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LTC2248/LTC2247/LTC2246 14-Bit, 65/40/25Msps Low Power 3V ADCs
FEATURES

DESCRIPTIO
Sample Rate: 65Msps/40Msps/25Msps Single 3V Supply (2.7V to 3.4V) Low Power: 205mW/120mW/75mW 74.3dB SNR 90dB SFDR No Missing Codes Flexible Input: 1VP-P to 2VP-P Range 575MHz Full Power Bandwidth S/H Clock Duty Cycle Stabilizer Shutdown and Nap Modes Pin Compatible Family 125Msps: LTC2253 (12-Bit), LTC2255 (14-Bit) 105Msps: LTC2252 (12-Bit), LTC2254 (14-Bit) 80Msps: LTC2229 (12-Bit), LTC2249 (14-Bit) 65Msps: LTC2228 (12-Bit), LTC2248 (14-Bit) 40Msps: LTC2227 (12-Bit), LTC2247 (14-Bit) 25Msps: LTC2226 (12-Bit), LTC2246 (14-Bit) 10Msps: LTC2225 (12-Bit), LTC2245 (14-Bit) 32-Pin (5mm x 5mm) QFN Package
The LTC(R)2248/LTC2247/LTC2246 are 14-bit 65Msps/ 40Msps/25Msps, low power 3V A/D converters designed for digitizing high frequency, wide dynamic range signals. The LTC2248/LTC2247/LTC2246 are perfect for demanding imaging and communications applications with AC performance that includes 74.3dB SNR and 90dB SFDR for signals at the Nyquist frequency. DC specs include 1LSB INL (typ), 0.5LSB DNL (typ) and no missing codes over temperature. The transition noise is a low 1LSBRMS. A single 3V supply allows low power operation. A separate output supply allows the outputs to drive 0.5V to 3.6V logic. A single-ended CLK input controls converter operation. An optional clock duty cycle stabilizer allows high performance at full speed for a wide range of clock duty cycles.
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
APPLICATIO S

Wireless and Wired Broadband Communication Imaging Systems Ultrasound Spectral Analysis Portable Instrumentation
TYPICAL APPLICATIO
REFH REFL FLEXIBLE REFERENCE
OVDD
+
ANALOG INPUT INPUT S/H
SNR (dBFS)
-
14-BIT PIPELINED ADC CORE
CORRECTION LOGIC
OUTPUT DRIVERS
D13 * * * D0 OGND
CLOCK/DUTY CYCLE CONTROL
2249 TA01a
CLK
U
LTC2248: SNR vs Input Frequency, -1dB, 2V Range, 65Msps
75 74 73 72 71 70 0 100 50 150 INPUT FREQUENCY (MHZ) 200
2249 TAO1b
U
U
224876fa
1
LTC2248/LTC2247/LTC2246
ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SENSE MODE VCM D13 D12 D11 VDD OF
OVDD = VDD (Notes 1, 2)
UH PACKAGE 32-LEAD (5mm x 5mm) PLASTIC QFN
TJMAX = 125C, JA = 34C/W EXPOSED PAD IS GND (PIN 33) MUST BE SOLDERED TO PCB
SHDN
Supply Voltage (VDD) ................................................. 4V Digital Output Ground Voltage (OGND) ....... -0.3V to 1V Analog Input Voltage (Note 3) ..... -0.3V to (VDD + 0.3V) Digital Input Voltage .................... -0.3V to (VDD + 0.3V) Digital Output Voltage ................ -0.3V to (OVDD + 0.3V) Power Dissipation ............................................ 1500mW Operating Temperature Range LTC2248C, LTC2247C, LTC2246C ........... 0C to 70C LTC2248I, LTC2247I, LTC2246I ..........-40C to 85C Storage Temperature Range ..................-65C to 125C
32 31 30 29 28 27 26 25 AIN+ 1 AIN- 2 REFH 3 REFH 4 REFL 5 REFL 6 VDD 7 GND 8 9 10 11 12 13 14 15 16
OE D0 D1 D2 D3 CLK D4
24 D10 23 D9 22 D8 33 21 OVDD 20 OGND 19 D7 18 D6 17 D5
ORDER PART NUMBER LTC2248CUH LTC2248IUH LTC2247CUH LTC2247IUH LTC2246CUH LTC2246IUH
QFN PART MARKING* 2248 2248 2247 2247 2246 2246
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.
CO VERTER CHARACTERISTICS
PARAMETER Resolution (No Missing Codes) Integral Linearity Error Differential Linearity Error Offset Error Gain Error Offset Drift Full-Scale Drift Transition Noise Internal Reference External Reference SENSE = 1V Differential Analog Input (Note 5) Differential Analog Input (Note 6) External Reference CONDITIONS

The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
MIN 14 -4 -1 -12 -2.5 1 0.5 2 0.5 10 30 5 1 4 1 12 2.5 LTC2248 TYP MAX MIN 14 -4 -1 -12 -2.5 1 0.5 2 0.5 10 30 5 1 4 1 12 2.5 LTC2247 TYP MAX MIN 14 -4 -1 -12 -2.5 1 0.5 2 0.5 10 30 5 1 4 1 12 2.5 LTC2246 TYP MAX UNITS Bits LSB LSB mV %FS V/C ppm/C ppm/C LSBRMS
224876fa
2
U
W
U
U
WW
W
U
LTC2248/LTC2247/LTC2246
A ALOG I PUT
SYMBOL VIN VIN,CM IIN ISENSE IMODE tAP tJITTER CMRR PARAMETER
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
CONDITIONS
+-
Analog Input Range (AIN
Analog Input Common Mode (AIN+ + AIN-)/2 Analog Input Leakage Current SENSE Input Leakage MODE Pin Leakage Sample-and-Hold Acquisition Delay Time Sample-and-Hold Acquisition Delay Time Jitter Analog Input Common Mode Rejection Ratio Full Power Bandwidth
DY A IC ACCURACY
SYMBOL SNR PARAMETER Signal-to-Noise Ratio
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. AIN = -1dBFS. (Note 4)
CONDITIONS 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input SFDR Spurious Free Dynamic Range 2nd or 3rd Harmonic 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input SFDR Spurious Free Dynamic Range 4th Harmonic or Higher 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input S/(N+D) Signal-to-Noise Plus Distortion Ratio 5MHz Input 12.5MHz Input 20MHz Input 30MHz Input 70MHz Input 140MHz Input IMD Intermodulation Distortion fIN1 = 28.2MHz fIN2 = 26.8MHz

U
WU
U
MIN

TYP 1.5 1.5
MAX 1.9 2 1 3 3
UNITS V V V A A A ns psRMS dB MHz
AIN
-)
2.7V < VDD < 3.4V (Note 7) Differential Input (Note 7) Single Ended Input (Note 7) 0V < AIN+, AIN- < VDD 0V < SENSE < 1V
0.5V to 1V 1 0.5 -1 -3 -3 0 0.2 80
Figure 8 Test Circuit
575
MIN
LTC2248 TYP MAX 74.3
MIN
LTC2247 TYP MAX 74.4
MIN 72.9
LTC2246 TYP MAX 74.5 74.2
UNITS dB dB dB dB
72.9 72.5 74.3 74.3 73.9 90 76 76 90 85 80 95 84 84 95 95 90 74.3 72.2 72 74.2 74.1 71.9 90
74.4 73.9 73.3 90 76 90 85 80 95 84 95 95 90 74.4 72.2 74.3 73.6 71.9 90 73.4 71.8 90 95 90 74.5 74.2 85 80 95 95 73.4 73 90 90
dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB
224876fa
3
LTC2248/LTC2247/LTC2246
I TER AL REFERE CE CHARACTERISTICS
PARAMETER VCM Output Voltage VCM Output Tempco VCM Line Regulation VCM Output Resistance CONDITIONS IOUT = 0
DIGITAL I PUTS A D DIGITAL OUTPUTS
SYMBOL VIH VIL IIN CIN LOGIC OUTPUTS OVDD = 3V COZ ISOURCE ISINK VOH VOL OVDD = 2.5V VOH VOL OVDD = 1.8V VOH VOL High Level Output Voltage Low Level Output Voltage IO = -200A IO = 1.6mA High Level Output Voltage Low Level Output Voltage IO = -200A IO = 1.6mA Hi-Z Output Capacitance Output Source Current Output Sink Current High Level Output Voltage Low Level Output Voltage PARAMETER High Level Input Voltage Low Level Input Voltage Input Current Input Capacitance CONDITIONS VDD = 3V VDD = 3V LOGIC INPUTS (CLK, OE, SHDN)
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
MIN

POWER REQUIRE E TS
SYMBOL VDD OVDD IVDD PDISS PSHDN PNAP PARAMETER Analog Supply Voltage Output Supply Voltage Supply Current Power Dissipation Shutdown Power Nap Mode Power SHDN = H, OE = H, No CLK SHDN = H, OE = L, No CLK CONDITIONS (Note 9) (Note 9)
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 8)
MIN

4
UW
U
U
U
U
U
(Note 4)
MIN 1.475 TYP 1.500 25 3 4 MAX 1.525 UNITS V ppm/C mV/V
2.7V < VDD < 3.4V -1mA < IOUT < 1mA
TYP
MAX
UNITS V
2 0.8 -10 3 10
V A pF
VIN = 0V to VDD (Note 7)
OE = High (Note 7) VOUT = 0V VOUT = 3V IO = -10A IO = -200A IO = 10A IO = 1.6mA

3 50 50 2.7 2.995 2.99 0.005 0.09 2.49 0.09 1.79 0.09 0.4
pF mA mA V V V V V V V V
LTC2248 TYP MAX 3 3 68.3 205 2 15 3.4 3.6 80 240
MIN 2.7 0.5
LTC2247 TYP MAX 3 3 40 120 2 15 3.4 3.6 48 144
MIN 2.7 0.5
LTC2246 TYP MAX 3 3 25 75 2 15 3.4 3.6 30 90
UNITS V V mA mW mW mW
2.7 0.5
224876fa
LTC2248/LTC2247/LTC2246
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
SYMBOL fs tL PARAMETER CLK Low Time CONDITIONS

TI I G CHARACTERISTICS
tH
tAP tD
Pipeline Latency Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: All voltage values are with respect to ground with GND and OGND wired together (unless otherwise noted). Note 3: When these pin voltages are taken below GND or above VDD, they will be clamped by internal diodes. This product can handle input currents of greater than 100mA below GND or above VDD without latchup. Note 4: VDD = 3V, fSAMPLE = 65MHz (LTC2248), 40MHz (LTC2247), or 25MHz (LTC2246), input range = 2VP-P with differential drive, unless otherwise noted.
TYPICAL PERFOR A CE CHARACTERISTICS
LTC2248: Typical INL, 2V Range, 65Msps
2.0 1.5 1.0 0.5 0 -0.5 -1.0 -1.5 -2.0 0 4096 8192 CODE 12288 16384
2248 G01
DNL ERROR (LSB)
INL ERROR (LSB)
UW
UW
MIN 1 7.3 5 7.3 5
LTC2248 TYP MAX 65 7.7 7.7 7.7 7.7 0 500 500 500 500
MIN 1 11.8 5 11.8 5
LTC2247 TYP MAX 40 12.5 12.5 12.5 12.5 0 500 500 500 500
MIN 1 18.9 5 18.9 5
LTC2246 TYP MAX 25 20 20 20 20 0 500 500 500 500
UNITS MHz ns ns ns ns ns
Sampling Frequency (Note 9) Duty Cycle Stabilizer Off Duty Cycle Stabilizer On (Note 7) Duty Cycle Stabilizer Off Duty Cycle Stabilizer On (Note 7)
CLK High Time
Sample-and-Hold Aperture Delay CLK to DATA delay Data Access Time After OE CL = 5pF (Note 7) CL = 5pF (Note 7)

1.4
2.7 4.3 3.3 5
5.4 10 8.5
1.4
2.7 4.3 3.3 5
5.4 10 8.5
1.4
2.7 4.3 3.3 5
5.4 10 8.5
ns ns ns Cycles
BUS Relinquish Time (Note 7)
Note 5: 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 6: Offset error is the offset voltage measured from -0.5 LSB when the output code flickers between 00 0000 0000 0000 and 11 1111 1111 1111. Note 7: Guaranteed by design, not subject to test. Note 8: VDD = 3V, fSAMPLE = 65MHz (LTC2248), 40MHz (LTC2247), or 25MHz (LTC2246), input range = 1VP-P with differential drive. Note 9: Recommended operating conditions.
LTC2248: Typical DNL, 2V Range, 65Msps
1.00 0.75 0.50 0.25 0 -0.25 -0.50 -0.75 -1.00 0 4096 8192 CODE 12288 16384
2248 G02
224876fa
5
LTC2248/LTC2247/LTC2246 TYPICAL PERFOR A CE CHARACTERISTICS
LTC2248: 8192 Point FFT, fIN = 5MHz, -1dB, 2V Range, 65Msps
0 -10 -20 -30 0 -10 -20 -30
AMPLITUDE (dB)
AMPLITUDE (dB)
-40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 20 25 FREQUENCY (MHz) 30
2248 G03
LTC2248: 8192 Point FFT, fIN = 70MHz, -1dB, 2V Range, 65Msps
0 -10 -20 -30 0 -10 -20 -30
AMPLITUDE (dB)
AMPLITUDE (dB)
-40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 20 25 FREQUENCY (MHz) 30
2248 G05
-40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 20 25 FREQUENCY (MHz) 30
2248 G06
AMPLITUDE (dB)
LTC2248: Grounded Input Histogram, 65Msps
25000 21824 20412 20000 74 75
COUNT
15000 10224
73
SFDR (dBFS)
SNR (dBFS)
10000
9042
5000 2116 172 0 1596 121
8196 8197 8198 8199 8200 8201 8202 8203 CODE 2248 G08
6
UW
LTC2248: 8192 Point FFT, fIN = 30MHz, -1dB, 2V Range, 65Msps
-40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 20 25 FREQUENCY (MHz) 30
2248 G04
LTC2248: 8192 Point FFT, fIN = 140MHz, -1dB, 2V Range, 65Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120
LTC2248: 8192 Point 2-Tone FFT, fIN = 28.2MHz and 26.8MHz, -1dB, 2V Range, 65Msps
0
5
10 15 20 25 FREQUENCY (MHz)
30
2248 G06a
LTC2248: SNR vs Input Frequency, -1dB, 2V Range, 65Msps
100 95 90 85 80 75 71 70 70 65 0 100 50 150 INPUT FREQUENCY (MHz) 200
2248 G09
LTC2248: SFDR vs Input Frequency, -1dB, 2V Range, 65Msps
72
0
50
100 150 INPUT FREQUENCY (MHz)
200
2248 G10
224876fa
LTC2248/LTC2247/LTC2246 TYPICAL PERFOR A CE CHARACTERISTICS
LTC2248: SNR and SFDR vs Sample Rate, 2V Range, fIN = 5MHz, -1dB
110 100 95
SNR AND SFDR (dBFS)
100
SNR AND SFDR (dBFS)
SFDR
90 SFDR: DCS OFF 85 80 75 SNR: DCS ON SNR: DCS OFF 70
SNR (dBc AND dBFS)
90
80 SNR 70
60 0 10 20 30 40 50 60 70 80 90 100 110 SAMPLE RATE (Msps) 2248 G11
LTC2248: SFDR vs Input Level, fIN = 30MHz, 2V Range, 65Msps
120 110 100
SFDR (dBc AND dBFS)
dBFS
90
IVDD (mA)
70 60 50 40 30 20 -60 -50
dBc 90dBc SFDR REFERENCE LINE
1V RANGE 65 60 55 50 2V RANGE
IOVDD (mA)
80
-40 -30 -20 INPUT LEVEL (dBFS)
LTC2247: Typical INL, 2V Range, 40Msps
2.0 1.5 1.0
DNL ERROR (LSB) INL ERROR (LSB)
AMPLITUDE (dB)
0.5 0 -0.5 -1.0 -1.5 -2.0 0 4096 8192 CODE 12288 16384
2247 G01
UW
-10
2248 G14
LTC2248: SNR and SFDR vs Clock Duty Cycle, 65Msps
SFDR: DCS ON 80 70 60 50
LTC2248: SNR vs Input Level, fIN = 30MHz, 2V Range, 65Msps
dBFS
dBc 40 30 20 10
30
35
40 45 50 55 60 CLOCK DUTY CYCLE (%)
65
70
0 -60
-50
2247 G12
- 40 -30 -20 INPUT LEVEL (dBFS)
-10
0
2248 G13
LTC2248: IVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB
80 75 70 6 5 4 3 2 1 0
LTC2248: IOVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB, OVDD = 1.8V
0
0
10
20 30 40 50 60 SAMPLE RATE (Msps)
70
80
0
10
2248 G15
20 30 40 50 60 SAMPLE RATE (Msps)
70
80
2248 G16
LTC2247: Typical DNL, 2V Range, 40Msps
1.00 0.75 0.50 0.25 0 -0.25 -0.50 -0.75 -1.00 0 4096 8192 CODE 12288 16384
2247 G02
LTC2247: 8192 Point FFT, fIN = 5MHz, -1dB, 2V Range, 40Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 FREQUENCY (MHz) 20
2247 G03
224876fa
7
LTC2248/LTC2247/LTC2246 TYPICAL PERFOR A CE CHARACTERISTICS
LTC2247: 8192 Point FFT, fIN = 30MHz, -1dB, 2V Range, 40Msps
0 -10 -20 -30
AMPLITUDE (dB) AMPLITUDE (dB)
AMPLITUDE (dB)
-40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 FREQUENCY (MHz) 20
2247 G04
LTC2247: 8192 Point 2-Tone FFT, fIN = 21.6MHz and 23.6MHz, -1dB, 2V Range, 40Msps
0 -10 -20 -30
AMPLITUDE (dB)
-40
COUNT
SNR (dBFS)
-50 -60 -70 -80 -90 -100 -110 -120
0
5
10 15 FREQUENCY (MHz)
LTC2247: SFDR vs Input Frequency, -1dB, 2V Range, 40Msps
100 95 100 90
SFDR (dBFS) SNR AND SFDR (dBFS)
SNR (dBc AND dBFS)
85 80 75 70 65 0 50 150 INPUT FREQUENCY (MHz) 100 200
2247 G10
8
UW
2247 G07
LTC2247: 8192 Point FFT, fIN = 70MHz, -1dB, 2V Range, 40Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 FREQUENCY (MHz) 20
2247 G05
LTC2247: 8192 Point FFT, fIN = 140MHz, -1dB, 2V Range, 40Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 5 10 15 FREQUENCY (MHz) 20
2247 G06
LTC2247: Grounded Input Histogram, 40Msps
30000 25000 20000 15000 10000 5000 0 30 640 4641 4520 546 36 70 71 14833 15714 73 24558 74 75
LTC2247: SNR vs Input Frequency, -1dB, 2V Range, 40Msps
72
20
8184 8185 8186 818781888189 8190 81918192 CODE 2247 G08
0
100 50 150 INPUT FREQUENCY (MHz)
200
2247 G09
LTC2247: SNR and SFDR vs Sample Rate, 2V Range, fIN = 5MHz, -1dB
110 SFDR 80 70 60 50
LTC2247: SNR vs Input Level, fIN = 5MHz, 2V Range, 40Msps
dBFS
90
dBc 40 30 20 10
80 SNR 70
60 0 40 20 60 SAMPLE RATE (Msps) 80
2247 G11
0 -60
-50
- 40 -30 -20 INPUT LEVEL (dBFS)
-10
0
2247 G12
224876fa
LTC2248/LTC2247/LTC2246 TYPICAL PERFOR A CE CHARACTERISTICS
LTC2247: SFDR vs Input Level, fIN = 5MHz, 2V Range, 40Msps
120 110 100
SNR (dBc AND dBFS)
dBFS 45
IOVDD (mA) IVDD (mA)
90 80 70 60 50 40 30 20 -60 -50 -40 -30 -20 INPUT LEVEL (dBFS) -10 0
2247 G13
dBc 90dBc SFDR REFERENCE LINE
LTC2246: Typical INL, 2V Range, 25Msps
2.0 1.5 1.0
DNL ERROR (LSB) INL ERROR (LSB)
AMPLITUDE (dB)
0.5 0 -0.5 -1.0 -1.5 -2.0 0 4096 8192 CODE 12288 16384
2246 G01
LTC2246: 8192 Point FFT, fIN = 30MHz, -1dB, 2V Range, 25Msps
0 -10 -20 -30
AMPLITUDE (dB) AMPLITUDE (dB)
AMPLITUDE (dB)
-40 -50 -60 -70 -80 -90 -100 -110 -120 0 2 4 6 8 10 FREQUENCY (MHz) 12
2246 G04
UW
LTC2247: IVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB
50 4
LTC2247: IOVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB, OVDD = 1.8V
3
2V RANGE 40
2
1V RANGE 35 1
30
0
10
30 40 20 SAMPLE RATE (Msps)
50
2247 G14
0
0
10
30 40 20 SAMPLE RATE (Msps)
50
2247 G15
LTC2246: Typical DNL, 2V Range, 25Msps
1.00 0.75 0.50 0.25 0 -0.25 -0.50 -0.75 -1.00 0 4096 8192 CODE 12288 16384
2246 G02
LTC2246: 8192 Point FFT, fIN = 5MHz, -1dB, 2V Range, 25Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 2 4 6 8 10 FREQUENCY (MHz) 12
2246 G03
LTC2246: 8192 Point FFT, fIN = 70MHz, -1dB, 2V Range, 25Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 2 4 6 8 10 FREQUENCY (MHz) 12
2246 G05
LTC2246: 8192 Point FFT, fIN = 140MHz, -1dB, 2V Range, 25Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 2 4 6 8 10 FREQUENCY (MHz) 12
2246 G06
224876fa
9
LTC2248/LTC2247/LTC2246 TYPICAL PERFOR A CE CHARACTERISTICS
LTC2246: 8192 Point 2-Tone FFT, fIN = 10.9MHz and 13.8MHz, -1dB, 2V Range, 25Msps
0 -10 -20 -30
AMPLITUDE (dB)
-40 -60 -70 -80 -90 -100 -110 -120 0 2 4 6 8 10 FREQUENCY (MHz) 12
2246 G07
COUNT
-50
15000
SNR (dBFS)
LTC2246: SFDR vs Input Frequency, -1dB, 2V Range, 25Msps
100 95 100 110
SNR (dBc AND dBFS)
90
SFDR (dBFS)
SNR AND SFDR (dBFS)
85 80 75 70 65 0 50 150 INPUT FREQUENCY (MHz) 100 200
2246 G10
LTC2246: SFDR vs Input Level, fIN = 5MHz, 2V Range, 25Msps
120 110 100
SFDR (dBc AND dBFS)
dBFS 30 2 dBc 90dBc SFDR REFERENCE LINE 2V RANGE 25 1V RANGE 20
IOVDD (mA) IVDD (mA)
90 80 70 60 50 40 30 20 -60 -50 -40 -30 -20 INPUT LEVEL (dBFS) -10 15 0
2246 G13
10
UW
LTC2246: Grounded Input Histogram, 25Msps
25000 22016 20000 18803 74 75
LTC2246: SNR vs Input Frequency, -1dB, 2V Range, 25Msps
13373
73
10000 6919 5000 43 853 3227 278
72
71
0
70
8179 8180 8181 8182 8183 8184 8185 8186 CODE 2246 G08
0
100 50 150 INPUT FREQUENCY (MHz)
200
2246 G09
LTC2246: SNR and SFDR vs Sample Rate, 2V Range, fIN = 5MHz, -1dB
80 70 SFDR 90 60 50
LTC2246: SNR vs Input Level, fIN = 5MHz, 2V Range, 25Msps
dBFS
dBc 40 30 20 10
80 SNR 70
60 0 10 20 30 40 SAMPLE RATE (Msps) 50
2246 G11
0 -60
-50
-30 -40 -20 INPUT LEVEL (dBFS)
-10
0
2246 G12
LTC2246: IVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB
35 3
LTC2246: IOVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB, OVDD = 1.8V
1
0
5
25 20 15 10 SAMPLE RATE (Msps)
30
35
0
0
5
2246 G14
25 20 15 10 SAMPLE RATE (Msps)
30
35
2246 G15
224876fa
LTC2248/LTC2247/LTC2246
PI FU CTIO S
AIN+ (Pin 1): Positive Differential Analog Input. AIN- (Pin 2): Negative Differential Analog Input. REFH (Pins 3, 4): ADC High Reference. Short together and bypass to pins 5, 6 with a 0.1F ceramic chip capacitor as close to the pin as possible. Also bypass to pins 5, 6 with an additional 2.2F ceramic chip capacitor and to ground with a 1F ceramic chip capacitor. REFL (Pins 5, 6): ADC Low Reference. Short together and bypass to pins 3, 4 with a 0.1F ceramic chip capacitor as close to the pin as possible. Also bypass to pins 3, 4 with an additional 2.2F ceramic chip capacitor and to ground with a 1F ceramic chip capacitor. VDD (Pins 7, 32): 3V Supply. Bypass to GND with 0.1F ceramic chip capacitors. GND (Pin 8): ADC Power Ground. CLK (Pin 9): Clock Input. The input sample starts on the positive edge. SHDN (Pin 10): Shutdown Mode Selection Pin. Connecting SHDN to GND and OE to GND results in normal operation with the outputs enabled. Connecting SHDN to GND and OE to VDD results in normal operation with the outputs at high impedance. Connecting SHDN to VDD and OE to GND results in nap mode with the outputs at high impedance. Connecting SHDN to VDD and OE to VDD results in sleep mode with the outputs at high impedance. OE (Pin 11): Output Enable Pin. Refer to SHDN pin function. D0 - D13 (Pins 12, 13, 14, 15, 16, 17, 18, 19, 22, 23, 24, 25, 26, 27): Digital Outputs. D13 is the MSB. OGND (Pin 20): Output Driver Ground. OVDD (Pin 21): Positive Supply for the Output Drivers. Bypass to ground with 0.1F ceramic chip capacitor. OF (Pin 28): Over/Under Flow Output. High when an over or under flow has occurred. MODE (Pin 29): Output Format and Clock Duty Cycle Stabilizer Selection Pin. Connecting MODE to GND selects offset binary output format and turns the clock duty cycle stabilizer off. 1/3 VDD selects offset binary output format and turns the clock duty cycle stabilizer on. 2/3 VDD selects 2's complement output format and turns the clock duty cycle stabilizer on. VDD selects 2's complement output format and turns the clock duty cycle stabilizer off. SENSE (Pin 30): Reference Programming Pin. Connecting SENSE to VCM selects the internal reference and a 0.5V input range. VDD selects the internal reference and a 1V input range. An external reference greater than 0.5V and less than 1V applied to SENSE selects an input range of VSENSE. 1V is the largest valid input range. VCM (Pin 31): 1.5V Output and Input Common Mode Bias. Bypass to ground with 2.2F ceramic chip capacitor. GND (Exposed Pad) (Pin 33): ADC Power Ground. The exposed pad on the bottom of the package needs to be soldered to ground.
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FUNCTIONAL BLOCK DIAGRA
AIN+ INPUT S/H FIRST PIPELINED ADC STAGE SECOND PIPELINED ADC STAGE THIRD PIPELINED ADC STAGE FOURTH PIPELINED ADC STAGE FIFTH PIPELINED ADC STAGE SIXTH PIPELINED ADC STAGE AIN-
VCM 2.2F
1.5V REFERENCE
RANGE SELECT
REFH SENSE REF BUF
DIFF REF AMP
REFH
0.1F
2.2F 1F 1F
Figure 1. Functional Block Diagram
TI I G DIAGRA
tAP ANALOG INPUT N tH tL CLK tD D0-D13, OF N-5 N-4 N-3 N-2 N-1 N
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SHIFT REGISTER AND CORRECTION REFL INTERNAL CLOCK SIGNALS OVDD OF CLOCK/DUTY CYCLE CONTROL D13 CONTROL LOGIC OUTPUT DRIVERS * * * D0 REFL CLK M0DE SHDN OE
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OGND
Timing Diagram
N+2 N+3
N+4 N+5
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DYNAMIC PERFORMANCE Signal-to-Noise Plus Distortion Ratio The signal-to-noise plus distortion ratio [S/(N + D)] is the ratio between the RMS amplitude of the fundamental input frequency and the RMS amplitude of all other frequency components at the ADC output. The output is band limited to frequencies above DC to below half the sampling frequency. Signal-to-Noise Ratio The signal-to-noise ratio (SNR) is the ratio between the RMS amplitude of the fundamental input frequency and the RMS amplitude of all other frequency components except the first five harmonics and DC. Total Harmonic Distortion 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 = 20Log ((V22 + V32 + V42 + . . . Vn2)/V1) where V1 is the RMS amplitude of the fundamental frequency and V2 through Vn are the amplitudes of the second through nth harmonics. The THD calculated in this data sheet uses all the harmonics up to the fifth. Intermodulation Distortion If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can produce intermodulation distortion (IMD) 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. The 3rd order intermodulation products are 2fa + fb, 2fb + fa, 2fa - fb and 2fb - fa. The intermodulation distortion is defined as the ratio of the RMS value of either Spurious Free Dynamic Range (SFDR) Spurious free dynamic range is the peak harmonic or spurious noise that is the largest spectral component excluding the input signal and DC. This value is expressed in decibels relative to the RMS value of a full scale input signal. Input Bandwidth The input bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full scale input signal. Aperture Delay Time The time from when CLK reaches mid-supply to the instant that the input signal is held by the sample and hold circuit. Aperture Delay Jitter The variation in the aperture delay time from conversion to conversion. This random variation will result in noise when sampling an AC input. The signal to noise ratio due to the jitter alone will be: SNRJITTER = -20log (2 * fIN * tJITTER) CONVERTER OPERATION As shown in Figure 1, the LTC2248/LTC2247/LTC2246 is a CMOS pipelined multistep converter. The converter has six pipelined ADC stages; a sampled analog input will result in a digitized value five cycles later (see the Timing Diagram section). For optimal AC performance the analog inputs should be driven differentially. For cost sensitive applications, the analog inputs can be driven single-ended with slightly worse harmonic distortion. The CLK input is single-ended. The LTC2248/LTC2247/LTC2246 has two phases of operation, determined by the state of the CLK input pin. Each pipelined stage shown in Figure 1 contains an ADC, a reconstruction DAC and an interstage residue amplifier. In operation, the ADC quantizes the input to the stage and the quantized value is subtracted from the input by the
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input tone to the RMS value of the largest 3rd order intermodulation product.
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DAC to produce a residue. The residue is amplified and output by the residue amplifier. Successive stages operate out of phase so that when the odd stages are outputting their residue, the even stages are acquiring that residue and vice versa. When CLK is low, the analog input is sampled differentially directly onto the input sample-and-hold capacitors, inside the "Input S/H" shown in the block diagram. At the instant that CLK transitions from low to high, the sampled input is held. While CLK is high, the held input voltage is buffered by the S/H amplifier which drives the first pipelined ADC stage. The first stage acquires the output of the S/H during this high phase of CLK. When CLK goes back low, the first stage produces its residue which is acquired by the second stage. At the same time, the input S/H goes back to acquiring the analog input. When CLK goes back high, the second stage produces its residue which is acquired by the third stage. An identical process is repeated for the third, fourth and fifth stages, resulting in a fifth stage residue that is sent to the sixth stage ADC for final evaluation. Each ADC stage following the first has additional range to accommodate flash and amplifier offset errors. Results from all of the ADC stages are digitally synchronized such that the results can be properly combined in the correction logic before being sent to the output buffer. SAMPLE/HOLD OPERATION AND INPUT DRIVE Sample/Hold Operation Figure 2 shows an equivalent circuit for the LTC2248/ LTC2247/LTC2246 CMOS differential sample-and-hold. The analog inputs are connected to the sampling capacitors (CSAMPLE) through NMOS transistors. The capacitors shown attached to each input (CPARASITIC) are the summation of all other capacitance associated with each input. During the sample phase when CLK is low, the transistors connect the analog inputs to the sampling capacitors and they charge to and track the differential input voltage. When CLK transitions from low to high, the sampled input voltage is held on the sampling capacitors. During the hold phase when CLK is high, the sampling capacitors are
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disconnected from the input and the held voltage is passed to the ADC core for processing. As CLK transitions from high to low, the inputs are reconnected to the sampling capacitors to acquire a new sample. Since the sampling capacitors still hold the previous sample, a charging glitch proportional to the change in voltage between samples will be seen at this time. If the change between the last sample and the new sample is small, the charging glitch seen at the input will be small. If the input change is large, such as the change seen with input frequencies near Nyquist, then a larger charging glitch will be seen. Single-Ended Input For cost sensitive applications, the analog inputs can be driven single-ended. With a single-ended input the harmonic distortion and INL will degrade, but the SNR and DNL will remain unchanged. For a single-ended input, AIN+ should be driven with the input signal and AIN- should be connected to 1.5V or VCM. Common Mode Bias For optimal performance the analog inputs should be driven differentially. Each input should swing 0.5V for the 2V range or 0.25V for the 1V range, around a common mode voltage of 1.5V. The VCM output pin (Pin 31) may be used to provide the common mode bias level. VCM can be tied directly to the center tap of a transformer to set the DC input level or as a reference level to an op amp differential driver circuit. The VCM pin must be bypassed to ground close to the ADC with a 2.2F or greater capacitor.
LTC2248/47/46 VDD 15 AIN+ VDD 15 CPARASITIC 1pF VDD CLK CPARASITIC 1pF CSAMPLE 4pF CSAMPLE 4pF AIN-
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Figure 2. Equivalent Input Circuit
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Input Drive Impedance As with all high performance, high speed ADCs, the dynamic performance of the LTC2248/LTC2247/LTC2246 can be influenced by the input drive circuitry, particularly the second and third harmonics. Source impedance and reactance can influence SFDR. At the falling edge of CLK, the sample-and-hold circuit will connect the 4pF sampling capacitor to the input pin and start the sampling period. The sampling period ends when CLK rises, holding the sampled input on the sampling capacitor. Ideally the input circuitry should be fast enough to fully charge the sampling capacitor during the sampling period 1/(2FENCODE); however, this is not always possible and the incomplete settling may degrade the SFDR. The sampling glitch has been designed to be as linear as possible to minimize the effects of incomplete settling. For the best performance, it is recommended to have a source impedance of 100 or less for each input. The source impedance should be matched for the differential inputs. Poor matching will result in higher even order harmonics, especially the second. Input Drive Circuits Figure 3 shows the LTC2248/LTC2247/LTC2246 being driven by an RF transformer with a center tapped secondary. The secondary center tap is DC biased with VCM, setting the ADC input signal at its optimum DC level. Terminating on the transformer secondary is desirable, as this provides a common mode path for charging glitches caused by the sample and hold. Figure 3 shows a 1:1 turns ratio transformer. Other turns ratios can be used if the source impedance seen by the ADC does not exceed 100 for each ADC input. A disadvantage of using a transformer is the loss of low frequency response. Most small RF transformers have poor performance at frequencies below 1MHz. Figure 4 demonstrates the use of a differential amplifier to convert a single ended input signal into a differential input signal. The advantage of this method is that it provides low frequency input response; however, the limited gain bandwidth of most op amps will limit the SFDR at high input frequencies.
1k 0.1F ANALOG INPUT 1k 25
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VCM 2.2F 0.1F ANALOG INPUT T1 1:1 25 25 T1 = MA/COM ETC1-1T 25 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE AIN-
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25 0.1F
AIN+
LTC2248/47/46
12pF
Figure 3. Single-Ended to Differential Conversion Using a Transformer
VCM HIGH SPEED DIFFERENTIAL 25 AMPLIFIER ANALOG INPUT 2.2F AIN+ LTC2248/47/46
+
CM
+
12pF
-
-
25 AIN-
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Figure 4. Differential Drive with an Amplifier
VCM 2.2F AIN+
LTC2248/47/46
12pF 25 0.1F AIN-
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Figure 5. Single-Ended Drive
Figure 5 shows a single-ended input circuit. The impedance seen by the analog inputs should be matched. This circuit is not recommended if low distortion is required. The 25 resistors and 12pF capacitor on the analog inputs serve two purposes: isolating the drive circuitry from the sample-and-hold charging glitches and limiting the wideband noise at the converter input.
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For input frequencies above 70MHz, the input circuits of Figure 6, 7 and 8 are recommended. The balun transformer gives better high frequency response than a flux coupled center tapped transformer. The coupling capacitors allow the analog inputs to be DC biased at 1.5V. In Figure 8, the series inductors are impedance matching elements that maximize the ADC bandwidth.
VCM 2.2F 0.1F ANALOG INPUT 25 T1 0.1F 25 12 AIN-
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AIN+ LTC2248/47/46 8pF
T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
Figure 6. Recommended Front End Circuit for Input Frequencies Between 70MHz and 170MHz
VCM 2.2F 0.1F ANALOG INPUT 25 T1 0.1F 25 T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE AIN-
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AIN+ 0.1F
LTC2248/47/46
Figure 7. Recommended Front End Circuit for Input Frequencies Between 170MHz and 300MHz
VCM 2.2F 0.1F ANALOG INPUT 25 T1 0.1F 25 6.8nH
-
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AIN+
LTC2248/47/46
AIN T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS, INDUCTORS ARE 0402 PACKAGE SIZE
Figure 8. Recommended Front End Circuit for Input Frequencies Above 300MHz
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Reference Operation Figure 9 shows the LTC2248/LTC2247/LTC2246 reference circuitry consisting of a 1.5V bandgap reference, a difference amplifier and switching and control circuit. The internal voltage reference can be configured for two pin selectable input ranges of 2V (1V differential) or 1V (0.5V differential). Tying the SENSE pin to VDD selects the 2V range; tying the SENSE pin to VCM selects the 1V range. The 1.5V bandgap reference serves two functions: its output provides a DC bias point for setting the common mode voltage of any external input circuitry; additionally, the reference is used with a difference amplifier to generate the differential reference levels needed by the internal ADC circuitry. An external bypass capacitor is required for the 1.5V reference output, VCM. This provides a high frequency low impedance path to ground for internal and external circuitry. The difference amplifier generates the high and low reference for the ADC. High speed switching circuits are connected to these outputs and they must be externally bypassed. Each output has two pins. The multiple output
LTC2248/47/46 1.5V VCM 2.2F 1V RANGE DETECT AND CONTROL SENSE BUFFER INTERNAL ADC HIGH REFERENCE REFH 0.5V 4 1.5V BANDGAP REFERENCE TIE TO VDD FOR 2V RANGE; TIE TO VCM FOR 1V RANGE; RANGE = 2 * VSENSE FOR 0.5V < VSENSE < VCM * 1.1 1.5 1F 2.2F 0.1F DIFF AMP 1F REFL INTERNAL ADC LOW REFERENCE
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Figure 9. Equivalent Reference Circuit
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APPLICATIO S I FOR ATIO
pins are needed to reduce package inductance. Bypass capacitors must be connected as shown in Figure 9. Other voltage ranges in-between the pin selectable ranges can be programmed with two external resistors as shown in Figure 10. An external reference can be used by applying its output directly or through a resistor divider to SENSE. It is not recommended to drive the SENSE pin with a logic device. The SENSE pin should be tied to the appropriate level as close to the converter as possible. If the SENSE pin is driven externally, it should be bypassed to ground as close to the device as possible with a 1F ceramic capacitor. Input Range The input range can be set based on the application. The 2V input range will provide the best signal-to-noise performance while maintaining excellent SFDR. The 1V input range will have better SFDR performance, but the SNR will degrade by 5.8dB. See the Typical Performance Characteristics section. Driving the Clock Input The CLK input can be driven directly with a CMOS or TTL level signal. A sinusoidal clock can also be used along with
1.5V VCM 2.2F 12k 0.75V 12k SENSE 1F LTC2248/47/46 CLK 100 LTC2248/ LTC2247/ LTC2246 4.7F FERRITE BEAD 0.1F
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Figure 10. 1.5V Range ADC
4.7F FERRITE BEAD 0.1F SINUSOIDAL CLOCK INPUT 0.1F 1k CLK 50 1k NC7SVU04
CLEAN SUPPLY
LTC2248/47/46
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Figure 11. Sinusoidal Single-Ended CLK Drive
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a low-jitter squaring circuit before the CLK pin (see Figure 11). The noise performance of the LTC2248/LTC2247/LTC2246 can depend on the clock signal quality as much as on the analog input. Any noise present on the clock signal will result in additional aperture jitter that will be RMS summed with the inherent ADC aperture jitter. In applications where jitter is critical, such as when digitizing high input frequencies, use as large an amplitude as possible. Also, if the ADC is clocked with a sinusoidal signal, filter the CLK signal to reduce wideband noise and distortion products generated by the source. Figures 12 and 13 show alternatives for converting a differential clock to the single-ended CLK input. The use of a transformer provides no incremental contribution to phase noise. The LVDS or PECL to CMOS translators provide little degradation below 70MHz, but at 140MHz will degrade the SNR compared to the transformer solution. The nature of the received signals also has a large
CLEAN SUPPLY
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IF LVDS USE FIN1002 OR FIN1018. FOR PECL, USE AZ1000ELT21 OR SIMILAR
Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter
ETC1-1T 5pF-30pF DIFFERENTIAL CLOCK INPUT
CLK
LTC2248/ LTC2247/ LTC2246
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FERRITE BEAD VCM
Figure 13. LVDS or PECL CLK Drive Using a Transformer
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bearing on how much SNR degradation will be experienced. For high crest factor signals such as WCDMA or OFDM, where the nominal power level must be at least 6dB to 8dB below full scale, the use of these translators will have a lesser impact. The transformer in the example may be terminated with the appropriate termination for the signaling in use. The use of a transformer with a 1:4 impedance ratio may be desirable in cases where lower voltage differential signals are considered. The center tap may be bypassed to ground through a capacitor close to the ADC if the differential signals originate on a different plane. The use of a capacitor at the input may result in peaking, and depending on transmission line length may require a 10 to 20 ohm series resistor to act as both a low pass filter for high frequency noise that may be induced into the clock line by neighboring digital signals, as well as a damping mechanism for reflections. Maximum and Minimum Conversion Rates The maximum conversion rate for the LTC2248/LTC2247/ LTC2246 is 65Msps (LTC2248), 40Msps (LTC2247), and 25Msps (LTC2246). For the ADC to operate properly, the CLK signal should have a 50% (5%) duty cycle. Each half cycle must have at least 7.3ns (LTC2248), 11.8ns (LTC2247), and 18.9ns (LTC2246) for the ADC internal circuitry to have enough settling time for proper operation. An optional clock duty cycle stabilizer circuit can be used if the input clock has a non 50% duty cycle. This circuit uses the rising edge of the CLK pin to sample the analog input. The falling edge of CLK is ignored and the internal falling edge is generated by a phase-locked loop. The input clock duty cycle can vary from 40% to 60% and the clock duty cycle stabilizer will maintain a constant 50% internal duty cycle. If the clock is turned off for a long period of time, the duty cycle stabilizer circuit will require a hundred clock cycles for the PLL to lock onto the input clock. To use the clock duty cycle stabilizer, the MODE pin should be connected to 1/3VDD or 2/3VDD using external resistors. The lower limit of the LTC2248/LTC2247/LTC2246 sample rate is determined by droop of the sample-and-hold circuits. The pipelined architecture of this ADC relies on
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storing analog signals on small valued capacitors. Junction leakage will discharge the capacitors. The specified minimum operating frequency for the LTC2248/LTC2247/ LTC2246 is 1Msps. DIGITAL OUTPUTS Table 1 shows the relationship between the analog input voltage, the digital data bits, and the overflow bit.
Table 1. Output Codes vs Input Voltage
AIN+ - AIN- (2V Range) >+1.000000V +0.999878V +0.999756V +0.000122V 0.000000V -0.000122V -0.000244V -0.999878V -1.000000V <-1.000000V OF 1 0 0 0 0 0 0 0 0 1 D13 - D0 (Offset Binary) 11 1111 1111 1111 11 1111 1111 1111 11 1111 1111 1110 10 0000 0000 0001 10 0000 0000 0000 01 1111 1111 1111 01 1111 1111 1110 00 0000 0000 0001 00 0000 0000 0000 00 0000 0000 0000 D13 - D0 (2's Complement) 01 1111 1111 1111 01 1111 1111 1111 01 1111 1111 1110 00 0000 0000 0001 00 0000 0000 0000 11 1111 1111 1111 11 1111 1111 1110 10 0000 0000 0001 10 0000 0000 0000 10 0000 0000 0000
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Digital Output Buffers Figure 14 shows an equivalent circuit for a single output buffer. Each buffer is powered by OVDD and OGND, isolated from the ADC power and ground. The additional N-channel transistor in the output driver allows operation down to low voltages. The internal resistor in series with the output makes the output appear as 50 to external circuitry and may eliminate the need for external damping resistors.
LTC2248/47/46 OVDD VDD VDD 0.5V TO 3.6V 0.1F OVDD DATA FROM LATCH OE OGND PREDRIVER LOGIC 43 TYPICAL DATA OUTPUT
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Figure 14. Digital Output Buffer
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As with all high speed/high resolution converters, the digital output loading can affect the performance. The digital outputs of the LTC2248/LTC2247/LTC2246 should drive a minimal capacitive load to avoid possible interaction between the digital outputs and sensitive input circuitry. The output should be buffered with a device such as an ALVCH16373 CMOS latch. For full speed operation the capacitive load should be kept under 10pF. Lower OVDD voltages will also help reduce interference from the digital outputs. Data Format Using the MODE pin, the LTC2248/LTC2247/LTC2246 parallel digital output can be selected for offset binary or 2's complement format. Connecting MODE to GND or 1/3VDD selects offset binary output format. Connecting MODE to 2/3VDD or VDD selects 2's complement output format. An external resistor divider can be used to set the 1/3VDD or 2/3VDD logic values. Table 2 shows the logic states for the MODE pin.
Table 2. MODE Pin Function
MODE Pin 0 1/3VDD 2/3VDD VDD Output Format Offset Binary Offset Binary 2's Complement 2's Complement Clock Duty Cycle Stablizer Off On On Off
Overflow Bit When OF outputs a logic high the converter is either overranged or underranged.
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Output Driver Power Separate output power and ground pins allow the output drivers to be isolated from the analog circuitry. The power supply for the digital output buffers, OVDD, should be tied to the same power supply as for the logic being driven. For example if the converter is driving a DSP powered by a 1.8V supply, then OVDD should be tied to that same 1.8V supply. OVDD can be powered with any voltage from 500mV up to 3.6V. OGND can be powered with any voltage from GND up to 1V and must be less than OVDD. The logic outputs will swing between OGND and OVDD. Output Enable The outputs may be disabled with the output enable pin, OE. OE high disables all data outputs including OF. The data access and bus relinquish times are too slow to allow the outputs to be enabled and disabled during full speed operation. The output Hi-Z state is intended for use during long periods of inactivity. Sleep and Nap Modes The converter may be placed in shutdown or nap modes to conserve power. Connecting SHDN to GND results in normal operation. Connecting SHDN to VDD and OE to VDD results in sleep mode, which powers down all circuitry including the reference and typically dissipates 1mW. When exiting sleep mode it will take milliseconds for the output data to become valid because the reference capacitors have to recharge and stabilize. Connecting SHDN to VDD and OE to GND results in nap mode, which typically dissipates 15mW. In nap mode, the on-chip reference circuit is kept on, so that recovery from nap mode is faster than that from sleep mode, typically taking 100 clock cycles. In both sleep and nap modes, all digital outputs are disabled and enter the Hi-Z state.
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Grounding and Bypassing The LTC2248/LTC2247/LTC2246 requires a printed circuit board with a clean, unbroken ground plane. A multilayer board with an internal ground plane is recommended. Layout for the printed circuit board should ensure that digital and analog signal lines are separated as much as possible. In particular, care should be taken not to run any digital track alongside an analog signal track or underneath the ADC. High quality ceramic bypass capacitors should be used at the VDD, OVDD, VCM, REFH, and REFL pins. Bypass capacitors must be located as close to the pins as possible. Of particular importance is the 0.1F capacitor between REFH and REFL. This capacitor should be placed as close to the device as possible (1.5mm or less). A size 0402 ceramic capacitor is recommended. The large 2.2F capacitor between REFH and REFL can be somewhat further away. The traces connecting the pins and bypass capacitors must be kept short and should be made as wide as possible. The LTC2248/LTC2247/LTC2246 differential inputs should run parallel and close to each other. The input traces should be as short as possible to minimize capacitance and to minimize noise pickup. Heat Transfer Most of the heat generated by the LTC2248/LTC2247/ LTC2246 is transferred from the die through the bottomside exposed pad and package leads onto the printed circuit board. For good electrical and thermal performance, the exposed pad should be soldered to a large grounded pad on the PC board. It is critical that all ground pins are connected to a ground plane of sufficient area. Clock Sources for Undersampling Undersampling raises the bar on the clock source and the higher the input frequency, the greater the sensitivity to clock jitter or phase noise. A clock source that degrades SNR of a full-scale signal by 1dB at 70MHz will degrade SNR by 3dB at 140MHz, and 4.5dB at 190MHz. In cases where absolute clock frequency accuracy is relatively unimportant and only a single ADC is required,
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a 3V canned oscillator from vendors such as Saronix or Vectron can be placed close to the ADC and simply connected directly to the ADC. If there is any distance to the ADC, some source termination to reduce ringing that may occur even over a fraction of an inch is advisable. You must not allow the clock to overshoot the supplies or performance will suffer. Do not filter the clock signal with a narrow band filter unless you have a sinusoidal clock source, as the rise and fall time artifacts present in typical digital clock signals will be translated into phase noise. The lowest phase noise oscillators have single-ended sinusoidal outputs, and for these devices the use of a filter close to the ADC may be beneficial. This filter should be close to the ADC to both reduce roundtrip reflection times, as well as reduce the susceptibility of the traces between the filter and the ADC. If you are sensitive to close-in phase noise, the power supply for oscillators and any buffers must be very stable, or propagation delay variation with supply will translate into phase noise. Even though these clock sources may be regarded as digital devices, do not operate them on a digital supply. If your clock is also used to drive digital devices such as an FPGA, you should locate the oscillator, and any clock fan-out devices close to the ADC, and give the routing to the ADC precedence. The clock signals to the FPGA should have series termination at the driver to prevent high frequency noise from the FPGA disturbing the substrate of the clock fan-out device. If you use an FPGA as a programmable divider, you must re-time the signal using the original oscillator, and the retiming flip-flop as well as the oscillator should be close to the ADC, and powered with a very quiet supply. For cases where there are multiple ADCs, or where the clock source originates some distance away, differential clock distribution is advisable. This is advisable both from the perspective of EMI, but also to avoid receiving noise from digital sources both radiated, as well as propagated in the waveguides that exist between the layers of multilayer PCBs. The differential pairs must be close together and distanced from other signals. The differential pair should be guarded on both sides with copper distanced at least 3x the distance between the traces, and grounded with vias no more than 1/4 inch apart.
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Evaluation Circuit Schematic of the LTC2248/LTC2247/LTC2246
VCC VCC
J1 ANALOG INPUT R2 12.4 74VCX16373MTD 34 45 39 42 25 48 C4 0.1F 1 OE1 VCC RN1D 33 RN1C 33 RN1B 33 RN1A 33 RN2D 33 RN2C 33 RN2B 33 RN2A 33 RN3D 33 RN3C 33 RN3B 33 RN3A 33 RN4D 33 RN4C 33 RN4B 33 RN4A 33 C17 0.1F VCC R11 10k 8 7 NC7SV86P5X 1 2 4 6 C26 10F 6.3V C20 0.1F R14 1k 3 5 7 GND 8 1/3VDD 2/3VDD R15 1k C19 0.1F R16 1k VDD VCC LT1763 R17 105k R18 100k VDD VDD E3 GND C27 0.01F C28 1F E2 VDD 3V 1 8 IN OUT 2 7 ADJ GND 3 6 GND GND 4 5 BYP SHDN C18 0.1F 6 5 R12 10k R13 10k 7 12 13 14 15 16 17 18 19 22 23 24 25 26 27 28 21 20 VCC C16 0.1F LTC2248/LTC2247/ LTC2246 1 2 3 REFH REFH REFL D4 D5 D6 D7 D8 D9 D10 D11 D12 32 VDD D13 OF OVDD OGND GND 33 VCM SENSE MODE C14 0.1F VCM 31 C15 2.2F 30 29 REFL VDD GND CLK SHDN OE D3 D2 C7 2.2F C8 0.1F 5 6 C9 1F VDD C11 0.1F 8 9 10 VDD VDD 11 GND JP2 OE GND VDD JP1 SHDN VDD 7 4 AIN- D1 AIN+ D0 R6 12.4 24 OE2 GND 4 LE1 GND 10 LE2 VCC 18 VCC GND 15 GND GND 21 GND VCC 31 C2 8.2pF GND GND 28 R3 24.9 R4 24.9 T1 ETC1-1T 5 1 2 4
*
R5 50
*3
3201S-40G1 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2
APPLICATIO S I FOR ATIO
L1 BEAD
C5 4.7F 6.3V C6 1F
R7 1k
C10 0.1F
J3 CLOCK INPUT
C12 0.1F
NC7SVU04
R8 49.9 C13 0.1F VDD
R9 1k
47 I0 46 I1 44 I2 43 I3 41 I4 40 I5 38 I6 37 I7 36 I8 35 I9 33 I10 32 I11 30 I12 29 I13 27 I14 26 I15 2 O0 3 O1 5 O2 6 O3 8 O4 9 O5 11 O6 12 O7 13 O8 14 O9 16 O10 17 O11 19 O12 20 O13 22 O14 23 O15
NC7SVU04 R10 33
JP3 SENSE JP4 MODE VDD
39 39 37 37 35 35 33 33 31 31 29 29 27 27 25 25 23 23 21 21 19 19 17 17 15 15 13 13 11 11 9 9 7 7 5 5 3 3 1 1
VDD
1
VDD
2
VCM
3
VCM
4
24LC025 1 VCC A0 2 WP A1 3 A2 SCL 4 A3 SDA
E1 EXT REF
EXT 5 REF 6
VCC
C21 0.1F
C22 0.1F
C23 0.1F
C24 0.1F
22554 TA02
C25 4.7F E4 PWR GND
U
VDD
W
C3 0.1F VCM
UU
R1 OPT
C1 0.1F
LTC2248/LTC2247/LTC2246
21
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LTC2248/LTC2247/LTC2246
APPLICATIO S I FOR ATIO
Silkscreen Top
Inner Layer 2 GND
22
U
Topside Inner Layer 3 Power
224876fa
W
U
U
LTC2248/LTC2247/LTC2246
APPLICATIO S I FOR ATIO
Bottomside
PACKAGE DESCRIPTIO
UH Package 32-Lead Plastic QFN (5mm x 5mm)
(Reference LTC DWG # 05-08-1693)
BOTTOM VIEW--EXPOSED PAD 5.00 0.10 (4 SIDES) 0.70 0.05 PIN 1 TOP MARK (NOTE 6) 0.75 0.05 0.00 - 0.05 R = 0.115 TYP 31 32 0.23 TYP (4 SIDES)
5.50 0.05 4.10 0.05 3.45 0.05 (4 SIDES) 3.45 0.10 (4-SIDES)
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD LAYOUT 0.200 REF NOTE: 1. DRAWING PROPOSED TO BE A JEDEC PACKAGE OUTLINE M0-220 VARIATION WHHD-(X) (TO BE APPROVED) 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.20mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE
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.
U
Silkscreen Bottom
1 2 0.40 0.10 0.25 0.05 0.50 BSC
(UH) QFN 0603
U
W
U
U
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LTC2248/LTC2247/LTC2246
RELATED PARTS
PART NUMBER LTC1748 LTC1750 LT1993-2 LT1994 LTC2202 LTC2208 LTC2220-1 LTC2224 LTC2225 LTC2226 LTC2227 LTC2228 LTC2229 LTC2236 LTC2237 LTC2238 LTC2239 LTC2245 LTC2246 LTC2247 LTC2248 LTC2249 LTC2250 LTC2251 LTC2252 LTC2253 LTC2254 LTC2255 LTC2284 LT5512 LT5514 LT5515 LT5516 LT5517 LT5522 DESCRIPTION 14-Bit, 80Msps, 5V ADC 14-Bit, 80Msps, 5V Wideband ADC High Speed Differential Op Amp Low Noise, Low Distortion Fully Differential Input/Output Amplifier/Driver 16-Bit, 10Msps, 3.3V ADC, Lowest Noise 16-Bit, 130Msps, 3.3V ADC, LVDS Outputs 12-Bit, 185Msps, 3.3V ADC, LVDS Outputs 12-Bit, 135Msps, 3V ADC, High IF Sampling 12-Bit, 10Msps, 3V ADC, Lowest Power 12-Bit, 25Msps, 3V ADC, Lowest Power 12-Bit, 40Msps, 3V ADC, Lowest Power 12-Bit, 65Msps, 3V ADC, Lowest Power 12-Bit, 80Msps, 3V ADC, Lowest Power 10-Bit, 25Msps, 3V ADC, Lowest Power 10-Bit, 40Msps, 3V ADC, Lowest Power 10-Bit, 65Msps, 3V ADC, Lowest Power 10-Bit, 80Msps, 3V ADC, Lowest Power 14-Bit, 10Msps, 3V ADC, Lowest Power 14-Bit, 25Msps, 3V ADC, Lowest Power 14-Bit, 40Msps, 3V ADC, Lowest Power 14-Bit, 65Msps, 3V ADC, Lowest Power 14-Bit, 80Msps, 3V ADC, Lowest Power 10-Bit, 105Msps, 3V ADC, Lowest Power 10-Bit, 125Msps, 3V ADC, Lowest Power 12-Bit, 105Msps, 3V ADC, Lowest Power 12-Bit, 125Msps, 3V ADC, Lowest Power 14-Bit, 105Msps, 3V ADC, Lowest Power 14-Bit, 125Msps, 3V ADC, Lowest Power 14-Bit, Dual, 105Msps, 3V ADC, Low Crosstalk DC-3GHz High Signal Level Downconverting Mixer Ultralow Distortion IF Amplifier/ADC Driver with Digitally Controlled Gain 1.5GHz to 2.5GHz Direct Conversion Quadrature Demodulator 800MHz to 1.5GHz Direct Conversion Quadrature Demodulator 40MHz to 900MHz Direct Conversion Quadrature Demodulator 600MHz to 2.7GHz High Linearity Downconverting Mixer COMMENTS 76.3dB SNR, 90dB SFDR, 48-Pin TSSOP Package Up to 500MHz IF Undersampling, 90dB SFDR 800MHz BW, 70dBc Distortion at 70MHz, 6dB Gain Low Distortion: -94dBc at 1MHz 150mW, 81.6dB SNR, 100dB SFDR, 48-Pin QFN 1250mW, 78dB SNR, 100dB SFDR, 64-Pin QFN 910mW, 67.7dB SNR, 80dB SFDR, 64-Pin QFN 630mW, 67.6dB SNR, 84dB SFDR, 48-Pin QFN 60mW, 71.3dB SNR, 90dB SFDR, 32-Pin QFN 75mW, 71.4dB SNR, 90dB SFDR, 32-Pin QFN 120mW, 71.4dB SNR, 90dB SFDR, 32-Pin QFN 205mW, 71.3dB SNR, 90dB SFDR, 32-Pin QFN 211mW, 70.6dB SNR, 90dB SFDR, 32-Pin QFN 75mW, 61.8dB SNR, 85dB SFDR, 32-Pin QFN 120mW, 61.8dB SNR, 85dB SFDR, 32-Pin QFN 205mW, 61.8dB SNR, 85dB SFDR, 32-Pin QFN 211mW, 61.8dB SNR, 85dB SFDR, 32-Pin QFN 60mW, 74.4dB SNR, 90dB SFDR, 32-Pin QFN 75mW, 74.5dB SNR, 90dB SFDR, 32-Pin QFN 120mW, 74.4dB SNR, 90dB SFDR, 32-Pin QFN 205mW, 74.3dB SNR, 90dB SFDR, 32-Pin QFN 222mW, 73dB SNR, 90dB SFDR, 32-Pin QFN 320mW, 61.6dB SNR, 85dB SFDR, 32-Pin QFN 395mW, 61.6dB SNR, 85dB SFDR, 32-Pin QFN 320mW, 70.2dB SNR, 88dB SFDR, 32-Pin QFN 395mW, 70.2dB SNR, 88dB SFDR, 32-Pin QFN 320mW, 72.4dB SNR, 88dB SFDR, 32-Pin QFN 395mW, 72.5dB SNR, 88dB SFDR, 32-Pin QFN 540mW, 72.4dB SNR, 88dB SFDR, 64-Pin QFN DC to 3GHz, 21dBm IIP3, Integrated LO Buffer 450MHz to 1dB BW, 47dB OIP3, Digital Gain Control 10.5dB to 33ddB in 1.5dB/Step High IIP3: 20dBm at 1.9GHz, Integrated LO Quadrature Generator High IIP3: 21.5dBm at 900MHz, Integrated LO Quadrature Generator High IIP3: 21dBm at 800MHz, Integrated LO Quadrature Generator 4.5V to 5.25V Supply, 25dBm IIP3 at 900MHz, NF = 12.5dB, 50 Single-Ended RF and LO Ports
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0106 REV A * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2004

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