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LTC1410 12-Bit, 1.25Msps Sampling A/D Converter with Shutdown FEATURES
s s s s s s
DESCRIPTIO
s s s s
1.25Msps Sample Rate Power Dissipation: 160mW 71dB S/(N + D) and 82dB THD at Nyquist No Pipeline Delay Nap (7mW) and Sleep (10W) Shutdown Modes Operates with Internal 15ppm/C Reference or External Reference True Differential Inputs Reject Common Mode Noise 20MHz Full Power Bandwidth Sampling 2.5V Bipolar Input Range 28-Pin SO Wide Package
The LTC(R)1410 is a 0.65s, 1.25Msps, 12-bit sampling A/D converter that draws only 160mW from 5V supplies. This easy-to-use device includes a high dynamic range sample-and-hold, a precision reference and requires no external components. Two digitally selectable power shutdown modes provide flexibility for low power systems. The LTC1410's full-scale input range is 2.5V. Maximum DC specifications include 1LSB INL and 1LSB DNL over temperature. Outstanding AC performance includes 71dB S/(N + D) and 82dB THD at the Nyquist input frequency of 625kHz. The unique differential input sample-and-hold can acquire single-ended or differential input signals up to its 20MHz bandwidth. The 60dB common mode rejection allows users to eliminate ground loops and common mode noise by measuring signals differentially from the source. The ADC has a P compatible, 12-bit parallel output port. There is no pipeline delay in the conversion results. A separate convert start input and a data ready signal (BUSY) ease connections to FIFOs, DSPs and microprocessors.
, LTC and LT are registered trademarks of Linear Technology Corporation.
APPLICATI
s s s s s s
S
Telecommunications Digital Signal Processing Multiplexed Data Acquisition Systems High Speed Data Acquisition Spectrum Analysis Imaging Systems
TYPICAL APPLICATI
Complete 1.25MHz, 12-Bit Sampling A/D Converter
LTC1410 DIFFERENTIAL 1 AVDD +AIN ANALOG INPUT (-2.5V TO 2.5V) 2 -AIN DVDD 2.50V 3 V VSS VREF OUTPUT 4 REF REFCOMP BUSY 5 0.1F AGND CS 6 D11(MSB) CONVST 7 D10 RD 8 D9 SHDN 9 D8 NAP/SLP 10 D7 OGND 11 12-BIT D6 D0 PARALLEL 12 D5 D1 BUS 13 D4 D2 14 DGND D3 5V 28 27 26 25 24 23 22 21 20 19 18 17 16 15 P CONTROL LINES 10F -5V
Effective Bits and Signal-to-(Noise + Distortion) vs Input Frequency
12 74 68 10 8 6 4 2 0 1k fSAMPLE = 1.25MHz 10k 100k 1M INPUT FREQUENCY (Hz) 10M
LTC1410 * TA02
+
10F 0.1F
EFFECTIVE BITS
0.1F
+
10F
1410 TA01
U
NYQUIST 62 56 50
S/(N + D) (dB)
UO
UO
1
LTC1410 ABSOLUTE AXI U RATI GS PACKAGE/ORDER I FOR ATIO
TOP VIEW +AIN 1 -AIN 2 VREF 3 REFCOMP 4 AGND 5 D11(MSB) 6 D10 7 D9 8 D8 9 D7 10 D6 11 D5 12 D4 13 DGND 14 G PACKAGE 28-LEAD PLASTIC SSOP 28 AVDD 27 DVDD 26 VSS 25 BUSY 24 CS 23 CONVST 22 RD 21 SHDN 20 NAP/SLP 19 OGND 18 D0 17 D1 16 D2 15 D3 SW PACKAGE 28-LEAD PLASTIC SO WIDE
AVDD = DVDD = VDD (Notes 1, 2)
Supply Voltage (VDD) ................................................ 6V Negative Supply Voltage (VSS) ............................... - 6V Total Supply Voltage (VDD to VSS) .......................... 12V Analog Input Voltage (Note 3) .................................. VSS - 0.3V to VDD + 0.3V Digital Input Voltage (Note 4) ............ VSS - 0.3V to 10V Digital Output Voltage ................... - 0.3V to VDD + 0.3V Power Dissipation ............................................. 500mW Operating Temperature Range LTC1410C .............................................. 0C to 70C LTC1410I ........................................... - 40C to 85C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LTC1410CG LTC1410CSW LTC1410IG LTC1410ISW
TJMAX = 110C, JA = 90C/W (SW) TJMAX = 110C, JA = 95C/W (G)
Consult factory for Military grade parts.
CO VERTER CHARACTERISTICS
PARAMETER Resolution (No Missing Codes) Integral Linearity Error Differential Linearity Error Offset Error Full-Scale Error Full-Scale Tempco
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. With Internal Reference (Notes 5, 6)
CONDITIONS
q
MIN 12
q q
TYP 0.3 0.3 2
MAX 1 1 6 8 15
UNITS Bits LSB LSB LSB LSB LSB ppm/C
(Note 7) (Note 8)
q
IOUT(REF) = 0
q
15
A ALOG I PUT
SYMBOL PARAMETER VIN IIN CIN t ACQ t AP tjitter CMRR
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 5)
CONDITIONS 4.75V VDD 5.25V, - 5.25V VSS - 4.75V CS = High Between Conversions During Conversions
q q q
MIN
TYP 2.5
MAX 1
UNITS V A pF pF
Analog Input Range (Note 9) Analog Input Leakage Current Analog Input Capacitance Sample-and-Hold Acquisition Time Sample-and-Hold Aperture Delay Time Sample-and-Hold Aperture Delay Time Jitter Analog Input Common Mode Rejection Ratio
17 5 50 -1.5 5 100
psRMS dB
- 2.5V < (- AIN = AIN) < 2.5V
60
2
U
ns ns
W
U
U
WW
W U
U
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LTC1410
DY A IC ACCURACY
SYMBOL S/(N + D) THD
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 5)
PARAMETER Signal-to-(Noise + Distortion) Ratio Total Harmonic Distortion Peak Harmonic or Spurious Noise Intermodulation Distortion Full Power Bandwidth Full Linear Bandwidth CONDITIONS 100kHz Input Signal (Note 12) 600kHz Input Signal (Note 12) 100kHz Input Signal, First 5 Harmonics 600kHz Input Signal, First 5 Harmonics 600kHz Input Signal fIN1 = 29.37kHz, fIN2 = 32.446kHz (S/(N + D) 68dB)
q q q q
IMD
I TER AL REFERE CE CHARACTERISTICS
PARAMETER VREF Output Voltage VREF Output Tempco VREF Line Regulation VREF Output Resistance COMP Output Voltage CONDITIONS IOUT = 0 IOUT = 0 4.75V VDD 5.25V - 5.25V VSS - 4.75V IOUT 0.1mA IOUT = 0
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 5)
MIN 2.480 TYP 2.500 15 0.01 0.01 2 4.06 MAX 2.520 UNITS V ppm/C LSB/V LSB/V k V
DIGITAL I PUTS A D DIGITAL OUTPUTS
SYMBOL VIH VIL IIN CIN VOH PARAMETER High Level Input Voltage Low Level Input Voltage Digital Input Current Digital Input Capacitance High Level Output Voltage CONDITIONS VDD = 5.25V VDD = 4.75V VIN = 0V to VDD VDD = 4.75V IO = - 10A IO = - 200A VDD = 4.75V IO = 160A IO = 1.6mA VOUT = 0V to VDD, CS High CS High (Note 9 ) VOUT = 0V VOUT = VDD
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 5)
q q q
VOL
Low Level Output Voltage
IOZ COZ ISOURCE ISINK
High-Z Output Leakage D11 to D0 High-Z Output Capacitance D11 to D0 Output Source Current Output Sink Current
POWER REQUIRE E TS
SYMBOL VDD VSS IDD PARAMETER Positive Supply Voltage Negative Supply Voltage Positive Supply Current Nap Mode Sleep Mode Negative Supply Current Nap Mode Sleep Mode
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 5)
CONDITIONS (Notes 10, 11) (Note 10) CS = RD = CONVST = 5V SHDN = 0V, NAP/SLP = 5V SHDN = 0V, NAP/SLP = 0V CS = RD = CONVST = 5V SHDN = 0V, NAP/SLP = 5V SHDN = 0V, NAP/SLP = 0V MIN 4.75 - 4.75
q
ISS
UW
U
U
U
WU
U
MIN 70 68
TYP 72.5 71.0 - 85 - 82 - 84 - 84 20 2.5
MAX
- 74 - 74
UNITS dB dB dB dB dB dB MHz MHz
U
MIN 2.4
TYP
MAX 0.8 10
5 4.5
q
UNITS V V A pF V V V V A pF mA mA
4.0 0.05 0.10
q q q
0.4 10 15
- 10 10
TYP
q
12 1.5 1 20 10 1
MAX 5.25 - 5.25 16 2.3 100 30 200 100
UNITS V V mA mA A mA A A
3
LTC1410
POWER REQUIRE E TS
SYMBOL PD PARAMETER Power Dissipation Nap Mode Sleep Mode
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 5)
CONDITIONS SHDN = 0V, NAP/SLP = 5V SHDN = 0V, NAP/SLP = 0V MIN TYP 160 7.5 0.01 MAX 230 12 1 UNITS mW mW mW
TI I G CHARACTERISTICS
SYMBOL fSAMPLE(MAX) tCONV tACQ tACQ+CONV t1 t2 t3 t4 t5 t6 t7 t8 t9 t10 PARAMETER Maximum Sampling Frequency Conversion Time Acquisition Time Throughput Time (Acquisition + Conversion) CS to RD Setup Time CS to CONVST Setup Time NAP/SLP to SHDN Setup Time
The q denotes specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 5)
CONDITIONS
q q q q
SHDN to CONVST Wake-Up Time (Note 10) CONVST Low Time (Notes 10, 11) CONVST to BUSY Delay Data Ready Before BUSY CL = 25pF
Delay Between Conversions Wait Time RD After BUSY Data Access Time After RD
t11
Bus Relinquish Time Commercial Industrial
q q q q
t12 t13 t14
RD Low Time CONVST High Time Aperture Delay of Sample-and-Hold
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: All voltage values are with respect to ground with DGND, OGND and AGND wired together unless otherwise noted. Note 3: When these pin voltages are taken below VSS or above VDD, they will be clamped by internal diodes. This product can handle input currents greater than 100mA below VSS or above VDD without latchup. Note 4: When these pin voltages are taken below VSS, they will be clamped by internal diodes. This product can handle input currents greater than 100mA below VSS without latchup. These pins are not clamped to VDD. Note 5: VDD = 5V, VSS = - 5V, fSAMPLE = 1.25MHz, tr = tf = 5ns unless otherwise specified. Note 6: Linearity, offset and full-scale specifications apply for a singleended +AIN input with - AIN grounded.
4
UW
UW
MIN 1.25
TYP 650 50
MAX 750 100 800
UNITS MHz ns ns ns ns ns ns
(Notes 9, 10) (Notes 9, 10) (Notes 9, 10)
q q q q q q
0 10 10 200 40 10 50 20 15 40 -5 15 25 35 35 50 20 25 30 35
ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns
(Note 10) (Note 10) CL = 25pF
q q q
CL = 100pF
q
20 8
t 10 40 - 1.5
Note 7: 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 8: Bipolar offset is the offset voltage measured from - 0.5LSB when the output code flickers between 0000 0000 0000 and 1111 1111 1111. Note 9: Guaranteed by design, not subject to test. Note 10: Recommended operating conditions. Note 11: The falling CONVST edge starts a conversion. If CONVST returns high at a critical point during the conversion it can create small errors. For best results ensure that CONVST returns high either within 425ns after the start of the conversion or after BUSY rises. Note 12: Signal-to-noise ratio (SNR) is measured at 100kHz and distortion is measured at 600kHz. These results are used to calculate signal-to-noise plus distortion (SINAD).
LTC1410 TYPICAL PERFORMANCE CHARACTERISTICS
S/(N + D) vs Input Frequency and Amplitude
80 SIGNAL/(NOISE + DISTORTION) (dB) 70 60 50 40 30 20 10 fSAMPLE = 1.25MHz 0 1k 1M 10k 100k INPUT FREQUENCY (Hz) 10M
1410 G01
AMPLITUDE (dB BELOW THE FUNDAMENTAL)
SIGNAL-TO-NOISE RATIO (dB)
VIN = 0dB VIN = -20dB
VIN = -60dB
Spurious-Free Dynamic Range vs Input Frequency
0
SPURIOUS-FREE DYNAMIC RANGE (dB)
-10 -20
-40 -50 -60 -70 -80 -90
AMPLITUDE (dB)
-40 -60 -80 -100 -120
DNL ERROR (LSB)
-30
-100 10k
100k 1M INPUT FREQUENCY (Hz)
AMPLITUDE OF POWER SUPPLY FEEDTHROUGH (dB)
Integral Nonlinearity vs Output Code
1.0
-20 -40 -60 -80 -100 -120 VSS VDD DGND
COMMON MODE REJECTION (dB)
0.5
INL ERROR (LSB)
0
-0.5
-1.0
0
512 1024 1536 2048 2560 3072 3504 4096 OUTPUT CODE
1410 G07
UW
1410 G04
Signal-to-Noise Ratio vs Input Frequency
80 70 60 50 40 30 20 10 0 1k 1M 10k 100k INPUT FREQUENCY (Hz) 10M
1410 G02
Distortion vs Input Frequency
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 1k 1M 10k 100k INPUT FREQUENCY (Hz) 10M
1410 G03
3RD 2ND
THD
-100
Intermodulation Distortion Plot
0 -20 fSAMPLE = 1.25MHz fIN1 = 88.19580078kHz fIN2 = 111.9995117kHz
0.5 1.0
Differential Nonlinearity vs Output Code
0
-0.5
10M
0
100
200
300
400
500
600
-1.0
0
FREQUENCY (kHz)
1410 G05
512 1024 1536 2048 2560 3072 3504 4096 OUTPUT CODE
1410 G06
Power Supply Feedthrough vs Ripple Frequency
0 VRIPPLE = 0.1V
80 70 60 50 40 30 20 10 0
Input Common Mode Rejection vs Input Frequency
1k
10k 100k 1M RIPPLE FREQUENCY (Hz)
10M
1410 G08
1k
1M 10k 100k INPUT FREQUENCY (Hz)
10M
1410 G09
5
LTC1410
PI FU CTIO S
+ AIN (Pin 1): Positive Analog Input, 2.5V. - AIN (Pin 2): Negative Analog Input, 2.5V. VREF (Pin 3): 2.50V Reference Output. REFCOMP (Pin 4): 4.06V Reference Bypass Pin. Bypass to AGND with 10F tantalum in parallel with 0.1F ceramic. AGND (Pin 5): Analog Ground. D11 to D4 (Pins 6 to 13): Three-State Data Outputs. DGND (Pin 14): Digital Ground for Internal Logic. Tie to AGND. D3 to D0 (Pins 15 to 18): Three-State Data Outputs. OGND (Pin 19): Digital Ground for Output Drivers. Tie to AGND. NAP/SLP (Pin 20): Power Shutdown Mode. Selects the mode invoked by the SHDN pin. Low selects Sleep mode and high selects quick wake-up Nap mode. SHDN (Pin 21): Power Shutdown Input. A low logic level will invoke the Shutdown mode selected by the NAP/SLP pin. RD (Pin 22): Read Input. This enables the output drivers when CS is low. CONVST (Pin 23): Conversion Start Signal. This active low signal starts a conversion on its falling edge. CS (Pin 24): The Chip Select input must be low for the ADC to recognize CONVST and RD inputs. BUSY (Pin 25): The BUSY output shows the converter status. It is low when a conversion is in progress. Data valid on the rising edge of BUSY. VSS (Pin 26): - 5V Negative Supply. Bypass to AGND with 10F tantalum in parallel 0.1F ceramic. DVDD (Pin 27): 5V Positive Supply. Short to Pin 28. AVDD (Pin 28): 5V Positive Supply. Bypass to AGND with 10F tantalum in parallel with 0.1F ceramic.
FU CTIO AL BLOCK DIAGRA
+AIN CSAMPLE - AIN VREF 2k 2.5V REF ZEROING SWITCHES AVDD DVDD VSS
REF AMP
REFCOMP (4V) AGND DGND INTERNAL CLOCK
NAP/SLP SHDN CONVST RD CS
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CSAMPLE
+
12-BIT CAPACITIVE DAC COMP
-
12 OUTPUT LATCHES
SUCCESSIVE APPROXIMATION REGISTER
* * *
D11 D0
CONTROL LOGIC
BUSY
LTC1410 * BD
LTC1410
TEST CIRCUITS
Load Circuits for Access Timing
5V 1k DBN 1k CL DBN CL DBN 1k 100pF DBN 100pF
Load Circuits for Output Float Delay
5V 1k
(A) Hi-Z TO VOH AND VOL TO VOH
(B) Hi-Z TO VOL AND VOH TO VOL
1410 TC01
(A) VOH TO Hi-Z
(B) VOL TO Hi-Z
1410 TC02
APPLICATIONS INFORMATION
CONVERSION DETAILS The LTC1410 uses a successive approximation algorithm and an internal sample-and-hold circuit to convert an analog signal to a 12-bit parallel output. The ADC is complete with a precision reference and an internal clock. The control logic provides easy interface to microprocessors and DSPs. (Please refer to the Digital Interface section for the data format.) Conversion start is controlled by the CS and CONVST inputs. At the start of the conversion the successive approximation register (SAR) is reset. Once a conversion cycle has begun it cannot be restarted. During the conversion, the internal differential 12-bit capacitive DAC output is sequenced by the SAR from the Most Significant Bit (MSB) to the Least Significant Bit (LSB). Referring to Figure 1, the + AIN and - AIN inputs are connected to the sample-and-hold capacitors (CSAMPLE) during the acquire phase and the comparator offset is nulled by the zeroing switches. In this acquire phase, a minimum duration of 100ns will provide enough time for the sample-and-hold capacitors to acquire the analog signal. During the convert phase the comparator zeroing switches open, putting the comparator into compare mode. The input switches connect the CSAMPLE capacitors to ground, transferring the differential analog input charge onto the summing junctions. This input charge is successively compared with the binarily-weighted charges supplied by the differential capacitive DAC. Bit decisions are made by the high speed comparator. At the end of a conversion, the differential DAC output balances the + AIN and - AIN input charges. The SAR contents (a 12-bit data word) which represent the difference of + AIN and - AIN are loaded into the 12-bit output latches.
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+CSAMPLE +AIN SAMPLE HOLD -CSAMPLE - AIN SAMPLE HOLD +CDAC ZEROING SWITCHES HOLD HOLD
+
-CDAC +VDAC -VDAC 12 SAR OUTPUT LATCHES * * * D11 D0
1410 F01
COMP
-
Figure 1. Simplified Block Diagram
7
LTC1410
APPLICATIONS INFORMATION
DYNAMIC PERFORMANCE The LTC1410 has excellent high speed sampling capability. Fast Four Transform (FFT) test techniques are used to test the ADC's frequency response, distortion and noise at the rated throughput. By applying a low distortion sine wave and analyzing the digital output using an FFT algorithm, the ADC's spectral content can be examined for frequencies outside the fundamental.
0 -20 -40 -60 -80 -100 12 -120 0 100 200 300 400 500 FREQUENCY (kHz) 600
1410 F02a
fSAMPLE = 1.25MHz fIN = 100.098kHz SFDR = 90.1dB SINAD = 72.4dB
AMPLITUDE (dB)
EFFECTIVE BITS
Figure 2a. LTC1410 Nonaveraged 4096 Point FFT, 100kHz Input
0 -20 -40 fSAMPLE = 1.25MHz fIN = 599.975kHz SFDR = 84.7dB SINAD = 71.7dB
AMPLITUDE (dB)
-60 -80 -100 -120 0 100 200 300 400 500 FREQUENCY (kHz) 600
1410 F02b
Figure 2b. LTC1410 Nonaveraged 4096 Point FFT, 600kHz Input
Signal-to-Noise Ratio The Signal-to-Noise plus Distortion ratio [S/(N + D)] is the ratio between the RMS amplitude of the fundamental input frequency to the RMS amplitude of all other frequency components at the ADC output. The output is band limited
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to frequencies from above DC and below half the sampling frequency. Figures 2a and 2b shows a typical spectral content with a 1.25MHz sampling rate for 100kHz and 600kHz inputs. The dynamic performance is excellent for input frequencies up to the Nyquist limit of 625kHz and beyond. Effective Number of Bits The Effective Number of Bits (ENOBs) is a measurement of the resolution of an ADC and is directly related to the S/(N + D) by the equation: N = [S/(N + D) - 1.76] /6.02 where N is the effective number of bits of resolution and S/(N + D) is expressed in dB. At the maximum sampling rate of 1.25MHz the LTC1410 maintains very good ENOBs up to the Nyquist input frequency of 625kHz and beyond. Refer to Figure 3.
74 68 10 8 6 4 2 0 1k fSAMPLE = 1.25MHz 10k 100k 1M INPUT FREQUENCY (Hz) 10M
LTC1410 * TA02
NYQUIST
62 56 50 S/(N + D) (dB)
Figure 3. Effective Bits and Signal/(Noise + Distortion) vs Input Frequency
Total Harmonic Distortion (THD) Total harmonic distortion is the ratio of the RMS sum of all harmonics of the input signal to the fundamental itself. The out-of-band harmonics alias into the frequency band between DC and half the sampling frequency. THD is expressed as:
THD = 20 log
2 2 2 2 V2 + V3 + V4 + . . .Vn
V1
LTC1410
APPLICATIONS INFORMATION
where V1 is the RMS amplitude of the fundamental frequency and V2 through Vn are the amplitudes of the second through nth harmonics. THD vs Input Frequency is shown in Figure 4. The LTC1410 has good distortion performance up to the Nyquist frequency and beyond.
AMPLITUDE (dB BELOW THE FUNDAMENTAL) 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 1k 1M 10k 100k INPUT FREQUENCY (Hz) 10M
1410 G03
AMPLITUDE (dB)
3RD 2ND
THD
-100
Figure 4. Distortion vs Input Frequency
Intermodulation Distortion (IMD) If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can produce Intermodulation Distortion in addition to THD. IMD is the change in one sinusoidal input caused by the presence of another sinusoidal input at a different frequency. If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer function can create distortion products at the sum and difference frequencies of mfa nfb, where m and n = 0, 1, 2, 3, etc. For example, the 2nd order IMD terms include (fa + fb). If the two input sine waves are equal in magnitude, the value (in decibels) of the 2nd order IMD products can be expressed by the following formula:
IMD fa + fb = 20 log
(
)
Amplitude at fa fb Amplitude at f a
(
)
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0 (fa) -20 -40 -60 -80 -100 -120 (fb) fSAMPLE = 1.25MHz fIN1 = 88.19580078kHz fIN2 = 111.9995117kHz
(2fa-fb) (fb-fa)
(2fb-fa) (fa+fb) (2fa+fb) (2fa) (2fb) (fa+2fb) (3fa) (3fb)
0
100
200
300 FREQUENCY (MHz)
400
500
600
1410 F05
Figure 5. Intermodulation Distortion Plot
Peak Harmonic or Spurious Noise The peak harmonic or spurious noise is the largest spectral component excluding the input signal and DC. This value is expressed in decibel relative to the RMS value of a full-scale input signal. Full Power and Full Linear Bandwidth The full power bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full-scale input signal. The full linear bandwidth is the input frequency at which the S/(N + D) has dropped to 68dB (11 effective bits). The LTC1410 has been designed to optimize input bandwidth, allowing the ADC to undersample input signals with frequencies above the converter's Nyquist frequency. The noise floor stays very low at high frequencies; S/(N + D) does not become dominated by distortion until frequencies far beyond Nyquist. Driving the Analog Input The differential analog inputs of the LTC1410 are easy to drive. The inputs may be driven differentially or as a single-ended input (i.e., the - AIN input is grounded). The + AIN and - AIN inputs are sampled at the same instant. Any unwanted signal that is common mode to both inputs will be reduced by the common mode rejection of the sample-and-hold circuit. The inputs draw only one small current spike while charging the sample-and-hold
9
LTC1410
APPLICATIONS INFORMATION
capacitors at the end of conversion. During conversion the analog inputs draw only a small leakage current. If the source impedance of the driving circuit is low then the LTC1410 inputs can be driven directly. As source impedance increases so will acquisition time (see Figure 6). For minimum acquisition time with high source impedance, a buffer amplifier should be used. The only requirement is that the amplifier driving the analog input(s) must settle after the small current spike before the next conversion starts (settling time must be 100ns for full throughput rate).
10
ACQUISITION TIME (s)
1
4 10F 0.1F 5
0.1
LTC1410
1410 F07
0.01
10
1k 10k 100 SOURCE RESISTANCE ()
100k
1410 F06
Figure 6. Acquisition Time vs Source Resistance
Choosing an input amplifier is easy if a few requirements are taken into consideration. First, choose an amplifier that has a low output impedance (< 100) at the closedloop bandwidth frequency. For example, if an amplifier is used in a gain of +1 and has a closed-loop bandwidth of 50MHz, then the output impedance at 50MHz must be less than 100. The second requirement is that the closed-loop bandwidth must be greater than 20MHz to ensure adequate small-signal settling for full throughput rate. If slower op amps are used, more settling time can be provided by increasing the time between conversions. Suitable devices capable of driving the ADC's inputs include the LT (R)1360, LT1220, LT1223, LT1224 and LT1227 op amps. The noise and the distortion of the input amplifier must also be considered since they will add to the LTC1410 noise and distortion. The small-signal bandwidth of the
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sample-and-hold circuit is 20MHz. Any noise that is present at the analog inputs will be summed over this entire bandwidth. Noisy input circuitry should be filtered prior to the analog inputs to minimize noise. A simple 1-pole RC filter is usually sufficient. For example, Figure 7 shows a 1000pF capacitor from + AIN to ground and a 100 source resistor will limit the input bandwidth to 1.6MHz. Simple RC filters work well for AC applications, but they will limit the transient response. For full speed operation, amplifiers with fast settling and low noise should be chosen.
ANALOG INPUT 100 1000pF 1 2 3 +AIN -AIN VREF REFCOMP AGND
Figure 7. RC Input Filter
Internal Reference The LTC1410 has an on-chip, temperature compensated, curvature corrected, bandgap reference which is factory trimmed to 2.500V. It is connected internally to a reference amplifier and is available at VREF (Pin 3). See Figure 8a. A 2k resistor is in series with the output so that it can be
ANALOG INPUT
1 2
+AIN -AIN
LTC1410
2.500V 4.06V
3 VREF 4 REFCOMP 0.1F 5 AGND R3 64k R2 40k
R1 2k
BANDGAP REFERENCE
+ -
10F
1410 F08a
Figure 8a. LTC1410 Reference Circuit
LTC1410
APPLICATIONS INFORMATION
easily overdriven in applications where an external reference is required. The reference amplifier provides buffering between the internal reference and the capacitive DAC. The reference amplifier compensation pin REFCOMP (Pin 4), must be bypassed with a capacitor to ground. The reference amplifier is stable with capacitors of 1F or greater. For the best noise performance, a 10F tantalum in parallel with 0.1F ceramic is recommended. The VREF pin can be driven with an external reference (Figure 8b), a DAC or other means to provide input span adjustment. The VREF should be kept in the range of 2.25V to 2.75V for specified linearity.
5V VIN LT1019A-2.5 VOUT ANALOG INPUT 1 2 3 4 10F 0.1F 5 +AIN -AIN VREF REFCOMP AGND LTC1410
1410 F08b
OUTPUT CODE
Figure 8b. Using the LT1019-2.5 as an External Reference
Full-Scale and Offset Adjustment
10F
Figure 9 shows the ideal input/output characteristics for the LTC1410. The code transitions occur midway between successive integer LSB values (i.e., - FS + 0.5LSB, - FS + 1.5LSB, - FS + 2.5LSB, . . . FS - 1.5LSB, FS - 0.5LSB).The output is two's complement binary with 1LSB = [(+FS) - (- FS)]/4096 = 5V/4096 = 1.22mV. In applications where absolute accuracy is important, offset and full-scale errors can be adjusted to zero. Offset error must be adjusted before full-scale error. Figure 10 shows the extra components required for full-scale error adjustment. Zero offset is achieved by adjusting the offset applied to the - AIN input. For zero offset error apply - 0.61mV (i.e., - 0.5LSB) at +AIN and adjust the offset at the - AIN input until the output code flickers between 0000 0000 0000 and 1111 1111 1111. For full-scale adjustment, an input voltage of 2.49817V (FS - 1.5LSBs) is
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applied to AIN and R2 is adjusted until the output code flickers between 0111 1111 1110 and 0111 1111 1111.
011...111 011...110 BIPOLAR ZERO
000...001 000...000 111...111 111...110 FS = 2.5V 2FS 1LSB = 4096 -FS -1 0V 1 FS - LSB LSB LSB INPUT VOLTAGE, (+AIN) - (-AIN) (V)
1410 F09
100...001 100...000
Figure 9. LTC1410 Transfer Characteristics
- 5V R1 50k R3 47k
ANALOG INPUT
1 2 3
+AIN -AIN VREF REFCOMP AGND LTC1410
1410 F10
R4 100 R5 R2 47k 50k R6 24k 0.1F
4 5
Figure 10. Offset and Full-Scale Adjust Circuit
BOARD LAYOUT AND BYPASSING Wire wrap boards are not recommended for high resolution or high speed A/D converters. To obtain the best performance from the LTC1410, a printed circuit board with ground plane is required. Layout for the printed circuit board should ensure that digital and analog signal lines are separated as much as possible. Particular care should be taken not to run any digital track alongside an analog signal track or underneath the ADC. The analog input should be screened by AGND.
11
LTC1410
APPLICATIONS INFORMATION
High quality tantalum and ceramic bypass capacitors should be used at the VDD, VSS and REFCOMP pins as shown in the Typical Application on the first page of this data sheet. Bypass capacitors must be located as close to the pins as possible. The traces connecting the pins and bypass capacitors must be kept short and should be made as wide as possible. The LTC1410 has differential inputs to minimize noise coupling. Common mode noise on the + AIN and - AIN leads will be rejected by the input CMRR. The - AIN input can be used as a ground sense for the + AIN input; the LTC1410 will hold and convert the difference voltage between + AIN and - AIN. The leads to + AIN (Pin 1) and - AIN (Pin 2) should be kept as short as possible. In applications where this is not possible, the + AIN and - AIN traces should be run side by side to equalize coupling. A single point analog ground separate from the logic system ground should be established with an analog ground plane at Pin 5 (AGND) or as close as possible to the ADC. Pin 14 and Pin 19 (ADC's DGND) and all other analog grounds should be connected to this single analog ground point. No other digital grounds should be connected to this analog ground point. Low impedance analog and digital power supply common returns are essential to low noise operation of the ADC and the foil width for these tracks should be as wide as possible. In applications where the ADC data outputs and control signals are connected to a continuously active microprocessor bus, it is possible to get errors in the conversion results. These errors are due to feedthrough from the microprocessor to the successive approximation comparator. The problem can be eliminated by forcing the microprocessor into a wait state during conversion or by using three-state buffers to isolate the ADC data bus. DIGITAL INTERFACE The A/D converter is designed to interface with microprocessors as a memory mapped device. The CS and RD control inputs are common to all peripheral memory interfacing. A separate CONVST is used to initiate a conversion. Internal Clock The A/D converter has an internal clock that eliminates the need of synchronization between the external clock and the CS and RD signals found in other ADCs. The internal clock is factory trimmed to achieve a typical conversion time of 0.65s and a maximum conversion time over the full operating temperature range of 0.75s. No external adjustments are required. The guaranteed maximum acquisition time is 100ns. In addition, throughput time of 800ns and a minimum sampling rate of 1.25Msps is guaranteed.
1
+AIN -AIN
REFCOMP 4 AGND
ANALOG INPUT CIRCUITRY
+ -
2
10F
0.1F
Figure 11. Power Supply Grounding Practice
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LTC1410 VSS 26 10F 0.1F 10F AVDD DVDD DGND OGND 28 27 0.1F 14 19
DIGITAL SYSTEM
1410 F11
LTC1410
APPLICATIONS INFORMATION
Power Shutdown The LTC1410 provides two power shutdown modes, Nap and Sleep, to save power during inactive periods. The Nap mode reduces the power by 95% and leaves only the digital logic and reference powered up. The wake-up time from Nap to active is 200ns. In Sleep mode all bias currents are shut down and only leakage current remains -- about 1A. Wake-up time from Sleep mode is
NAP/SLP t3 SHDN
1410 F12a
Figure 12a. NAP/SLP to SHDN Timing
SHDN t4 CONVST
1410 F12b
Figure 12b. SHDN to CONVST Wake-Up Timing
much slower since the reference circuit must power up and settle to 0.01% for full 12-bit accuracy. Sleep mode wake-up time is dependent on the value of the capacitor connected to the REFCOMP (Pin 4). The wake-up time is 10ms with the recommended 10F capacitor. Shutdown is controlled by Pin 21 (SHDN), the ADC is in shutdown when it is low. The shutdown mode is selected with Pin 20 (NAP/SLP); high selects Nap.
CS t2 CONVST t1 RD
1410 F12
Figure 13. CS to CONVST Setup Timing
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Timing and Control Conversion start and data read operations are controlled by three digital inputs: CONVST, CS and RD. A logic "0" applied to the CONVST pin will start a conversion after the ADC has been selected (i.e., CS is low). Once initiated, it cannot be restarted until the conversion is complete. Converter status is indicated by the BUSY output. BUSY is low during a conversion. Figures 14 through 18 show several different modes of operation. In modes 1a and 1b (Figures 14 and 15) CS and RD are both tied low. The falling edge of CONVST starts the conversion. The data outputs are always enabled and data can be latched with the BUSY rising edge. Mode 1a shows operation with a narrow logic low CONVST pulse. Mode 1b shows a narrow logic high CONVST pulse. In mode 2 (Figure 16) CS is tied low. The falling edge of CONVST signal again starts the conversion. Data outputs are in three-state until read by the MPU with the RD signal. Mode 2 can be used for operation with a shared MPU databus. In slow memory and ROM modes (Figures 17 and 18) CS is tied low and CONVST and RD are tied together. The MPU starts the conversion and reads the output with the RD signal. Conversions are started by the MPU or DSP (no external sample clock). In slow memory mode the processor applies a logic low to RD (= CONVST), starting the conversion. BUSY goes low forcing the processor into a wait state. The previous conversion result appears on the data outputs. When the conversion is complete, the new conversion results appear on the data outputs; BUSY goes high releasing the processor and the processor takes RD (= CONVST) back high and reads the new conversion data. In ROM mode, the processor takes RD (= CONVST) low, starting a conversion and reading the previous conversion result. After the conversion is complete, the processor can read the new result and initiate another conversion.
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LTC1410
APPLICATIONS INFORMATION
CS = RD = 0 t5 CONVST t6 BUSY t7 DATA DATA (N - 1) DB11 TO DB0 DATA N DB11 TO DB0 DATA (N + 1) DB11 TO DB0
1410 F14
Figure 14. Mode 1a. CONVST Starts a Conversion. Data Outputs Always Enabled
(CONVST =
CS = RD = 0 t13 CONVST t6 BUSY
tCONV t5
DATA
DATA (N - 1) DB11 TO DB0
Figure 15. Mode 1b. CONVST Starts a Conversion. Data Outputs Always Enabled
(CONVST = )
tCONV t5 CONVST t6 BUSY t9 RD t 10 DATA
Figure 16. Mode 2. CONVST Starts a Conversion. Data is Read by RD
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t CONV
t8
)
t8
t6
t7 DATA N DB11 TO DB0 DATA (N + 1) DB11 TO DB0
1410 F15
t13 t8
t 12
t 11
DATA N DB11 TO DB0
1410 F16
LTC1410
APPLICATI
S I FOR ATIO
t CONV
RD = CONVST t6 BUSY t 10 DATA DATA (N - 1) DB11 TO DB0 t7 DATA N DB11 TO DB0 DATA N DB11 TO DB0 DATA (N + 1) DB11-DB0
1410 F17
Figure 17. Slow Memory Mode Timing
t CONV RD = CONVST t6 BUSY t 10 DATA DATA (N - 1) DB11 TO DB0 DATA N DB11 TO DB0
1410 F18
t 11
Figure 18. ROM Mode Timing
PACKAGE DESCRIPTIO
Dimensions in inches (millimeters) unless otherwise noted. G Package 28-Lead Plastic SSOP (0.209)
(LTC DWG # 05-08-1640)
0.397 - 0.407* (10.07 - 10.33) 28 27 26 25 24 23 22 21 20 19 18 17 16 15
0.205 - 0.212** (5.20 - 5.38)
0 - 8
0.005 - 0.009 (0.13 - 0.22)
0.022 - 0.037 (0.55 - 0.95)
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.006" (0.152mm) PER SIDE **DIMENSIONS DO NOT INCLUDE INTERLEAD FLASH. INTERLEAD FLASH SHALL NOT EXCEED 0.010" (0.254mm) PER SIDE
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.
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t8
0.301 - 0.311 (7.65 - 7.90)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 0.068 - 0.078 (1.73 - 1.99)
0.0256 (0.65) BSC
0.010 - 0.015 (0.25 - 0.38)
0.002 - 0.008 (0.05 - 0.21)
G28 SSOP 0694
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LTC1410
PACKAGE DESCRIPTIO U
Dimensions in inches (millimeters) unless otherwise noted.
SW Package 28-Lead Plastic Small Outline (Wide 0.300)
(LTC DWG # 05-08-1620)
0.697 - 0.712* (17.70 - 18.08) 28 27 26 25 24 23 22 21 20 19 18 17 16 15
NOTE 1
0.394 - 0.419 (10.007 - 10.643)
0.291 - 0.299** (7.391 - 7.595) 0.010 - 0.029 x 45 (0.254 - 0.737) 0 - 8 TYP
1 0.093 - 0.104 (2.362 - 2.642)
2
3
4
5
6
7
8
9
10
11
12
13
14 0.037 - 0.045 (0.940 - 1.143)
0.009 - 0.013 (0.229 - 0.330)
NOTE 1 0.016 - 0.050 (0.406 - 1.270)
0.050 (1.270) TYP
NOTE: 1. PIN 1 IDENT, NOTCH ON TOP AND CAVITIES ON THE BOTTOM OF PACKAGES ARE THE MANUFACTURING OPTIONS. THE PART MAY BE SUPPLIED WITH OR WITHOUT ANY OF THE OPTIONS *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
0.014 - 0.019 (0.356 - 0.482) TYP
0.004 - 0.012 (0.102 - 0.305)
S28 (WIDE) 0996
RELATED PARTS
12-Bit Sampling A/D Converters
PART NUMBER LTC1273/75/76 LTC1274/77 LTC1278/79 LTC1282 DESCRIPTION Complete 5V Sampling 12-Bit ADCs with 70dB SINAD at Nyquist Low Power 12-Bit ADCs with Nap and Sleep Mode Shutdown High Speed Sampling 12-Bit ADCs with Shutdown Complete 3V 12-Bit ADCs with 12mW Power Dissipation COMMENTS Lower Power and Cost Effective for fSAMPLE 300ksps Lowest Power for fSAMPLE 100ksps Cost Effective 12-Bit ADCs -- Best for 2-Pair HDSL, fSAMPLE 500ksps/600ksps Fully Specified for 3V-Powered Applications, fSAMPLE 140ksps
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408)432-1900 q FAX: (408) 434-0507 q www.linear-tech.com
1410fa LT/TP 0399 2K REV A * PRINTED IN USA
(c) LINEAR TECHNOLOGY CORPORATION 1995

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