5 0.5 1.5 INPUT VOLTAGE (V)
Noise Histogram (Output Rate = 15Hz, VCC = 5V, VREF = 5V)
30 10,000 CONSECUTIVE READINGS VCC = 5V 25 VREF = 5V GAUSSIAN VIN = 0V DISTRIBUTION VINCM = 2.5V m = -325.4ppm 20 FO = GND = 0.79ppm TA = 25C 15 10 5 0 -330 -329 -328 -327 -326 -325 -324 -323 -322 -321 OUTPUT CODE(ppm OF VREF)
2435 G07
NUMBER OF READINGS (%)
NUMBER OF READINGS (%)
6
UW
2435 G01
Total Unadjusted Error (VCC = 5V, VREF = 2.5V)
FO = GND VCC = 5V -685 VREF = 2.5V VINCM = VINCM = 1.25V
Total Unadjusted Error (VCC = 2.7V, VREF = 2.5V)
-320 FO = GND VCC = 2.7V VREF = 2.5V -330 VINCM = VINCM = 1.25V TA = -45C -340 TA = 25C TA = 85C -350
TA = -45C
2.5
-710 -1.25
-0.75
-0.25 0.25 0.75 INPUT VOLTAGE (V)
1.25
2435 G02
-360 -1.25
-0.75
-0.25 0.25 0.75 INPUT VOLTAGE (V)
1.25
2435 G03
Integral Nonlinearity (VCC = 5V, VREF = 2.5V)
3 2 TA = -45C
10
Integral Nonlinearity (VCC = 2.7V, VREF = 2.5V)
FO = GND 8 VCC = 2.7V VREF = 2.5V 6V INCM = VINCM = 1.25V 4 2 0 -2 -4 -6 -8 TA = 85C
TA = 25C TA = -45C
2.5
2435 G04
TA = 25C FO = GND -2 VCC = 5V VREF = 2.5V VINCM = VINCM = 1.25V -3 -1.25 -0.75 -0.25 0.25 0.75 INPUT VOLTAGE (V)
1.25
2435 G05
-10 -1.25
-0.75
-0.25 0.25 0.75 INPUT VOLTAGE (V)
1.25
2435 G06
Noise Histogram (Output Rate = 15Hz, VCC = 2.7V, VREF = 2.5V)
14 10,000 CONSECUTIVE READINGS VCC = 2.7V GAUSSIAN 12 V DISTRIBUTION REF = 2.5V VIN = 0V m = -365ppm 10 VINCM = 2.5V = 1.55ppm FO = GND 8 TA = 25C 6 4 2 0 -372 -370 -368 -366 -364 -362 -360 -358 OUTPUT CODE (ppm OF VREF)
2435 G08
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LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS
RMS Noise vs Input Differential Voltage
1.5 VCC = 5V 1.4 VREF = 5V VINCM = 2.5V 1.3 F = GND O 1.2 TA = 25C 1.1 1.0 0.9 0.8 0.7 0.6 0.5 -2.5 -2 -1.5 -1 - 0.5 0 0.5 1 1.5 2 INPUT DIFFERENTIAL VOLTAGE (V) 2.5
5.0
RMS NOISE (ppm OF VREF)
RMS NOISE (V)
RMS NOISE (V)
RMS Noise vs VCC = VREF
5.0 FO = GND 4.8 REF+ = VCC REF- = GND 4.6 T = 25C A 4.4 VIN = 0V VINCM = GND 4.2 4.0 3.8 3.6 3.4 3.2 3.0 2.7 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 5.5 5.0
OFFSET ERROR (ppm OF VREF)
RMS NOISE (V)
RMS NOISE (V)
Offset Error vs Temperature
-320 -322 VCC = 5V VREF = 5V VIN = 0V VINCM = GND FO = GND -320
OFFSET ERROR (ppm OF VREF)
-324 -326 -328 -330 -332 -334 -336 -338 -340 - 45 -30 -15
OFFSET ERROR (ppm OF VREF)
OFFSET ERROR (mV)
0 15 30 45 60 TEMPERATURE (C)
UW
2435 G10 2435 G13
RMS Noise vs VINCM
FO = GND 4.8 REF+ = 5V REF - = GND 4.6 T = 25C A 4.4 VCC = 5V VIN = 0V 4.2 VINCM = GND 4.0 3.8 3.6 3.4 3.2 3.0 -1 0 1 3 2 VINCM (V) 4 5 6
2435 G11
RMS Noise vs Temperature (TA)
5.0 FO = GND 4.8 VCC = 5V = 5V V 4.6 VREF 0V IN = 4.4 VINCM = GND 4.2 4.0 3.8 3.6 3.4 3.2 3.0 -50 -25 50 25 0 TEMPERATURE (C) 75 100
2435 G12
RMS Noise vs VREF
FO = GND 4.8 REF- = GND T = 25C 4.6 VA = 5V CC 4.4 VIN = 0V VINCM = GND 4.2 4.0 3.8 3.6 3.4 3.2 3.0 0 1 3 2 VREF (V) 4 5
2435 G14
Offset Error vs VINCM
-320 -322 -324 -326 -328 -330 -332 VCC = 5V -334 REF+ = 5V REF - = GND -336 VIN = 0V -338 FO = GND TA = 25C -340 1 -1 0
3 2 VINCM (V)
4
5
6
2435 G15
Offset Error vs VCC = VREF
REF+ = VCC -322 REF - = GND V = 0V -324 IN VINCM = GND -326 FO = GND TA = 25C -328 -330 -332 -334 -336 -338 75 90 -340 2.7 3.1 3.5 3.9 4.3 VCC (V) 4.7 5.1 5.5 -1.60 -1.61 -1.62 -1.63 -1.64 -1.65 -1.66 -1.67 -1.68 -1.69 -1.70
Offset Error vs VREF
FO = GND REF- = GND TA = 25C VCC = 5V VIN = 0V VINCM = GND 0 1 3 2 VREF (V) 4 5
2435 G18
2435 G16
2435 G17
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LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS
Full-Scale Error vs Temperature
-330
FULL-SCALE ERROR (ppm OF VREF)
FULL-SCALE ERROR(ppm OF VREF)
-340
+FS ERROR -350 -FS ERROR -360
-500 -600 -700 -800 -900 2.7
-FS ERROR
+FS GAIN ERROR (ppm OF VREF)
FO = GND VCC = 5V VREF = 5V VINCM = 2.5V
-370 -60 -40 -20
20 40 60 0 TEMPERATURE (C)
PSRR vs Frequency at VCC (LTC2435-1)
VCC = 4.1VDC 1.4V REF+ = 2.5V -20 REF - = GND IN+ = GND - -40 IN = GND FO = GND T = 25C -60 A -80 -100 -120 -140 0 20 40 60 80 100 120 140 160 180 200 220 FREQUENCY AT VCC (Hz)
2435 G22
0
REJECTION (dB)
REJECTION (dB)
-80 -100 -120 -140 1 10 100 1000 10000 100000 1000000 FREQUENCY AT VCC (Hz)
2435 G23
REJECTION (dB)
PSRR vs Frequency at VCC (LTC2435)
VCC = 4.1VDC 1.4V REF+ = 2.5V -20 REF - = GND IN+ = GND -40 IN- = GND FO = GND -60 TA = 25C -80 -100 -120 -140 0 40 80 120 160 200 FREQUENCY AT VCC (Hz) 240
2435 G25
0
REJECTION (dB)
REJECTION (dB)
-80 -100 -120 -140 1 10 100 1000 10000 100000 1000000 FREQUENCY AT VCC (Hz)
2435 G26
REJECTION (dB)
8
UW
80
2435 G19
Full-Scale Error vs VCC
-300 +FS ERROR -400 VREF = 2.5V REF - = GND VINCM = 0.5VREF FO = GND TA = 25C
20
+Full-Scale Gain Error vs VCC
VREF = 2.5V REF - = GND = 0.5VREF V 15 INCM FO = GND TA = 25C 10
5
0
100
3.1
3.5
3.9 4.3 VCC (V)
4.7
5.1
5.5
-5 2.7
3.1
3.5
3.9 4.3 VCC (V)
4.7
5.1
5.5
2435 G20
2435 G21
PSRR vs Frequency at VCC (LTC2435-1)
VCC = 4.1VDC REF+ = 2.5V -20 REF - = GND IN+ = GND -40 IN- = GND FO = GND -60 TA = 25C 0
PSRR vs Frequency at VCC (LTC2435-1)
VCC = 4.1VDC 0.7V REF+ = 2.5V -20 REF - = GND IN+ = GND -40 IN- = GND FO = GND -60 TA = 25C -80 -100 -120 -140 13800 0
13850
13900 13950 FREQUENCY AT VCC (Hz)
14000
2435 G24
PSRR vs Frequency at VCC (LTC2435)
VCC = 4.1VDC REF+ = 2.5V -20 REF - = GND IN+ = GND -40 IN- = GND FO = GND -60 TA = 25C 0
0
PSRR vs Frequency at VCC (LTC2435)
VCC = 4.1VDC 0.7V REF+ = 2.5V -20 REF - = GND IN+ = GND -40 IN- = GND FO = GND -60 TA = 25C -80 -100 -120 -140 15250
15300
15350 15400 FREQUENCY AT VCC (Hz)
15450
2435 G27
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LTC2435/LTC2435-1 TYPICAL PERFOR A CE CHARACTERISTICS
Conversion Current vs Temperature
240 VCC = 5.5V 230
CONVERSION CURRENT (A)
SUPPLY CURRENT (A)
220 210 FO = GND CS = GND 200 SCK = NC SDO = NC 190 180 170 160 -45 -30 -15 0 15 30 45
VCC = 5V
800 700 600 500 400 300 200
VCC = 5V
SLEEP-MODE CURRENT (A)
TEMPERATURE (C)
2435 G28
Offset Change* vs Output Data Rate
50
OFFSET CHANGE* (ppm OF VREF)
RESOLUTION (BITS)
10 0 -10 -20 -30
VCC = VREF = 5V
RESOLUTION (BITS)
VINCM = VREFCM 40 VIN = 0V REF - = GND 30 F = EXT OSC O 20 TA = 25C
VCC = 2.7V VREF = 2.5V
-40 * RELATIVE TO OFFSET AT NORMAL OUTPUT RATE -50 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 G31
UW
VCC = 3V VCC = 2.7V 60 75
Conversion Current vs Output Data Rate
1000 900 VREF = VCC IN+ = GND IN- = GND SCK = NC SDO = NC SDI = GND CS = GND FO = EXT OSC TA = 25C
6 5 4 3 2 1
Sleep-Mode Current vs Temperature
FO = GND CS = VCC SCK = NC SDO = NC VCC = 5.5V
VCC = 5V VCC = 3V
VCC = 3V
VCC = 2.7V
90
100 0 10 20 30 40 50 60 70 80 90 100 OUTPUT DATA RATE (READINGS/SEC)
2435 G29
0 -45 -30 -15
0
15
30
45
60
75
90
TEMPERATURE (C)
2435 G30
Resolution (NoiseRMS 1LSB) vs Output Data Rate
22 21 20 19 18 VINCM = VREFCM 17 VIN = 0V REF - = GND FO = EXT OSC 16 TA = 25C RES = LOG2 (VREF/NOISERMS) 15 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 G32
Resolution (INLMAX 1LSB) vs Output Data Rate
21
VCC = VREF = 5V
20 VCC = VREF = 5V 19 18 17 VINCM = VREFCM 16 VIN = 0V REF - = GND FO = EXT OSC 15 TA = 25C RES = LOG2 (VREF/INLMAX) 14 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 G33
VCC = 2.7V VREF = 2.5V
VCC = 2.7V VREF = 2.5V
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LTC2435/LTC2435-1
PI FU CTIO S
GND (Pins 1, 7, 8, 9, 10, 15, 16): Ground. Multiple ground pins internally connected for optimum ground current flow and VCC decoupling. Connect each one of these pins to a ground plane through a low impedance connection. All seven pins must be connected to ground for proper operation. VCC (Pin 2): Positive Supply Voltage. Bypass to GND (Pin 1) with a 10F tantalum capacitor in parallel with 0.1F ceramic capacitor as close to the part as possible. REF + (Pin 3), REF - (Pin 4): Differential Reference Input. The voltage on these pins can have any value between GND and VCC as long as the reference positive input, REF +, is maintained more positive than the reference negative input, REF -, by at least 0.1V. IN + (Pin 5), IN- (Pin 6): Differential Analog Input. The voltage on these pins can have any value between GND - 0.3V and VCC + 0.3V. Within these limits the converter bipolar input range (VIN = IN+ - IN-) extends from - 0.5 * (VREF ) to 0.5 * (VREF ). Outside this input range the converter produces unique overrange and underrange output codes. CS (Pin 11): Active LOW Digital Input. A LOW on this pin enables the SDO digital output and wakes up the ADC. Following each conversion, the ADC automatically enters the Sleep mode and remains in this low power state as long as CS is HIGH. A LOW-to-HIGH transition on CS during the Data Output transfer aborts the data transfer and starts a new conversion. SDO (Pin 12): Three-State Digital Output. During the Data Output period, this pin is used as serial data output. When the chip select CS is HIGH (CS = VCC) the SDO pin is in a high impedance state. During the Conversion and Sleep periods, this pin is used as the conversion status output. The conversion status can be observed by pulling CS LOW. SCK (Pin 13): Bidirectional Digital Clock Pin. In Internal Serial Clock Operation mode, SCK is used as digital output for the internal serial interface clock during the Data Output period. In External Serial Clock Operation mode, SCK is used as digital input for the external serial interface clock during the Data Output period. A weak internal pullup is automatically activated in Internal Serial Clock Operation mode. The Serial Clock Operation mode is determined by the logic level applied to the SCK pin at power up or during the most recent falling edge of CS. FO (Pin 14): Frequency Control Pin. Digital input that controls the ADC's notch frequencies and conversion time. When the FO pin is connected to VCC (LTC2435 only), the converter uses its internal oscillator and the digital filter first null is located at 50Hz. When the FO pin is connected to GND (FO = OV), the converter uses its internal oscillator and the digital filter first null is located at 60Hz (LTC2435) or simultaneous 50Hz/60Hz (LTC2435-1). When FO is driven by an external clock signal with a frequency fEOSC, the converter uses this signal as its system clock and the digital filter first null is located at a frequency fEOSC/2560.
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LTC2435/LTC2435-1
FU CTIO AL BLOCK DIAGRA
VCC GND AUTOCALIBRATION AND CONTROL
IN + IN -
+ -

ADC SERIAL INTERFACE DECIMATING FIR
REF + REF -
-+
DAC
Figure 1. Functional Block Diagram
TEST CIRCUITS
SDO 1.69k CLOAD = 20pF
Hi-Z TO VOH VOL TO VOH VOH TO Hi-Z
2435 TA03
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INTERNAL OSCILLATOR FO (INT/EXT) SDO SCK CS
2435 F01
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VCC 1.69k SDO CLOAD = 20pF
Hi-Z TO VOL VOH TO VOL VOL TO Hi-Z
2435 TA04
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
CONVERTER OPERATION Converter Operation Cycle The LTC2435/LTC2435-1 are low power, delta-sigma analog-to-digital converters with an easy to use 3-wire serial interface (see Figure 1). Their operation is made up of three states. The converter operating cycle begins with the conversion, followed by the sleep state and ends with the data output (see Figure 2). The 3-wire interface consists of serial data output (SDO), serial clock (SCK) and chip select (CS).
CONVERT
SLEEP
FALSE
CS = LOW AND SCK TRUE DATA OUTPUT
2435 F02
Figure 2. LTC2435 State Transition Diagram
Initially, the LTC2435/LTC2435-1 perform a conversion. Once the conversion is complete, the device enters the sleep state. While in this sleep state, power consumption is reduced by an order of magnitude if CS is HIGH. The part remains in the sleep state as long as CS is HIGH. The conversion result is held indefinitely in a static shift register while the converter is in the sleep state. Once CS is pulled LOW, the device exits the low power sleep mode and enters the data output state. If CS is pulled HIGH before the first rising edge of SCK, the device returns to the sleep mode and the conversion result is still held in the internal static shift register. If CS remains LOW after the first rising edge of SCK, the device begins outputting the conversion result. Taking CS HIGH at this point will terminate the data output state and start a new conversion.
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There is no latency in the conversion result. The data output corresponds to the conversion just performed. This result is shifted out on the serial data out pin (SDO) under the control of the serial clock (SCK). Data is updated on the falling edge of SCK allowing the user to reliably latch data on the rising edge of SCK (see Figure 3). The data output state is concluded once 24 bits are read out of the ADC or when CS is brought HIGH. The device automatically initiates a new conversion and the cycle repeats. Through timing control of the CS and SCK pins, the LTC2435/LTC2435-1 offer several flexible modes of operation (internal or external SCK and free-running conversion modes). These various modes do not require programming configuration registers; moreover, they do not disturb the cyclic operation described above. These modes of operation are described in detail in the Serial Interface Timing Modes section. Conversion Clock A major advantage the delta-sigma converter offers over conventional type converters is an on-chip digital filter (commonly implemented as a Sinc or Comb filter). For high resolution, low frequency applications, this filter is typically designed to reject line frequencies of 50Hz or 60Hz plus their harmonics. The filter rejection performance is directly related to the accuracy of the converter system clock. The LTC2435/LTC2435-1 incorporate a highly accurate on-chip oscillator. This eliminates the need for external frequency setting components such as crystals or oscillators. Clocked by the on-chip oscillator, the LTC2435 achieves a minimum of 110dB rejection at the line frequency (50Hz or 60Hz 2%), while the LTC2435-1 achieves a minimum of 87db rejection at 50Hz 2% and 60Hz 2% simultaneously. Ease of Use The LTC2435/LTC2435-1 data output has no latency, filter settling delay or redundant data associated with the conversion cycle. There is a one-to-one correspondence between the conversion and the output data. Therefore, multiplexing multiple analog voltages is easy.
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
The LTC2435/LTC2435-1 perform a full-scale calibration every conversion cycle. This calibration is transparent to the user and has no effect on the cyclic operation described above. The advantage of continuous calibration is extreme stability of full-scale readings with respect to time, supply voltage change and temperature drift. Unlike the LTC2430, the LTC2435 and LTC2435-1 do not perform an offset calibration every conversion cycle. This enables the LTC2435/LTC2435-1 to double their output rate while maintaining line frequency rejection. The initial offset of the LTC2435/LTC2435-1 is within 5mV independent of VREF. Based on the LTC2435/LTC2435-1 new modulator architecture, the temperature drift of the offset is less than 100nV/C. More information on the LTC2435/ LTC2435-1 offset is described in the Offset Accuracy and Drift section of this data sheet. Power-Up Sequence The LTC2435/LTC2435-1 automatically enter an internal reset state when the power supply voltage VCC drops below approximately 2.2V. This feature guarantees the integrity of the conversion result and of the serial interface mode selection. (See the 2-wire I/O sections in the Serial Interface Timing Modes section.) When the VCC voltage rises above this critical threshold, the converter creates an internal power-on-reset (POR) signal with a duration of approximately 1ms. The POR signal clears all internal registers. Following the POR signal, the LTC2435/LTC2435-1 start a normal conversion cycle and follow the succession of states described above. The first conversion result following POR is accurate within the specifications of the device if the power supply voltage is restored within the operating range (2.7V to 5.5V) before the end of the POR time interval. Reference Voltage Range These converters accept a truly differential external reference voltage. The absolute/common mode voltage specification for the REF + and REF - pins covers the entire range from GND to VCC. For correct converter operation, the REF + pin must always be more positive than the REF - pin.
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The LTC2435/LTC2435-1 can accept a differential reference voltage from 0.1V to VCC. The converter output noise is determined by the thermal noise of the front-end circuits, and as such, its value is nearly constant with reference voltage. A decrease in reference voltage will not significantly improve the converter's effective resolution. On the other hand, a reduced reference voltage will improve the converter's overall INL performance. A reduced reference voltage will also improve the converter performance when operated with an external conversion clock (external FO signal) at substantially higher output data rates (see the Output Data Rate section). Input Voltage Range The analog input is truly differential with an absolute/ common mode range for the IN+ and IN- input pins extending from GND - 0.3V to VCC + 0.3V. Outside these limits, the ESD protection devices begin to turn on and the errors due to input leakage current increase rapidly. Within these limits, the LTC2435/LTC2435-1 convert the bipolar differential input signal, VIN = IN+ - IN-, from - FS = - 0.5 * VREF to +FS = 0.5 * VREF where VREF = REF+ - REF-. Outside this range, the converters indicate the overrange or the underrange condition using distinct output codes. Input signals applied to IN+ and IN- pins may extend by 300mV below ground and above VCC. In order to limit any fault current, resistors of up to 5k may be added in series with the IN+ and IN- pins without affecting the performance of the device. In the physical layout, it is important to maintain the parasitic capacitance of the connection between these series resistors and the corresponding pins as low as possible; therefore, the resistors should be located as close as practical to the pins. The effect of the series resistance on the converter accuracy can be evaluated from the curves presented in the Input Current/ Reference Current sections. In addition, series resistors will introduce a temperature dependent offset error due to the input leakage current. A 1nA input leakage current will develop a 1ppm offset error on a 5k resistor if VREF = 5V. This error has a very strong temperature dependency.
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
Output Data Format The LTC2435/LTC2435-1 serial output data stream is 24 bits long. The first 3 bits represent status information indicating the sign and conversion state. The next 21 bits are the conversion result, MSB first. The third and fourth bit together are also used to indicate an underrange condition (the differential input voltage is below -FS) or an overrange condition (the differential input voltage is above +FS). Bit 23 (first output bit) is the end of conversion (EOC) indicator. This bit is available at the SDO pin during the conversion and sleep states whenever the CS pin is LOW. This bit is HIGH during the conversion and goes LOW when the conversion is complete. Bit 22 (second output bit) is a dummy bit (DMY) and is always LOW. Bit 21 (third output bit) is the conversion result sign indicator (SIG). If VIN is >0, this bit is HIGH. If VIN is <0, this bit is LOW. Bit 20 (fourth output bit) is the most significant bit (MSB) of the result. This bit in conjunction with Bit 21 also provides the underrange or overrange indication. If both Bit 21 and Bit 20 are HIGH, the differential input voltage is above +FS. If both are LOW, the differential input voltage is below -FS. The function of these bits is summarized in Table 1.
Table 1. LTC2435/LTC2435-1 Status Bits
Input Range VIN 0.5 * VREF 0V VIN < 0.5 * VREF -0.5 * VREF VIN < 0V VIN < - 0.5 * VREF Bit 23 Bit 22 Bit 21 Bit 20 EOC DMY SIG MSB 0 0 0 0 0 0 0 0 1 1 0 0 1 0 1 0
Bits 20-0 are the 21-bit conversion result MSB first. Bit 0 is the least significant bit (LSB). Data is shifted out of the SDO pin under control of the serial clock (SCK), see Figure 3. Whenever CS is HIGH, SDO remains high impedance and any externally generated
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SCK clock pulses are ignored by the internal data out shift register. In order to shift the conversion result out of the device, CS must first be driven LOW. EOC is seen at the SDO pin of the device once CS is pulled LOW. EOC changes real time from HIGH to LOW at the completion of a conversion. This signal may be used as an interrupt for an external microcontroller. Bit 23 (EOC) can be captured on the first rising edge of SCK. Bit 22 is shifted out of the device on the first falling edge of SCK. The final data bit (Bit 0) is shifted out on the falling edge of the 23rd SCK and may be latched on the rising edge of the 24th SCK pulse. On the falling edge of the 24th SCK pulse, SDO goes HIGH indicating the initiation of a new conversion cycle. This bit serves as EOC (Bit 23) for the next conversion cycle. Table 2 summarizes the output data format. As long as the voltage on the IN+ and IN- pins is maintained within the - 0.3V to (VCC + 0.3V) absolute maximum operating range, a conversion result is generated for any differential input voltage VIN from -FS = -0.5 * VREF to +FS = 0.5 * VREF. For differential input voltages greater than +FS, the conversion result is clamped to the value corresponding to the +FS. For differential input voltages below -FS, the conversion result is clamped to the value corresponding to -FS - 1LSB. Offset Accuracy and Drift Unlike the LTC2430 and most of the LTC2400 family, the LTC2435/LTC2435-1 do not perform an offset calibration every cycle. The reason for this is to increase the data output rate while maintaining line frequency rejection. While the initial accuracy of the LTC2435/LTC2435-1 offset is within 5mV (see Figure 4), several unique properties of the LTC2435/LTC2435-1 architecture nearly eliminate the drift of the offset error with respect to temperature and supply. As shown in Figure 5, the offset variation with temperature is less than 3ppm over the complete temperature range of -50C to 100C. This corresponds to a temperature drift of 0.022ppm/C. While the variation in offset with supply voltage is propor24351fa
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
tional to VCC (see Figure 4), several characteristics of this variation can be used to eliminate the effects. First, the variation with respect to supply voltage is linear. Second, the magnitude of the offset error decreases with decreased supply voltage. Third, the offset error in microvolts is almost independent with reference and therefore
Table 2. LTC2435/LTC2435-1 Output Data Format
Differential Input Voltage VIN * VIN* 0.5 * VREF** 0.5 * VREF** - 1LSB 0.25 * VREF** 0.25 * VREF** - 1LSB 0 -1LSB - 0.25 * VREF** - 0.25 * VREF** - 1LSB - 0.5 * VREF** VIN* < -0.5 * VREF** *The differential input voltage VIN Bit 23 EOC 0 0 0 0 0 0 0 0 0 0 = IN+ - IN-. Bit 22 DMY 0 0 0 0 0 0 0 0 0 0 Bit 21 SIG 1 1 1 1 1 0 0 0 0 0
CS BIT 23 SDO Hi-Z SCK EOC BIT 22 "0" BIT 21 SIG BIT 20 MSB BIT 19 BIT 5 BIT 0 LSB
SLEEP
Figure 3. Output Data Timing
-350 -400 REF+ = 2.5V REF- = GND VIN = 0V VINCM = GND FO = GND TA = 25C -324 -325 -326 -327 -328 -329 -330 15 30 45 60 -45 -30 -15 0 TEMPERATURE (C) VCC = 5V VREF = 5V VIN = 0V VINCM = GND FO = GND -300 -305
OFFSET ERROR (ppm OF VREF)
OFFSET ERROR (ppm OF VREF)
OFFSET ERROR (ppm OF VREF)
-450 -500 -550 -600 -650 -700 -750 2.5 3.0 3.5 4.0 VCC (V)
4.5
5.0
5.5
2435 F04
Figure 4. Offset vs VCC
Figure 5. Offset vs Temperature
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the offset in ppm is inverse proportional to reference voltage. As a result, by tying VCC to VREF, the variation with supply can be reduced, see Figure 6. The variation with supply is less than 15ppm over the entire 2.7V to 5.5V supply range. Frequency Rejection Selection LTC2435 (FO)
Bit 20 MSB 1 0 0 0 0 1 1 1 1 0 Bit 19 0 1 1 0 0 1 1 0 0 1 Bit 18 0 1 0 1 0 1 0 1 0 1 Bit 17 0 1 0 1 0 1 0 1 0 1 ... ... ... ... ... ... ... ... ... ... ... = REF+ Bit 0 0 1 0 1 0 1 0 1 0 1 - REF-. **The differential reference voltage VREF
DATA OUTPUT CONVERSION
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-310 -315 -320 -325 -330 -335 -340 -345 -
REF+ = VCC REF- = GND VIN = 0V VINCM = GND FO = GND TA = 25C
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-350 2.5
3.0
3.5 4.0 4.5 VCC and VREF (V)
5.0
5.5
2435 F06
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Figure 6. Offset vs VCC (VREF = VCC)
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APPLICATIO S I FOR ATIO
The LTC2435 internal oscillator provides better than 110dB normal mode rejection at the line frequency and its harmonics for 50Hz 2% or 60Hz 2%. For 60Hz rejection, FO should be connected to GND while for 50Hz rejection the FO pin should be connected to VCC. The selection of 50Hz or 60Hz rejection can also be made by driving FO to an appropriate logic level. A selection change during the sleep or data output states will not disturb the converter operation. If the selection is made during the conversion state, the result of the conversion in progress may be outside specifications but the following conversions will not be affected. When a fundamental rejection frequency different from 50Hz or 60Hz is required or when the converter must be synchronized with an outside source, the LTC2435 can operate with an external conversion clock. The converter automatically detects the presence of an external clock signal at the FO pin and turns off the internal oscillator. The frequency fEOSC of the external signal must be at least 5kHz to be detected. The external clock signal duty cycle is not significant as long as the minimum and maximum specifications for the high and low periods tHEO and tLEO are observed. While operating with an external conversion clock of a frequency fEOSC, the LTC2435 provides better than 110dB normal mode rejection in a frequency range fEOSC/2560 4% and its harmonics. The normal mode rejection as a function of the input frequency deviation from fEOSC/2560 is shown in Figure 7a. Whenever an external clock is not present at the FO pin, the converter automatically activates its internal oscillator and enters the Internal Conversion Clock mode. The LTC2435 operation will not be disturbed if the change of conversion clock source occurs during the sleep state or during the data output state while the converter uses an external serial clock. If the change occurs during the conversion state, the result of the conversion in progress may be outside specifications but the following conversions will not be affected.
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If the change occurs during the data output state and the converter is in the Internal SCK mode, the serial clock duty cycle may be affected but the serial data stream will remain valid. Table 3a summarizes the duration of each state and the achievable output data rate as a function of FO. Frequency Rejection Selection LTC2435-1 (FO) The LTC2435-1 internal oscillator provides better than 87dB normal mode rejection over the range of 49Hz to 61.2Hz as shown in Figure 7b. For simultaneous 50Hz/60Hz rejection, FO should be connected to GND. In order to achieve 87dB normal mode rejection of 50Hz 2% and 60Hz 2%, two consecutive conversions must be averaged. By performing a continuous running average of the two most current results, both simultaneous rejection is achieved and a nearly 2x increase in throughput is realized relative to the LTC2430 (see Normal Mode Rejection, Ouput Rate and Running Averages sections of this data sheet). When a fundamental rejection frequency different from the range 49Hz to 61.2Hz is required or when the converter must be synchronized with an outside source, the LTC2435-1 can operate with an external conversion clock. The performance of the LTC2435-1 is the same as the LTC2435 when driven by an external conversion clock at the FO pin. Table 3b summarizes the duration of each state and the achievable output data rate as a function of FO. Serial Interface Pins The LTC2435/LTC2435-1 transmit the conversion results and receive the start of conversion command through a synchronous 3-wire interface. During the conversion and sleep states, this interface can be used to assess the converter status and during the data output state it is used to read the conversion result.
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
-60 -70 -80 REJECTION (dB) -90 -100 -110 -120 -130 -80
NORMAL MODE REECTION RATIO (dB)
NORMAL MODE REJECTION (dB)
2435 F07b
-90 -100 -100 -120 -130
-140 -140 -12 -8 -4 0 4 8 12 48 50 52 54 56 58 60 62 INPUT FREQUENCY DEVIATION FROM NOTCH FREQUENCY (%) DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
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Figure 7a. LTC2435/LTC2435-1 Normal Mode Rejection When Using an External Oscillator of Frequency fEOSC without Running Averages
Figure 7b. LTC2435-1 Normal Mode Rejection When Using an Internal Oscillator with Running Averages
Table 3a. LTC2435 State Duration
State CONVERT Operating Mode Internal Oscillator External Oscillator SLEEP DATA OUTPUT Internal Serial Clock FO = LOW/HIGH, (Internal Oscillator) FO = External Oscillator with Frequency fEOSC kHz External Serial Clock with Frequency fSCK kHz FO = LOW, (60Hz Rejection) FO = HIGH, (50Hz Rejection) Duration 67ms, Output Data Rate 15 Readings/s 80ms, Output Data Rate 12.4 Readings/s
FO = External Oscillator with Frequency 10278/fEOSCs, Output Data Rate fEOSC/10278 Readings/s fEOSC kHz (fEOSC/2560 Rejection) As Long As CS = HIGH As Long As CS = LOW But Not Longer Than 1.25ms (24 SCK cycles) As Long As CS = LOW But Not Longer Than 192/fEOSCms (24 SCK cycles) As Long As CS = LOW But Not Longer Than 24/fSCKms (24 SCK cycles)
Table 3b. LTC2435-1 State Duration
State CONVERT Operating Mode Internal Oscillator External Oscillator SLEEP DATA OUTPUT Internal Serial Clock FO = LOW (Internal Oscillator) FO = External Oscillator with Frequency fEOSC kHz External Serial Clock with Frequency fSCK kHz FO = LOW Simultaneous 50Hz/60Hz Rejection Duration 73ms, Output Data Rate 14 Readings/s
FO = External Oscillator with Frequency 10278/fEOSCs, Output Data Rate fEOSC/10278 Readings/s fEOSC kHz (fEOSC/2560 Rejection) As Long As CS = HIGH As Long As CS = LOW But Not Longer Than 1.4ms (24 SCK cycles) As Long As CS = LOW But Not Longer Than 192/fEOSCms (24 SCK cycles) As Long As CS = LOW But Not Longer Than 24/fSCKms (24 SCK cycles)
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-80 -85 -90 -95 -100 -105 -110 -115 -120 -125 -130 -135 -140 -12 -8 -4 0 4 8 12 DIFFERENTIAL INPUT SIGNAL FREQUENCY DEVIATION FROM NOTCH FREQUENCY fEOSC/2560(%)
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Figure 7c. LTC2435/LTC2435-1 Normal Mode Rejection When Using an External Oscillator of Frequency fEOSC with Running Averages
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APPLICATIO S I FOR ATIO
Serial Clock Input/Output (SCK)
The serial clock signal present on SCK (Pin 13) is used to synchronize the data transfer. Each bit of data is shifted out the SDO pin on the falling edge of the serial clock. In the Internal SCK mode of operation, the SCK pin is an output and the LTC2435/LTC2435-1 create their own serial clock by dividing the internal conversion clock by 8. In the External SCK mode of operation, the SCK pin is used as input. The internal or external SCK mode is selected on power-up and then reselected every time a HIGH-to-LOW transition is detected at the CS pin. If SCK is HIGH or floating at power-up or during this transition, the converter enters the internal SCK mode. If SCK is LOW at power-up or during this transition, the converter enters the external SCK mode. Serial Data Output (SDO) The serial data output pin, SDO (Pin 12), provides the result of the last conversion as a serial bit stream (MSB first) during the data output state. In addition, the SDO pin is used as an end of conversion indicator during the conversion and sleep states. When CS (Pin 11) is HIGH, the SDO driver is switched to a high impedance state. This allows sharing the serial interface with other devices. If CS is LOW during the convert or sleep state, SDO will output EOC. If CS is LOW during the conversion phase, the EOC bit appears HIGH on the SDO pin. Once the conversion is complete, EOC goes LOW.
Table 4. LTC2435/LTC2435-1 Interface Timing Modes
SCK Source External External Internal Internal
Configuration External SCK, Single Cycle Conversion External SCK, 2-Wire I/O Internal SCK, Single Cycle Conversion Internal SCK, 2-Wire I/O, Continuous Conversion
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Chip Select Input (CS) The active LOW chip select, CS (Pin 11), is used to test the conversion status and to enable the data output transfer as described in the previous sections. In addition, the CS signal can be used to trigger a new conversion cycle before the entire serial data transfer has been completed. The LTC2435/LTC2435-1 will abort any serial data transfer in progress and start a new conversion cycle anytime a LOW-to-HIGH transition is detected at the CS pin after the converter has entered the data output state (i.e., after the first rising edge of SCK occurs with CS = LOW). Finally, CS can be used to control the free-running modes of operation, see Serial Interface Timing Modes section. Grounding CS will force the ADC to continuously convert at the maximum output rate selected by FO. SERIAL INTERFACE TIMING MODES The LTC2435/LTC2435-1 3-wire interface is SPI and MICROWIRE compatible. This interface offers several flexible modes of operation. These include internal/external serial clock, 2- or 3-wire I/O, single cycle conversion and autostart. The following sections describe each of these serial interface timing modes in detail. In all these cases, the converter can use the internal oscillator (FO = LOW or FO = HIGH) or an external oscillator connected to the FO pin. Refer to Table 4 for a summary.
Conversion Cycle Control CS and SCK SCK CS Continuous Data Output Control CS and SCK SCK CS Internal Connection and Waveforms Figures 8, 9 Figure 10 Figures 11, 12 Figure 13
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External Serial Clock, Single Cycle Operation (SPI/MICROWIRE Compatible) This timing mode uses an external serial clock to shift out the conversion result and a CS signal to monitor and control the state of the conversion cycle, see Figure 8. The serial clock mode is selected on the falling edge of CS. To select the external serial clock mode, the serial clock pin (SCK) must be LOW during each CS falling edge. The serial data output pin (SDO) is Hi-Z as long as CS is HIGH. At any time during the conversion cycle, CS may be pulled LOW in order to monitor the state of the converter. While CS is pulled LOW, EOC is output to the SDO pin. EOC = 1 while a conversion is in progress and EOC = 0 if the conversion is over. With CS HIGH, the device automatically enters the sleep state once the conversion is complete. When CS is low, the devcice enters the data output mode. The result is held in the internal static shift register until the first SCK rising edge is seen while CS is LOW. Data is shifted out the SDO pin on each falling edge of SCK. This enables external circuitry to latch the output on the rising edge of SCK. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result can be
2.7V TO 5.5V 1F 2 VCC
REFERENCE VOLTAGE 0.1V TO VCC ANALOG INPUT RANGE -0.5VREF TO 0.5VREF
1, 7, 8, 9, 10, 15, 16
CS TEST EOC SDO Hi-Z Hi-Z TEST EOC
BIT 23 EOC
BIT 22
SCK (EXTERNAL) CONVERSION SLEEP SLEEP TEST EOC DATA OUTPUT CONVERSION
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Figure 8. External Serial Clock, Single Cycle Operation
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latched on the 24th rising edge of SCK. On the 24th falling edge of SCK, the device begins a new conversion. SDO goes HIGH (EOC = 1) indicating a conversion is in progress. At the conclusion of the data cycle, CS may remain LOW and EOC monitored as an end-of-conversion interrupt. Alternatively, CS may be driven HIGH setting SDO to Hi-Z. As described above, CS may be pulled LOW at any time in order to monitor the conversion status. Typically, CS remains LOW during the data output state. However, the data output state may be aborted by pulling CS HIGH anytime between the first rising edge and the 24th falling edge of SCK, see Figure 9. On the rising edge of CS, the device aborts the data output state and immediately initiates a new conversion. This is useful for systems not requiring all 24 bits of output data, aborting an invalid conversion cycle or synchronizing the start of a conversion. External Serial Clock, 2-Wire I/O This timing mode utilizes a 2-wire serial I/O interface. The conversion result is shifted out of the device by an externally generated serial clock (SCK) signal, see Figure 10. CS may be permanently tied to ground, simplifying the user interface or isolation barrier.
VCC
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FO
14
LTC2435/ LTC2435-1 3 4 5 6 REF + REF - IN + IN - GND SDO CS 12 11 SCK 13
= 50Hz REJECTION (LTC2435) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2435) = 50Hz/60Hz REJECTION (LTC2435-1)
3-WIRE SPI INTERFACE
BIT 21 SIG
BIT 20 MSB
BIT 19
BIT 18
BIT 5
BIT 0 LSB
Hi-Z
19
LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
REFERENCE VOLTAGE 0.1V TO VCC ANALOG INPUT RANGE -0.5VREF TO 0.5VREF
1, 7, 8, 9, 10, 15, 16
CS TEST EOC TEST EOC
BIT 0 SDO EOC
BIT 23 EOC
Hi-Z
Hi-Z
Hi-Z
SCK (EXTERNAL) SLEEP DATA OUTPUT CONVERSION SLEEP TEST EOC SLEEP DATA OUTPUT CONVERSION
2435 F09
Figure 9. External Serial Clock, Reduced Data Output Length
The external serial clock mode is selected at the end of the power-on reset (POR) cycle. The POR cycle is concluded approximately 1ms after VCC exceeds 2.2V. The level applied to SCK at this time determines if SCK is internal or external. SCK must be driven LOW prior to the end of POR in order to enter the external serial clock timing mode. Since CS is tied LOW, the end-of-conversion (EOC) can be continuously monitored at the SDO pin during the convert and sleep states. EOC may be used as an interrupt to an external controller indicating the conversion result is ready. EOC = 1 while the conversion is in progress and EOC = 0 once the conversion is over. On the falling edge of EOC, the conversion result is loaded into an internal static shift register. Data is shifted out the SDO pin on each falling edge of SCK enabling external circuitry to latch data on the rising edge of SCK. EOC can be latched on the first rising edge of SCK. On the 24th falling edge of SCK, SDO goes HIGH (EOC = 1) indicating a new conversion has begun.
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2.7V TO 5.5V 1F 2 VCC FO 14
VCC
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LTC2435/ LTC2435-1 3 4 5 6 REF + REF IN + IN - GND
-
= 50Hz REJECTION (LTC2435) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2435) = 50Hz/60Hz REJECTION (LTC2435-1)
SCK
13 3-WIRE SPI INTERFACE
SDO CS
12 11
BIT 22
BIT 21 SIG
BIT 20 MSB
BIT 19
BIT 9
BIT 8
Hi-Z
Internal Serial Clock, Single Cycle Operation This timing mode uses an internal serial clock to shift out the conversion result and a CS signal to monitor and control the state of the conversion cycle, see Figure 11. In order to select the internal serial clock timing mode, the serial clock pin (SCK) must be floating (Hi-Z) or pulled HIGH prior to the falling edge of CS. The device will not enter the internal serial clock mode if SCK is driven LOW on the falling edge of CS. An internal weak pull-up resistor is active on the SCK pin during the falling edge of CS; therefore, the internal serial clock timing mode is automatically selected if SCK is not externally driven. The serial data output pin (SDO) is Hi-Z as long as CS is HIGH. At any time during the conversion cycle, CS may be pulled LOW in order to monitor the state of the converter. Once CS is pulled LOW, SCK goes LOW and EOC is output to the SDO pin. EOC = 1 while a conversion is in progress and EOC = 0 if the conversion is over.
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
When testing EOC, if the conversion is complete (EOC = 0), the device will exit the sleep state and enter the data output state. In order to allow the device to return to the sleep state, CS must be pulled HIGH before the first rising edge of SCK. In the internal SCK timing mode, SCK goes HIGH
2.7V TO 5.5V 1F 2 VCC
ANALOG INPUT RANGE -0.5VREF TO 0.5VREF
1, 7, 8, 9, 10, 15, 16
CS
BIT 23 SDO EOC
BIT 22
SCK (EXTERNAL) CONVERSION DATA OUTPUT CONVERSION
2435 F10
Figure 10. External Serial Clock, CS = 0 Operation (2-Wire)
2.7V TO 5.5V 1F 2 VCC FO 14
VCC
ANALOG INPUT RANGE -0.5VREF TO 0.5VREF
1, 7, 8, 9, 10, 15, 16
BIT 23 SDO Hi-Z Hi-Z EOC
BIT 22
SCK (INTERNAL) CONVERSION SLEEP TEST EOC SLEEP DATA OUTPUT CONVERSION
2435 F11
Figure 11. Internal Serial Clock, Single Cycle Operation
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and the device begins outputting data at time tEOCtest after the falling edge of CS (if EOC = 0) or tEOCtest after EOC goes LOW (if CS is LOW during the falling edge of EOC). The value of tEOCtest is 23s (LTC2435), 26s (LTC2435-1) if the device is using its internal oscillator (F0 = logic LOW or
VCC
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FO
14
LTC2435/ LTC2435-1 REFERENCE VOLTAGE 0.1V TO VCC 3 4 5 6 REF + REF - IN
+
= 50Hz REJECTION (LTC2435) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2435) = 50Hz/60Hz REJECTION (LTC2435-1)
SCK
13 2-WIRE INTERFACE
SDO CS
12 11
IN - GND
BIT 21 SIG
BIT 20 MSB
BIT 19
BIT 18
BIT 5
BIT 0 LSB
VCC = 50Hz REJECTION (LTC2435) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2435) = 50Hz/60Hz REJECTION (LTC2435-1)
10k
LTC2435/ LTC2435-1 REFERENCE VOLTAGE 0.1V TO VCC 3 4 5 6 REF + REF - IN + IN - GND SDO CS 12 11 SCK 13
3-WIRE SPI INTERFACE
BIT 21 SIG
BIT 20 MSB
BIT 19
BIT 18
BIT 5
BIT 0 LSB
Hi-Z
Hi-Z
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
HIGH). If FO is driven by an external oscillator of frequency fEOSC, then tEOCtest is 3.6/fEOSC. If CS is pulled HIGH before time tEOCtest, the device returns to the sleep state. The conversion result is held in the internal static shift register. If CS remains LOW longer than tEOCtest, the first rising edge of SCK will occur and the conversion result is serially shifted out of the SDO pin. The data output cycle begins on this first rising edge of SCK and concludes after the 24th rising edge. Data is shifted out the SDO pin on each falling edge of SCK. The internally generated serial clock is output to the SCK pin. This signal may be used to shift the conversion result into external circuitry. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result on the 24th rising edge of SCK. After the 24th rising edge, SDO goes HIGH (EOC = 1), SCK stays HIGH and a new conversion starts. Typically, CS remains LOW during the data output state. However, the data output state may be aborted by pulling CS HIGH anytime between the first and 24th rising edge of SCK, see Figure 12. On the rising edge of CS, the device aborts the data output state and immediately initiates a
2.7V TO 5.5V 1F 2 VCC
REFERENCE VOLTAGE 0.1V TO VCC ANALOG INPUT RANGE -0.5VREF TO 0.5VREF
1, 7, 8, 9, 10, 15, 16
> tEOCtest CS TEST EOC
BIT 0 SDO Hi-Z EOC Hi-Z
BIT 23 EOC Hi-Z Hi-Z
SCK (INTERNAL) SLEEP DATA OUTPUT CONVERSION SLEEP TEST EOC SLEEP DATA OUTPUT CONVERSION
2435 F12
Figure 12. Internal Serial Clock, Reduced Data Output Length
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new conversion. This is useful for systems not requiring all 24 bits of output data, aborting an invalid conversion cycle, or synchronizing the start of a conversion. If CS is pulled HIGH while the converter is driving SCK LOW, the internal pull-up is not available to restore SCK to a logic HIGH state. This will cause the device to exit the internal serial clock mode on the next falling edge of CS. This can be avoided by adding an external 10k pull-up resistor to the SCK pin or by never pulling CS HIGH when SCK is LOW. Whenever SCK is LOW, the LTC2435/LTC2435-1 internal pull-up at pin SCK is disabled. Normally, SCK is not externally driven if the device is in the internal SCK timing mode. However, certain applications may require an external driver on SCK. If this driver goes Hi-Z after outputting a LOW signal, the LTC2435/LTC2435-1 internal pull-up remains disabled. Hence, SCK remains LOW. On the next falling edge of CS, the device is switched to the external SCK timing mode. By adding an external 10k pull-up resistor to SCK, this pin goes HIGH once the external driver goes Hi-Z. On the next CS falling edge, the device will remain in the internal SCK timing mode.
VCC
VCC
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FO
14
LTC2435/ LTC2435-1 3 4 5 6 REF + REF - IN + IN - GND SDO CS 12 11 SCK 13
= 50Hz REJECTION (LTC2435) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2435) = 50Hz/60Hz REJECTION (LTC2435-1)
10k
3-WIRE SPI INTERFACE
BIT 22
BIT 21 SIG
BIT 20 MSB
BIT 19
BIT 18
BIT 8
TEST EOC
Hi-Z
LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
A similar situation may occur during the sleep state when CS is pulsed HIGH-LOW-HIGH in order to test the conversion status. If the device is in the sleep state (EOC = 0), SCK will go LOW. Once CS goes HIGH (within the time period defined above as tEOCtest), the internal pull-up is activated. For a heavy capacitive load on the SCK pin, the internal pull-up may not be adequate to return SCK to a HIGH level before CS goes low again. This is not a concern under normal conditions where CS remains LOW after detecting EOC = 0. This situation is easily overcome by adding an external 10k pull-up resistor to the SCK pin. Internal Serial Clock, 2-Wire I/O, Continuous Conversion This timing mode uses a 2-wire, all output (SCK and SDO) interface. The conversion result is shifted out of the device by an internally generated serial clock (SCK) signal, see Figure 13. CS may be permanently tied to ground, simplifying the user interface or isolation barrier. The internal serial clock mode is selected at the end of the power-on reset (POR) cycle. The POR cycle is concluded
REFERENCE VOLTAGE 0.1V TO VCC ANALOG INPUT RANGE -0.5VREF TO 0.5VREF
1, 7, 8, 9, 10, 15, 16 CS
SDO
BIT 23 EOC
BIT 22
BIT 21 SIG
SCK (INTERNAL) CONVERSION DATA OUTPUT CONVERSION
2435 F13
Figure 13. Internal Serial Clock, Continuous Operation
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approximately 1ms after VCC exceeds 2.2V. An internal weak pull-up is active during the POR cycle; therefore, the internal serial clock timing mode is automatically selected if SCK is not externally driven LOW (if SCK is loaded such that the internal pull-up cannot pull the pin HIGH, the external SCK mode will be selected). During the conversion, the SCK and the serial data output pin (SDO) are HIGH (EOC = 1). Once the conversion is complete, SCK and SDO go LOW (EOC = 0) indicating the conversion has finished. The data output cycle begins on the first rising edge of SCK and ends after the 24th rising edge. Data is shifted out the SDO pin on each falling edge of SCK. The internally generated serial clock is output to the SCK pin. This signal may be used to shift the conversion result into external circuitry. EOC can be latched on the first rising edge of SCK and the last bit of the conversion result can be latched on the 24th rising edge of SCK. After the 24th rising edge, SDO goes HIGH (EOC = 1) indicating a new conversion is in progress. SCK remains HIGH during the conversion.
2.7V TO 5.5V 1F 2 VCC FO 14
VCC
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LTC2435/ LTC2435-1 3 4 5 6 REF + REF - IN + IN - GND SDO CS 12 11 SCK 13
= 50Hz REJECTION (LTC2435) = EXTERNAL OSCILLATOR = 60Hz REJECTION (LTC2435) = 50Hz/60Hz REJECTION (LTC2435-1)
2-WIRE INTERFACE
BIT 20 MSB
BIT 19
BIT 18
BIT 5
BIT 0 LSB
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
PRESERVING THE CONVERTER ACCURACY
The LTC2435/LTC2435-1 are designed to reduce as much as possible conversion result sensitivity to device decoupling, PCB layout, antialiasing circuits, line frequency perturbations and so on. Nevertheless, in order to preserve the extreme accuracy capability of this part, some simple precautions are desirable. Digital Signal Levels The LTC2435/LTC2435-1 digital interface is easy to use. Its digital inputs (FO, CS and SCK in External SCK mode of operation) accept standard TTL/CMOS logic levels and the internal hysteresis receivers can tolerate edge rates as slow as 100s. However, some considerations are required to take advantage of the exceptional accuracy and low supply current of this converter. The digital output signals (SDO and SCK in Internal SCK mode of operation) are less of a concern because they are not generally active during conversion. While a digital input signal is in the range 0.5V to (VCC - 0.5V), the CMOS input receiver draws additional current from the power supply. It should be noted that, when any one of the digital input signals (FO, CS and SCK in External SCK mode of operation) is within this range, the LTC2435/LTC2435-1 power supply current may increase even if the signal in question is at a valid logic level. For micropower operation, it is recommended to drive all digital input signals to full CMOS levels [VIL < 0.4V and VOH > (VCC - 0.4V)]. During the conversion period, the undershoot and/or overshoot of a fast digital signal connected to the LTC2435/ LTC2435-1 pins may severely disturb the analog to digital conversion process. Undershoot and overshoot can occur because of the impedance mismatch at the converter pin when the transition time of an external control signal is less than twice the propagation delay from the driver to LTC2435/LTC2435-1. For reference, on a regular FR-4 board, signal propagation velocity is approximately 183ps/inch for internal traces and 170ps/inch for surface traces. Thus, a driver generating a control signal with a minimum transition time of 1ns must be connected to the
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converter pin through a trace shorter than 2.5 inches. This problem becomes particularly difficult when shared control lines are used and multiple reflections may occur. The solution is to carefully terminate all transmission lines close to their characteristic impedance. Parallel termination near the LTC2435/LTC2435-1 pins will eliminate this problem but will increase the driver power dissipation. A series resistor between 27 and 56 placed near the driver or near the LTC2435/LTC2435-1 pins will also eliminate this problem without additional power dissipation. The actual resistor value depends upon the trace impedance and connection topology. An alternate solution is to reduce the edge rate of the control signals. It should be noted that using very slow edges will increase the converter power supply current during the transition time. The multiple ground pins used in this package configuration, as well as the differential input and reference architecture, reduce substantially the converter's sensitivity to ground currents. Particular attention must be given to the connection of the FO signal when the LTC2435/LTC2435-1 are used with an external conversion clock. This clock is active during the conversion time and the normal mode rejection provided by the internal digital filter is not very high at this frequency. A normal mode signal of this frequency at the converter reference terminals may result in DC gain and INL errors. A normal mode signal of this frequency at the converter input terminals may result in a DC offset error. Such perturbations may occur due to asymmetric capacitive coupling between the FO signal trace and the converter input and/or reference connection traces. An immediate solution is to maintain maximum possible separation between the FO signal trace and the input/reference signals. When the FO signal is parallel terminated near the converter, substantial AC current is flowing in the loop formed by the FO connection trace, the termination and the ground return path. Thus, perturbation signals may be inductively coupled into the converter input and/or reference. In this situation, the user must reduce to a minimum the loop area for the FO signal as well as the loop area for the differential input and reference connections.
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
Driving the Input and Reference
The input and reference pins of the LTC2435/LTC2435-1 converters are directly connected to a network of sampling capacitors. Depending upon the relation between the differential input voltage and the differential reference voltage, these capacitors are switching between these four pins transferring small amounts of charge in the process. A simplified equivalent circuit is shown in Figure 14. For a simple approximation, the source impedance RS driving an analog input pin (IN+, IN-, REF+ or REF-) can be considered to form, together with RSW and CEQ (see Figure 14), a first order passive network with a time constant = (RS + RSW) * CEQ. The converter is able to sample the input signal with better than 1ppm accuracy if the sampling period is at least 14 times greater than the input circuit time constant . The sampling process on the four input analog pins is quasi-independent so each time constant should be considered by itself and, under worstcase circumstances, the errors may add. When using the internal oscillator (FO = LOW or HIGH), the LTC2435's front-end switched-capacitor network is clocked at 76800Hz corresponding to a 13s sampling period and the LTC2435-1's front end is clocked at 69900Hz corresponding to 14.2s. Thus, for settling errors of less than 1ppm, the driving source impedance should be chosen such that 13s/14 = 920ns (LTC2435) and <14.2s/ 14 = 1.01s (LTC2435-1). When an external oscillator of frequency fEOSC is used, the sampling period is 2/fEOSC and, for a settling error of less than 1ppm, 0.14/fEOSC. Input Current If complete settling occurs on the input, conversion results will be unaffected by the dynamic input current. An incomplete settling of the input signal sampling process may result in gain and offset errors, but it will not degrade the INL performance of the converter. Figure 14 shows the mathematical expressions for the average bias currents flowing through the IN + and IN - pins as a result of the sampling charge transfers when integrated over a substantial time period (longer than 64 internal clock cycles).
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The effect of this input dynamic current can be analyzed using the test circuit of Figure 15. The CPAR capacitor includes the LTC2435/LTC2435-1 pin capacitance (5pF typical) plus the capacitance of the test fixture used to obtain the results shown in Figures 16 and 17. A careful implementation can bring the total input capacitance (CIN + CPAR) closer to 5pF thus achieving better performance than the one predicted by Figures 16 and 17. For simplicity, two distinct situations can be considered. For relatively small values of input capacitance (CIN < 0.01F), the voltage on the sampling capacitor settles almost completely and relatively large values for the source impedance result in only small errors. Such values for CIN will deteriorate the converter offset and gain performance without significant benefits of signal filtering and the user is advised to avoid them. Nevertheless, when small values of CIN are unavoidably present as parasitics of input multiplexers, wires, connectors or sensors, the LTC2435/LTC2435-1 can maintain their exceptional accuracy while operating with relative large values of source resistance as shown in Figures 16 and 17. These measured results may be slightly different from the first order approximation suggested earlier because they include the effect of the actual second order input network together with the nonlinear settling process of the input amplifiers. For small CIN values, the settling on IN+ and IN - occurs almost independently and there is little benefit in trying to match the source impedance for the two pins. Larger values of input capacitors (CIN > 0.01F) may be required in certain configurations for antialiasing or general input signal filtering. Such capacitors will average the input sampling charge and the external source resistance will see a quasi constant input differential impedance. When FO = LOW (internal oscillator and 60Hz notch), the typical differential input resistance is 22M (LTC2435) or 24M (LTC2435-1) which will generate a +FS gain error of approximately 0.023ppm (LTC2435) or 0.021ppm (LTC2435-1) for each ohm of source resistance driving IN+ or IN -. For the LTC2435, when FO = HIGH (internal oscillator and 50Hz notch), the typical differential input resistance is 26M which will generate a +FS gain error of approximately 0.019ppm for each ohm of source resis24351fa
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
IREF+ VREF+ ILEAK IIN+ VIN+ ILEAK IIN - VIN - ILEAK IREF - VREF - ILEAK VCC ILEAK RSW (TYP) 20k VCC ILEAK RSW (TYP) 20k VCC ILEAK RSW (TYP) 20k CEQ 18pF (TYP) VCC ILEAK RSW (TYP) 20k
SWITCHING FREQUENCY fSW = 76800Hz INTERNAL OSCILLATOR (LTC2435) (FO = LOW OR HIGH) fSW = 69900Hz INTERNAL OSCILLATOR (LTC2435-1) (FO = LOW) fSW = 0.5 * fEOSC EXTERNAL OSCILLATOR
2435 F18
Figure 14. LTC2435/LTC2435-1 Equivalent Analog Input Circuit
VINCM + 0.5VIN
VINCM - 0.5VIN
Figure 15. An RC Network at IN + and IN -
10 0 +FS ERROR VARIATION (ppm) -20 -30 -40 -50 -60 -70 -80 -90 -100 1 VCC = 5V VREF+ = 5V VREF- = GND VIN+ = 3.75V VIN- = 1.25V FO = GND TA = 25C 10 -10
CIN = 0pF -FS ERROR VARIATION (ppm) CIN = 0.01F
CIN = 0.001F CIN = 100pF
1000 100 RSOURCE (W)
10000 100000
2435 F16
Figure 16. +FS Error vs RSOURCE at IN+ or IN- (Small CIN)
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V ( )AVG = VIN + V0INCMR-EQREFCM .5 * -V + V -V = IN INCM REFCM I(IN- ) AVG 0.5 * REQ I IN+ I REF + I REF - where: VREF = REF + - REF - REF + + REF - VREFCM = 2 VIN = IN+ - IN- IN+ - IN- VINCM = 2 REQ = 43.2M INTERNAL OSCILLATOR 60Hz NOTCH (FO = LOW) LTC2435 REQ = 52M INTERNAL OSCILLATOR 50Hz NOTCH (FO = HIGH) LTC2435 REQ = 48M INTERNAL OSCILLATOR 50Hz/60Hz NOTCH (FO = LOW) LTC2435-1 REQ = (6.7 * 1012)/fEOSC EXTERNAL OSCILLATOR
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(
)AVG = 1.5 * VREF0-5V*INCM + VREFCM - VREF INREQ * . REQ )AVG = -1.5 * VREF0.- VINCM + VREFCM + VREF INREQ 5 * REQ *
V2
V2
(
RSOURCE CPAR 20pF
IN + CIN
LTC2435/ LTC2435-1 IN -
RSOURCE CPAR 20pF
CIN
2435 F19
100 90 80 70 60 50 40 30 20 10 0 -10 1
VCC = 5V VREF+ = 5V VREF- = GND VIN+ = 1.25V VIN- = 3.75V FO = GND TA = 25C
CIN = 0.01F CIN = 100pF CIN = 0.001F
CIN = 0pF 10 1000 100 RSOURCE (W) 10000 100000
2435 F17
Figure 17. -FS Error vs RSOURCE at IN+ or IN- (Small CIN)
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
tance driving IN+ or IN -. When FO is driven by an external oscillator with a frequency fEOSC (external conversion clock operation), the typical differential input resistance is 3.3 * 1012/fEOSC and each ohm of source resistance driving IN+ or IN - will result in 0.15 * 10-6 * fEOSC ppm +FS gain error. The effect of the source resistance on the two input pins is additive with respect to this gain error. The typical +FS and -FS errors as a function of the sum of the source resistance seen by IN+ and IN- for large values of CIN are shown in Figures 18 and 19. In addition to this gain error, an offset error term may also appear. The offset error is proportional to the mismatch between the source impedance driving the two input pins IN+ and IN- and with the difference between the input and reference common mode voltages. While the input drive circuit nonzero source impedance combined with the converter average input current will not degrade the INL performance, indirect distortion may result from the modulation of the offset error by the common mode component of the input signal. Thus, when using large CIN capacitor values, it is advisable to carefully match the source impedance seen by the IN+ and IN- pins. When FO = LOW (internal oscillator and 60Hz notch), every 1 mismatch in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 0.023ppm. When FO = HIGH (internal oscillator and 50Hz
0 -10
+FS ERROR VARIATION (ppm)
100
CIN = 0.01F
-20 -30 -40 -50 CIN = 1F, 10F -60 VCC = 5V VREF+ = 5V -70 VREF- = GND VIN+ = 3.75V -80 V - = 1.25V IN -90 FO = GND TA = 25C -100 800 0 400 1200 RSOURCE () CIN = 0.1F
-FS ERROR VARIATION (ppm)
OFFSET ERROR (ppm)
VCC = 5V 90 VREF+ = 5V V - = GND 80 VREF = 1.25V IN+ 70 VIN- = 3.75V FO = GND 60 TA = 25C 50 40 30 20 10 0 CIN = 1F, 10F CIN = 0.1F CIN = 0.01F
1600
2000
2435 F18
0
Figure 18. +FS Error vs RSOURCE at IN+ or IN- (Large CIN)
Figure 19. -FS Error vs RSOURCE at IN+ or IN- (Large CIN)
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notch), every 1 mismatch in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 0.02ppm. When FO is driven by an external oscillator with a frequency fEOSC, every 1 mismatch in source impedance transforms a full-scale common mode input signal into a differential mode input signal of 0.15 * 10-6 * fEOSCppm. Figure 20 shows the typical offset error due to input common mode voltage for various values of source resistance imbalance between the IN+ and IN- pins when large CIN values are used. If possible, it is desirable to operate with the input signal common mode voltage very close to the reference signal common mode voltage as is the case in the ratiometric measurement of a symmetric bridge. This configuration eliminates the offset error caused by mismatched source impedances. The magnitude of the dynamic input current depends upon the size of the very stable internal sampling capacitors and upon the accuracy of the converter sampling clock. The accuracy of the internal clock over the entire temperature and power supply range is typical better than 0.5%. Such a specification can also be easily achieved by an external clock. When relatively stable resistors (50ppm/C) are used for the external source impedance seen by IN+ and IN-, the expected drift of the dynamic current, offset and
-310 -320 -330 -340 -350 -360 -370 -380
400 800 1200 RSOURCE () 1600 2000
2435 F19
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E: RIN = -200 A: RIN = 1k B: RIN = 500 F: RIN = -500 C: RIN = 200 G: RIN = -1k D: RIN = 0
A
B C D E VCC = 5V FO = GND VREF+ = 5V TA = 25C - = GND C = 10F VREF IN VIN+ = VIN- = VINCM F G
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 VINCM (V)
2435 F20
Figure 20. Offset Error vs Common Mode Voltage (VINCM = VIN+ = VIN-) and Input Source Resistance Imbalance (RIN = RSOURCEIN+ - RSOURCEIN-) for Large CIN Values (CIN 1F)
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
gain errors will be insignificant (about 1% of their respective values over the entire temperature and voltage range). Even for the most stringent applications, a one-time calibration operation may be sufficient. In addition to the input sampling charge, the input ESD protection diodes have a temperature dependent leakage current. This current, nominally 1nA (10nA max), results in a small offset shift. A 100 source resistance will create a 0.1V typical and 1V maximum offset voltage. Reference Current In a similar fashion, the LTC2435/LTC2435-1 sample the differential reference pins REF+ and REF- transferring small amount of charge to and from the external driving circuits thus producing a dynamic reference current. This current does not change the converter offset, but it may degrade the gain and INL performance. The effect of this current can be analyzed in the same two distinct situations. For relatively small values of the external reference capacitors (CREF < 0.01F), the voltage on the sampling capacitor settles almost completely and relatively large values for the source impedance result in only small errors. Such values for CREF will deteriorate the converter offset and gain performance without significant benefits of reference filtering and the user is advised to avoid them.
100 90
+FS ERROR VARIATION (ppm)
70 60 50 40 30 20 10 0 -10 1
-FS ERROR VARIATION (ppm)
80
VCC = 5V VREF+ = 5V VREF- = GND VIN+ = 3.75V VIN- = 1.25V FO = GND TA = 25C
CIN = 0.01F CIN = 100pF CIN = 0.001F
CIN = 0pF 10 1000 100 RSOURCE () 10000 100000
2435 F21
Figure 21. +FS Error vs RSOURCE at REF+ or REF- (Small CIN)
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Larger values of reference capacitors (CREF > 0.01F) may be required as reference filters in certain configurations. Such capacitors will average the reference sampling charge and the external source resistance will see a quasi constant reference differential impedance. For the LTC2435, when FO = LOW (internal oscillator and 60Hz notch), the typical differential reference resistance is 15.6M which will generate a +FS gain error of approximately 0.032ppm for each ohm of source resistance driving REF+ or REF-. When FO = HIGH (internal oscillator and 50Hz notch), the typical differential reference resistance is 18.7M which will generate a +FS gain error of approximately 0.027ppm for each ohm of source resistance driving REF+ or REF-. For the LTC2435-1, the typical differential reference resistance is 17.1M which will generate a +FS gain error of approximately 0.029ppm for each ohm of source resistance driving REF + or REF -. When FO is driven by an external oscillator with a frequency fEOSC (external conversion clock operation), the typical differential reference resistance is 2.4 * 1012/fEOSC and each ohm of source resistance driving REF + or REF - will result in 0.21 * 10-6 * fEOSCppm +FS gain error. The effect of the source resistance on the two reference pins is additive with respect to this gain error. The typical +FS and -FS errors for various combinations of source resistance seen by the REF+ and REF- pins and external capacitance CREF connected to these pins are shown in Figures 21, 22, 23 and 24.
10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 1 VCC = 5V VREF+ = 5V VREF- = GND VIN+ = 1.25V VIN- = 3.75V FO = GND TA = 25C 10 CIN = 0.001F CIN = 100pF CIN = 0.01F CIN = 0pF 1000 100 RSOURCE () 10000 100000
2435 F22
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Figure 22. -FS Error vs RSOURCE at REF+ or REF- (Small CIN)
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
100
+FS ERROR VARIATION (ppm)
-FS ERROR VARIATION (ppm)
VCC = 5V 90 VREF+ = 5V V - = GND 80 VREF = 3.75V IN+ 70 VIN- = 1.25V FO = GND 60 TA = 25C 50 40 30 20 10 0 0 400 800
CIN = 1F, 10F
CIN = 0.1F
CIN = 0.01F
1200 RSOURCE ()
1600
2000
2435 F23
Figure 23. +FS Error vs RSOURCE at REF+ and REF- (Large CREF)
In addition to this gain error, the converter INL performance is degraded by the reference source impedance. When FO = LOW (internal oscillator and 60Hz notch), every 100 of source resistance driving REF+ or REF- translates into about 0.11ppm additional INL error. For the LTC2435, when FO = HIGH (internal oscillator and 50Hz notch), every 100 of source resistance driving REF+ or REF- translates into about 0.092ppm additional INL error; and for the LTC2435-1 operating with simultaneous 50Hz/60Hz rejection, every 100 of source resistance leads to an additional 0.10ppm of additional INL error. When FO is driven by an external oscillator with a frequency fEOSC, every 100 of source resistance driving REF+ or REF- translates into about 0.73 * 10-6 * fEOSCppm additional INL error. Figure 25 shows the typical INL error due to the source resistance driving the REF+ or REF- pins when large CREF values are used. The effect of the source resistance on the two reference pins is additive with respect to this INL error. In general, matching of source impedance for the REF+ and REF- pins does not help the gain or the INL error. The user is thus advised to minimize the combined source impedance driving the REF+ and REF- pins rather than to try to match it. The magnitude of the dynamic reference current depends upon the size of the very stable internal sampling capacitors and upon the accuracy of the converter sampling clock. The accuracy of the internal clock over the entire temperature and power supply range is typical better than 0.5%. Such a specification can also be easily achieved by
INL (ppm OF VREF)
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0 -10 -20 -30 -40 -50 -60 VCC = 5V VREF+ = 5V -70 VREF- = GND V + = 1.25V -80 VIN- = 3.75V IN -90 FO = GND TA = 25C -100 0 400 CIN = 1F, 10F CIN = 0.1F CIN = 0.01F 800 1200 RSOURCE () 1600 2000
2435 F24
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Figure 24. -FS Error vs RSOURCE at REF+ and REF- (Large CREF)
an external clock. When relatively stable resistors (50ppm/C) are used for the external source impedance seen by REF+ and REF-, the expected drift of the dynamic current gain error will be insignificant (about 1% of its value over the entire temperature and voltage range). Even for the most stringent applications a one-time calibration operation may be sufficient. In addition to the reference sampling charge, the reference pins ESD protection diodes have a temperature dependent leakage current. This leakage current, nominally 1nA (10nA max), results in a small gain error. A 100 source resistance will create a 0.05V typical and 0.5V maximum full-scale error.
15 VINCM = 0.5 * (IN + + IN -) = 2.5V 12 VCC = 5V REF+ = 5V 9 REF- = GND 6 FO = GND CREF = 10F 3 TA = 25C 0 -3 -6 -9 RSOURCE = 10k -15 -0.5 -0.4 -0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 VINDIF /VREFDIF (V)
2435 F25
RSOURCE = 1k RSOURCE = 5k
-12
Figure 25. INL vs Differential Input Voltage (VIN = IN+ - IN-) and Reference Source Resistance (RSOURCE at REF+ and REF- for Large CREF Values (CREF 1F)
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
Output Data Rate
When using its internal oscillator, the LTC2435 can produce up to 15 readings per second with a notch frequency of 60Hz (FO = LOW) and 12.5 readings per second with a notch frequency of 50Hz (FO = HIGH) and the LTC2435-1 can produce up to 13.6 readings per second with FO = LOW. The actual output data rate will depend upon the length of the sleep and data output phases which are controlled by the user and which can be made insignificantly short. When operated with an external conversion clock (FO connected to an external oscillator), the LTC2435/ LTC2435-1 output data rate can be increased as desired. The duration of the conversion phase is 10278/fEOSC. If fEOSC = 153600Hz, the converter behaves as if the internal oscillator is used and the notch is set at 60Hz. There is no significant difference in the LTC2435/LTC2435-1 performance between these two operation modes. An increase in fEOSC over the nominal 153600Hz will translate into a proportional increase in the maximum output data rate. This substantial advantage is nevertheless accompanied by three potential effects, which must be carefully considered. First, a change in fEOSC will result in a proportional change in the internal notch position and in a reduction of the converter differential mode rejection at the power line frequency. In many applications, the subsequent performance degradation can be substantially reduced by relying upon the LTC2435/LTC2435-1's exceptional common mode rejection and by carefully eliminating common mode to differential mode conversion sources in the input circuit. The user should avoid single-ended input filters and should maintain a very high degree of matching and symmetry in the circuits driving the IN+ and IN- pins. Second, the increase in clock frequency will increase proportionally the amount of sampling charge transferred through the input and the reference pins. If large external input and/or reference capacitors (CIN, CREF) are used, the previous section provides formulae for evaluating the effect of the source resistance upon the converter performance for any value of fEOSC. If small external input and/ or reference capacitors (CIN, CREF) are used, the effect of the external source resistance upon the LTC2435/ LTC2435-1 typical performance can be inferred from Figures 16, 17, 21 and 22 in which the horizontal axis is scaled by 153600/fEOSC.
OFFSET ERROR (ppm OF VREF)
+FS ERROR (ppm OF VREF)
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Third, the internal analog circuits are optimized for normal operation; therefore an increase in the frequency of the external oscillator will start to decrease the effectiveness of the internal analog circuits. This will result in a progressive degradation in the converter accuracy and linearity. Typical measured performance curves for output data rates up to 200 readings per second are shown in Figures 26 to 33. The degradation becomes more obvious above output data rate of 150Hz, which corresponds to an external oscillator of 1.536MHz. In order to obtain the highest possible level of accuracy from this converter at output data rates above 150 readings per second, the user is advised to maximize the power supply voltage used and to limit the maximum ambient operating temperature. In certain circumstances, a reduction of the differential reference voltage may be beneficial.
-300 VINCM = VREFCM VCC = VREF = 5V VIN = 0V FO = EXT OSC TA = 25C -310 -320 -330 TA = 85C -340 -350 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F26
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Figure 26. Offset Error vs Output Data Rate and Temperature
-300 -320 -340 -360 -380 -400 -420 -440 -460 VINCM = VREFCM -480 VCC = VREF = 5V FO = EXT OSC -500 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F27
TA = 25C
TA = 85C
Figure 27. + FS Error vs Output Data Rate and Temperature
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
-200
-FS ERROR (ppm OF VREF)
VINCM = VREFCM -220 VCC = VREF = 5V FO = EXT OSC -240
RESOLUTION (BITS)
-260 -280 -300 -320 -340 -360 -380 -400 0 TA = 25C
TA = 85C 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F28
Figure 28. - FS Error vs Output Data Rate and Temperature
21
OFFSET CHANGE* (ppm OF VREF)
20
RESOLUTION (BITS)
19 TA = 25C 18 17 16 VCC = VREF = 5V VINCM = VREFCM REF - = GND 15 FO = EXT OSC RES = LOG2 (VREF/INLMAX) 14 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F30
TA = 85C
Figure 30. Resolution (INLRMS 1LSB) vs Output Data Rate and Temperature
22 21 RESOLUTION (BITS) 20 19 18 17 16 15 0 VINCM = VREFCM VIN = 0V REF - = GND FO = EXT OSC TA = 25C RES = LOG2(VREF/NOISERMS) 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F32
VCC = VREF = 5V
VCC = 2.7V VREF = 2.5V
RESOLUTION (BITS)
Figure 32. Resolution (NoiseRMS 1LSB) vs Output Data Rate and Reference Voltage
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22 21 20 19 18 17 16 15 0 VCC = VREF = 5V VINCM = VREFCM VIN = 0V REF - = GND FO = EXT OSC RES = LOG2(VREF/NOISERMS) 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F29
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TA = 25C TA = 85C
Figure 29. Resolution (NoiseRMS 1LSB) vs Output Data Rate and Temperature
50 40 30 20 10 0 -10 -20 -30 -40 * RELATIVE TO OFFSET AT NORMAL OUTPUT RATE -50 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F31
VINCM = VREFCM VIN = 0V REF - = GND FO = EXT OSC TA = 25C VCC = VREF = 5V VCC = 2.7V VREF = 2.5V
Figure 31. Offset Change* vs Output Data Rate and Reference Voltage
21 20 VCC = VREF = 5V 19 18 17 VINCM = VREFCM 16 VIN = 0V REF - = GND F = EXT OSC 15 O TA = 25C RES = LOG2 (VREF/INLMAX) 14 0 20 40 60 80 100 120 140 160 180 200 OUTPUT DATA RATE (READINGS/SEC)
2435 F33
VCC = 2.7V VREF = 2.5V
Figure 33. Resolution (INLMAX 1LSB) vs Output Data Rate and Reference Voltage
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
Normal Mode Rejection and Antialiasing One of the advantages delta-sigma ADCs offer over conventional ADCs is on-chip digital filtering. Combined with a large oversampling ratio, the LTC2435/LTC2435-1 significantly simplifies antialiasing filter requirements. The sinc4 digital filter provides greater than 120dB normal mode rejection at all frequencies except DC and integer multiples of the modulator sampling frequency (fS). Independent of the operating mode, fS = 256 * fN = 1024 * fOUTMAX where fN is the notch frequency and fOUTMAX is the maximum output data rate. In the internal oscillator mode, for the LTC2435, FS = 12800Hz with a 50Hz notch setting and fS = 15360Hz with a 60Hz notch setting. For the LTC2435-1, fS = 13980Hz (FO = LOW). In the external oscillator mode, fS = fEOSC/10. The normal mode rejection performance is shown in Figure 34. The regions of low rejection occurring at integer multiples of fS have a very narrow bandwidth. Magnified details of the normal mode rejection curves are shown in Figure 35 (rejection near DC) and Figure 36 (rejection at fS = 256fN) where fN represents the notch frequency. For the LTC2435, the bandwidth is 13.6Hz (FO = GND) and 11.4Hz (FO = VCC). The Bandwidth is 12.4Hz for the LTC2435-1 (FO = GND).
0
INPUT NORMAL MODE REJECTION (dB)
-10 -20 -30 -50 -60 -70 -80 -90 -100 -110 -120
FO = HIGH
REJECTION (dB)
0 fS 2fS 3fS 4fS 5fS 6fS 7fS 8fS 9fS 10fS11fS12fS DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2435 F34a
-40
Figure 34a. Input Normal Mode Rejection, Internal Oscillator and 50Hz Notch (LTC2435)
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Through FO connection, the LTC2435 provides better than 110dB input differential mode rejection at 50Hz or 60Hz 2%. While for the LTC2435-1, it has a notch frequency of about 55Hz with better than 70db rejection over 48Hz to 62.4Hz, which covers both 50Hz 2% and 60Hz 2%. In order to achieve better rejection over the range of 48Hz to 62.4Hz, a running average can be performed. By averaging two consecutive LTC2435-1 readings, a sinc1 notch is combined with the sinc4 digital filter, yielding the frequency response shown in Figure 37. The averaging operation still keeps the output rate with the following algorithm: Result 1 = average (sample 0, sample 1) Result 2 = average (sample 1, sample 2) Result n = average (sample n-1, sample n) The user can expect to achieve in practice this level of performance using the internal oscillator as it is demonstrated by Figures 38 to 40. Typical measured values of the normal mode rejection of the LTC2435-1 operating with an internal oscillator and a 54.6Hz notch setting are shown in Figure 38 and 39 superimposed over the theoretical calculated curve. The same normal mode rejection performance is obtained for the LTC2435 with the frequency scaled to have the notch frequency at 60Hz (FO = GND) or 50Hz (FO = VCC).
0 -20 -40 -60 -80 -100 -120 -140 0 fS/2 INPUT FREQUENCY
2435 F34b
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Figure 34b. Input Normal Mode Rejection, Internal Oscillator and FO = Low or External Oscillator
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
As a result of these remarkable normal mode specifications, minimal (if any) antialias filtering is required in front of the LTC2435/LTC2435-1. If passive RC components are placed in front of the LTC2435/LTC2435-1, the input dynamic current should be considered (see Input Current section). In cases where large effective RC time constants are used, an external buffer amplifier may be required to minimize the effects of dynamic input current. Traditional high order delta-sigma modulators, while providing very good linearity and resolution, suffer from potential instabilities at large input signal levels. The proprietary architecture used for the LTC2435/LTC2435-1 third order modulator resolves this problem and guarantees a predictable stable behavior at input signal levels of up to 150% of full scale. In many industrial applications, it is not uncommon to have to measure microvolt level signals superimposed over volt level perturbations and LTC2435/LTC2435-1 are eminently suited for such tasks. When the perturbation is differential, the specification of interest is the normal mode rejection for large input signal levels. With a reference voltage VREF = 5V, the LTC2435/ LTC2435-1 have a full-scale differential input range of 5V peak-to-peak. Figure 40 shows measurement results for the LTC2435-1 normal mode rejection ratio with a 7.5V peak-to-peak (150% of full scale) input signal superim-
INPUT NORMAL REJECTION (dB)
0 INPUT NORMAL REJECTION (dB)
NORMAL MODE REJECTION (dB)
-20 -40 -60 -80
-100 -120
0
fN
2fN 3fN 4fN 5fN 6fN 7fN INPUT SIGNAL FREQUENCY (fN)
8fN
2435 F35
Figure 35. Input Normal Mode Rejection
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posed over the more traditional normal mode rejection ratio results obtained with a 5V peak-to-peak (full scale) input signal. The same performance is obtained for the LTC2435 with the frequency scaled to have the notch frequency at 60Hz (FO = GND) or 50Hz (FO = VCC). It is clear that the LTC2435/LTC2435-1 rejection performance is maintained with no compromises in this extreme situation. When operating with large input signal levels, the user must observe that such signals do not violate the device absolute maximum ratings.
0 -20 -40 -60 -80 -100 -120 248 250 252 254 256 258 260 262 264 INPUT SIGNAL FREQUENCY (fN)
2435 F36
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Figure 36. Input Normal Mode Rejection
-70 -80 NO AVERAGE -90 -100 -110 -120 -130 -140 60 62 54 56 58 48 50 52 DIFFERENTIAL INPUT SIGNAL FREQUENCY (Hz)
2435 F37
WITH RUNNING AVERAGE
Figure 37. LTC2435-1 Input Normal Mode Rejection
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
0
NORMAL MODE REJECTION (dB)
-40 -60 -80
NORMAL MODE REJECTION (dB)
-20
MEASURED DATA VCC = 5V CALCULATED DATA VREF = 5V REF- = GND VINCM = 2.5V VIN(P-P) = 5V FO = GND TA = 25C
-100 -120
0
25
50 75 100 125 150 175 200 225 INPUT FREQUENCY (Hz)
2435 F38
Figure 38. Input Normal Mode Rejection vs Input Frequency (LTC2435-1)
0
NORMAL MODE REJECTION (dB)
-20 -40 -60 -80
-100 -120
0
Figure 40. Measured Input Normal Mode Rejection vs Input Frequency (fN = 54.6Hz)
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0 -20 -40 -60 -80 MEASURED DATA VCC = 5V CALCULATED DATA VREF = 5V REF- = GND VINCM = 2.5V VIN(P-P) = 5V FO = GND TA = 25C -100 -120 0 25 50 75 100 125 150 175 200 225 INPUT FREQUENCY (Hz)
2435 F39
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Figure 39. Input Normal Mode Rejection vs Input Frequency with Running Average
VIN(P-P) = 5V VCC = 5V VIN(P-P) = 7.5V VREF = 5V (150% OF FULL SCALE) REF- = GND VINCM = 2.5V FO = GND TA = 25C
25
50 75 100 125 150 175 200 225 INPUT FREQUENCY (Hz)
2435 F40
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO U
The listing in Figure 43 is a data collection program for the LTC2435/LTC2435-1 using the PIC16F73 microcontroller. The microcontroller is configured to transfer data through the SPI serial interface. Figure 42 shows the connection. The LT1180A is a dual RS232 driver/receiver pair with integral charge pump that generates RS232 voltage levels from a single 5V supply. The program begins by declaring variables and allocating memory locations to store the 24-bit conversion result. The main sequence starts with pulling CS LOW. It then waits for SDO to go LOW to start reading data. Three bytes are read to the MCU and the LTC2435/LTC2435-1 will automatically start a new conversion. CS is also raised to HIGH to ensure that a new conversion is started. The collected data are sent out through the serial port at 57600 baud. This can be captured with a terminal program and analyzed with a spreadsheet using the HEX2DEC function.
5V 16 12 14 15 11 1 5 TO OTHER DEVICES 2 4 8 9 10
2435 F41
Sample Driver for LTC2435/LTC2435-1 SPI Interface Figure 41 shows the use of an LTC2435/LTC2435-1 with a differential multiplexer. This is an inexpensive multiplexer that will contribute some error due to leakage if used directly with the output from the bridge, or if resistors are inserted as a protection mechanism from overvoltage. Although the bridge output may be within the input range of the A/D and multiplexer in normal operation, some thought should be given to fault conditions that could result in full excitation voltage at the inputs to the multiplexer or ADC. The use of amplification prior to the multiplexer will largely eliminate errors associated with channel leakage developing error voltages in the source impedance. The LTC2435/LTC2435-1 have a very simple serial interface that makes interfacing to microprocessors and microcontrollers very easy.
5V
Figure 41. Use a Differential Multiplexer to Expand Channel Capability
VCC 20 SCK LTC2435/ SDO LTC2435-1 CS 13 12 11 PIC16F73 13 17 RC2 RC6 14 RC3 18 15 RC7 RC4 8 19 VCC LT1180A 12 11 13 10 18 2 C1 T1IN T2IN R1OUT R2OUT SHDN C1+ T1OUT T2OUT R1IN R2IN VCC V+ V- 15 8 14 9 17 C3 3 7 C4 GND 16
2435 F42
Figure 42. Connecting the LTC2435/LTC2435-1 to a PIC16F73 MCU Using the SPI Serial Interface
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+
47F 3 4 REF + REF -
2 VCC
74HC4052 13 3 6 5 6
LTC2435/ LTC2435-1 IN + IN - GND 1, 7, 8, 9, 10, 15, 16 A0 A1
X1 1 2 3 4 5 VCC 6 7 8 9
4 C1- 5 C2+ 6 C2-
C5
C2
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
// Basic data collection program for the LTC2435 using the // PIC16F73 microcontroller. Collects data as fast as possible // and sends it out the serial port at 57600 baud as six // hexadecimal characters, followed by a carriage return. // This can be captured with a terminal program and analyzed // with a spreadsheet using the HEX2DEC function (in Excel.) // // Written for the CCS compiler, version 3.049. //////////////////////////////////////////////////////////////////// #include <16F73.h> #byte SSPCON = 0x14 #byte SSPSTAT = 0x94 #bit CKE = SSPSTAT.6 #bit CKP = SSPCON.4 #bit SSPEN = SSPCON.5 #fuses HS,NOWDT,PUT #use delay(clock=10000000) // Synchronous serial port control // registers.
// For baud rate calculation.
#use rs232(baud=57600,parity=N,xmit=PIN_C6,rcv=PIN_C7) // Serial data is sent on pin C6. #define CS_ PIN_C2 // Chip select connected to pin C2 #define CLOCK PIN_C // Clock connected to pin C3 #define SDO PIN_C4 // SDO on the LTC2435 connected to pin C4 // (this is SDI on the PIC; // Master In, Slave Out (MISO) is less ambiguous) void main() { // Basic configuration, no bearing on operation of LTC2435 setup_adc_ports(NO_ANALOGS); setup_adc(ADC_CLOCK_DIV_2); setup_counters(RTCC_INTERNAL,RTCC_DIV_2); setup_timer_1(T1_DISABLED); setup_timer_2(T2_DISABLED,0,1); setup_ccp1(CCP_OFF); setup_ccp2(CCP_OFF); // LTC2435 is connected to the processor's hardware SPI port. // This sets the port such that data is shifted on clock falling edges and // valid on rising edges. For a 10 MHz master clock, the SPI clock frequency // wil be 2.5 MHz. setup_spi(SPI_MASTER|SPI_L_TO_H|SPI_CLK_DIV_4|SPI_SS_DISABLED); CKP = 0; // Set up clock edges - clock idles low, data changes on CKE = 1; // falling edges, valid on rising edges.
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO U
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while(1) { output_low(CS_); // Enable LTC2435 while(input(SDO)) { /* Wait for SDO to fall, indicating end of conversion.*/ } printf("%2X",spi_read(0)); // Read first byte, send 2 hex characters. printf("%2X",spi_read(0)); // Read second byte, send 2 hex characters. printf("%2X",spi_read(0)); / Read third byte, send 2 hex characters. printf("\r"); // Send carriage return. output_high(CS_); // Conversion actually started after last data byte was read, // but raising CS_ ensures the loop will never lock up waiting for // a low on SDO if a clock pulse is missed for some reason. } }
Figure 43. A Sample Program for Data Collection from the LTC2435/LTC2435-1 Using the PIC16F73 Microcontroller.
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LTC2435/LTC2435-1
APPLICATIO S I FOR ATIO
Correlated Double Sampling with the LTC2435/LTC2435-1
The Typical Application on the back page of this data sheet shows the LTC2435/LTC2435-1 in a correlated double sampling circuit that achieves a noise floor of under 100nV. In this scheme, the polarity of the bridge is alternated every other sample and the result is the average of a pair of samples of opposite sign. This technique has the benefit of canceling any fixed DC error components in the bridge, amplifiers and the converter, as these will alternate in polarity relative to the signal. Offset voltages and currents, thermocouple voltages at junctions of dissimilar metals and the lower frequency components of 1/f noise are virtually eliminated. The LTC2435/LTC2435-1 have the virtue of being able to digitize an input voltage that is outside the range defined by the reference, thereby providing a simple means to implement a ratiometric example of correlated double sampling. This circuit uses a bipolar amplifier (LT1219--U1 and U2) that has neither the lowest noise nor the highest gain. It does, however, have an output stage that can effectively suppress the conversion spikes from the LTC2435/ LTC2435-1. The LT1219 is a C-LoadTM stable amplifier that, by design, needs at least 0.1F output capacitance to remain stable. The 0.1F ceramic capacitors at the outputs (C1 and C2) should be placed and routed to minimize lead inductance or their effectiveness in preventing envelope detection in the input stage will be reduced. Alternatively, several smaller capacitors could be placed so that lead inductance is further reduced. This is a consideration because the frequency content of the conversion spikes extends to 50MHz or more. The output impedance of most op amps increases dramatically with frequency but the effective output impedance of the LT1219 remains
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low, determined by the ESR and inductance of the capacitors above 10MHz. The conversion spikes that remain at the output of other bipolar amplifiers pass through the feedback network and often overdrive the input of the amplifier, producing envelope detection. RFI may also be present on the signal lines from the bridge; C3 and C4 provide RFI suppression at the signal input, as well as suppressing transient voltages during bridge commutation. The wideband noise density of the LT1219 is 33nV/Hz, seemingly much noisier than the lowest noise amplifiers. However, in the region just below the 1/f corner that is not well suppressed by the correlated double sampling, the average noise density is similar to the noise density of many low noise amplifiers. If the amplifier is rolled off below about 1500Hz, the total noise bandwidth is determined by the converter's Sinc4 filter at about 12Hz. The use of correlated double sampling involves averaging even numbers of samples; hence, in this situation, two samples would be averaged to give an input-referred noise level of about 100nVRMS. Level shift transistors Q4 and Q5 are included to allow excitation voltages up to the maximum recommended for the bridge. In the case shown, if a 10V supply is used, the excitation voltage to the bridge is 8.5V and the outputs of the bridge are above the supply rail of the ADC. U1 and U2 are also used to produce a level shift to bring the outputs within the input range of the converter. This instrumentation amplifier topology does not require well-matched resistors in order to produce good CMRR. However, the use of R2 requires that R3 and R6 match well, as the common mode gain is approximately -12dB. If the bridge is composed of four equal 350 resistors, the differential component associated with mismatch of R3 and R6 is nearly constant with either polarity of excitation and, as with offset, its contribution is canceled.
C-Load is a trademark of Linear Technology Corporation.
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LTC2435/LTC2435-1
PACKAGE DESCRIPTIO U
GN Package 16-Lead Plastic SSOP (Narrow .150 Inch)
(Reference LTC DWG # 05-08-1641)
.189 - .196* (4.801 - 4.978) 16 15 14 13 12 11 10 9
.045 .005 .009 (0.229) REF .150 - .165
.254 MIN
.229 - .244 (5.817 - 6.198)
.150 - .157** (3.810 - 3.988)
.0165 .0015
.0250 BSC
RECOMMENDED SOLDER PAD LAYOUT
1
23
4
56
7
8
.004 - .0098 (0.102 - 0.249)
.015 .004 x 45 (0.38 0.10)
.007 - .0098 (0.178 - 0.249) 0 - 8 TYP
.0532 - .0688 (1.35 - 1.75)
.016 - .050 (0.406 - 1.270)
NOTE: 1. CONTROLLING DIMENSION: INCHES INCHES 2. DIMENSIONS ARE IN (MILLIMETERS) 3. DRAWING NOT TO SCALE *DIMENSION DOES NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSION DOES NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
.008 - .012 (0.203 - 0.305) TYP
.0250 (0.635) BSC
GN16 (SSOP) 0204
24351fa
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
39
LTC2435/LTC2435-1
TYPICAL APPLICATIO
ELIMINATE FOR 5V OPERATION (CONNECT 2.7k RESISTORS TO 100 RESISTORS) 100 1.5k
Q2 100 22
5V
Q4
22 5V Q5
2.7k
2.7k C3 2.2nF C4 2.2nF R5 499 1000pF R6 10k 10V 0.1f 1k 5 IN+
POL
22
R1 61.9 0.1% 22
33
100
Q1: SILICONIX Si9802DY Q2, Q3: MMBD2907 Q4, Q5: MMBD3904
(800) 554-5565
RELATED PARTS
PART NUMBER LT1019 LT1025 LTC1043 LTC1050 LT1236A-5 LT1460 LTC2400 DESCRIPTION Precision Bandgap Reference, 2.5V, 5V Micropower Thermocouple Cold Junction Compensator Dual Precision Instrumentation Switched Capacitor Building Block Precision Chopper Stabilized Op Amp Precision Bandgap Reference, 5V Micropower Series Reference 24-Bit, No Latency ADC in SO-8 COMMENTS 3ppm/C Drift, 0.05% Max Initial Accuracy 80A Supply Current, 0.5C Initial Accuracy Precise Charge, Balanced Switching, Low Power No External Components 5V Offset, 1.6VP-P Noise 0.05% Max Initial Accuracy, 5ppm/C Drift 0.075% Max Initial Accuracy, 10ppm/C Max Drift, 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200A 0.6ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200A 800nVRMS Noise, Pin Compatible with LTC2435 1.45VRMS Noise, 4ppm INL, Simultaneous 50Hz/60Hz Rejection (LTC2411-1) Simultaneous 50Hz/60Hz Rejection, 800nVRMS Noise Pin Compatible with the LTC2435/LTC2435-1 0.2ppm Noise, 2ppm INL, 3ppm Total Unadjusted Errors 200A 1.2ppm Noise, 8ppm INL, Pin Compatible with LTC2400 2.8V Noise, SSOP-16/MSOP Package
24351fa LT/TP 0804 1K REV A * PRINTED IN USA
LTC2401/LTC2402 1-/2-Channel, 24-Bit, No Latency ADCs in MSOP LTC2410 LTC2411/ LTC2411-1 LTC2413 LTC2415/ LTC2415-1 LTC2420 24-Bit, No Latency ADC with Differential Inputs 24-Bit, No Latency ADC with Differential Inputs in MSOP 24-Bit, No Latency ADC with Differential Inputs 24-Bit, No Latency ADC with 15Hz Output Rate
LTC2404/LTC2408 4-/8-Channel, 24-Bit, No Latency ADCs with Differential Inputs 0.3ppm Noise, 4ppm INL, 10ppm Total Unadjusted Error, 200A
LTC2414/LTC2418 8-/16-Channel 24-Bit, No Latency ADC 20-Bit, No Latency ADC in SO-8 LTC2430/LTC2431 20-Bit, No Latency ADC with Differential Inputs
40
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
www.linear.com
+
Q1
3
-
74HC04
U
Correlated Double Sampling Resolves 100nV
10V 1.5k DIFFERENCE AMP Q3 3 R2 27k 1k 1000pF R4 499 10V 0.1f 7 U1 LT1219 6 5 4 SHDN 5k
+ -
2
C1 0.1F 5V
R3 10k
350 x4
6
IN- LTC2435/ LTC2435-1 3 REF+ 4
2
7 U2 LT1219 5 4 SHDN 6 5k
REF- GND
C2 0.1F
30pF
30pF
2435 F46
(c) LINEAR TECHNOLOGY CORPORATION 2001

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