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LTC2281 Dual 10-Bit, 125Msps Low Power 3V ADC FEATURES
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DESCRIPTION
The LTC(R)2281 is a 10-bit 125Msps, low power dual 3V A/D converter designed for digitizing high frequency, wide dynamic range signals. The LTC2281 is perfect for demanding imaging and communications applications with AC performance that includes 61.6dB SNR and 82dB SFDR for signals at the Nyquist frequency. Typical DC specs include 0.1LSB INL, 0.1LSB DNL. The transition noise is a low 0.08LSBRMS. 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. A data ready output clock (CLKOUT) can be used to latch the output data.
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
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Integrated Dual 10-Bit ADCs Sample Rate: 125Msps Single 3V Supply (2.85V to 3.4V) Low Power: 790mW 61.6dB SNR, 88dB SFDR 110dB Channel Isolation at 100MHz Flexible Input: 1VP-P to 2VP-P Range 640MHz Full Power Bandwidth S/H Clock Duty Cycle Stabilizer Shutdown and Nap Modes Data Ready Output Clock Pin Compatible Family 125Msps: LTC2283 (12-Bit), LTC2281 (10-Bit) 105Msps: LTC2282 (12-Bit), LTC2280 (10-Bit) 80Msps: LTC2294 (12-Bit), LTC2289 (10-Bit) 65Msps: LTC2293 (12-Bit), LTC2288 (10-Bit) 40Msps: LTC2292 (12-Bit), LTC2287 (10-Bit) 64-Pin (9mm x 9mm) QFN Package
APPLICATIONS
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Wireless and Wired Broadband Communication Imaging Systems Spectral Analysis Portable Instrumentation
TYPICAL APPLICATION
+
ANALOG INPUT A INPUT S/H OVDD 10-BIT PIPELINED ADC CORE OUTPUT DRIVERS D9A D0A OGND SNR (dBFS)
* * *
SNR vs Input Frequency, -1dB, 2V Range
65 64 63 62 61 60 59 58
-
CLK A
CLOCK/DUTY CYCLE CONTROL CLOCK/DUTY CYCLE CONTROL
OF MUX
CLK B
CLKOUT
57 56
OVDD
55 0 50 100 150 200 250 300 350 INPUT FREQUENCY (MHz) 2281 TA01b
+
ANALOG INPUT B INPUT S/H
-
10-BIT PIPELINED ADC CORE
OUTPUT DRIVERS
D9B D0B OGND
2281 TA01
* * *
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LTC2281 ABSOLUTE MAXIMUM RATINGS
OVDD = VDD (Notes 1, 2)
PIN CONFIGURATION
TOP VIEW 64 GND 63 VDD 62 SENSEA 61 VCMA 60 MODE 59 SHDNA 58 OEA 57 OF 56 DA9 55 DA8 54 DA7 53 DA6 52 DA5 51 DA4 50 OGND 49 OVDD AINA+ 1 AINA- 2 REFHA 3 REFHA 4 REFLA 5 REFLA 6 VDD 7 CLKA 8 CLKB 9 VDD 10 REFLB 11 REFLB 12 REFHB 13 REFHB 14 AINB- 15 AINB+ 16 65 48 DA3 47 DA2 46 DA1 45 DA0 44 NC 43 NC 42 NC 41 NC 40 CLKOUT 39 DB9 38 DB8 37 DB7 36 DB6 35 DB5 34 DB4 33 DB3
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 LTC2281C ................................................ 0C to 70C LTC2281I.............................................. -40C to 85C Storage Temperature Range................... -65C to 150C
TJMAX = 150C, JA = 20C/W EXPOSED PAD (PIN 65) IS GND AND MUST BE SOLDERED TO PCB
ORDER INFORMATION
LEAD FREE FINISH LTC2281CUP#PBF LTC2281IUP#PBF LEAD BASED FINISH LTC2281CUP LTC2281IUP TAPE AND REEL LTC2281CUP#TRPBF LTC2281IUP#TRPBF TAPE AND REEL LTC2281CUP#TR LTC2281IUP#TR PART MARKING* LTC2281UP LTC2281UP PART MARKING* LTC2281UP LTC2281UP PACKAGE DESCRIPTION 64-Lead (9mm x 9mm) Plastic QFN 64-Lead (9mm x 9mm) Plastic QFN PACKAGE DESCRIPTION 64-Lead (9mm x 9mm) Plastic QFN 64-Lead (9mm x 9mm) Plastic QFN TEMPERATURE RANGE 0C to 70C -40C to 85C TEMPERATURE RANGE 0C to 70C -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
PARAMETER Resolution (No Missing Codes) Integral Linearity Error Differential Linearity Error Offset Error Gain Error Offset Drift Full-Scale Drift Internal Reference External Reference Differential Analog Input (Note 5) Differential Analog Input (Note 6) External Reference CONDITIONS

CONVERTER CHARACTERISTICS
GND 17 VDD 18 SENSEB 19 VCMB 20 MUX 21 SHDNB 22 OEB 23 NC 24 NC 25 NC 26 NC 27 DB0 28 DB1 29 DB2 30 OGND 31 OVDD 32 UP PACKAGE 64-LEAD (9mm 9mm) PLASTIC QFN
MIN 10 -0.7 -0.7 -12 -2.5
TYP 0.1 0.1 2 0.5 10 30 5
MAX 0.7 0.7 12 2.5
UNITS Bits LSB LSB mV %FS V/C ppm/C ppm/C
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LTC2281 CONVERTER CHARACTERISTICS The l denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25C. (Note 4)
CONDITIONS External Reference SENSE = 1V PARAMETER Gain Matching Offset Matching Transition Noise MIN TYP 0.3 2 0.08 MAX UNITS %FS mV LSBRMS
ANALOG INPUT
SYMBOL VIN VIN,CM IIN ISENSE IMODE tAP tJITTER CMRR PARAMETER
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
CONDITIONS 2.85V < VDD < 3.4V (Note 7) Differential Input Drive (Note 7) Single Ended Input Drive (Note 7) 0V < AIN+, AIN- < VDD 0V < SENSEA, SENSEB < 1V 0V < MODE < VDD

MIN 1 0.5 -1 -3 -3
TYP 0.5V to 1V 1.5 1.5
MAX 1.9 2 1 3 3
UNITS V V V A A A ns psRMS dB MHz
Analog Input Range (AIN+ -AIN-) Analog Input Common Mode (AIN+ +AIN-)/2 Analog Input Leakage Current SENSEA, SENSEB Input Leakage MODE Input Leakage Current Sample-and-Hold Acquisition Delay Time Sample-and-Hold Acquisition Delay Time Jitter Analog Input Common Mode Rejection Ratio Full Power Bandwidth
0 0.2 80 Figure 8 Test Circuit 640
DYNAMIC ACCURACY
SYMBOL SNR PARAMETER Signal-to-Noise Ratio
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. AIN = -1dBFS. (Note 4)
CONDITIONS 5MHz Input 30MHz Input 70MHz Input 140MHz Input SFDR Spurious Free Dynamic Range 2nd or 3rd Harmonic 5MHz Input 30MHz Input 70MHz Input 140MHz Input SFDR Spurious Free Dynamic Range 4th Harmonic or Higher 5MHz Input 30MHz Input 70MHz Input 140MHz Input S/(N+D) Signal-to-Noise Plus Distortion Ratio 5MHz Input 30MHz Input 70MHz Input 140MHz Input IMD Intermodulation Distortion Crosstalk fIN = 40MHz, 41MHz fIN = 100MHz

MIN
TYP 61.6 61.6
MAX
UNITS dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB dB
60
61.5 61.4 85 85
69
82 77 85 85
75
85 85 61.5 61.5
60
61.4 61.2 80 -110
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LTC2281 INTERNAL REFERENCE CHARACTERISTICS
PARAMETER VCM Output Voltage VCM Output Tempco VCM Line Regulation VCM Output Resistance 2.85V < VDD < 3.4V CONDITIONS IOUT = 0
(Note 4)
MIN 1.475 TYP 1.500 25 3 4 MAX 1.525 UNITS V ppm/C mV/V
|IOUT| < 1mA
DIGITAL INPUTS AND 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 OE = High (Note 7) VOUT = 0V VOUT = 3V IO = -10A IO = -200A IO = 10A IO = 1.6mA PARAMETER High Level Input Voltage Low Level Input Voltage Input Current Input Capacitance CONDITIONS VDD = 3V VDD = 3V VIN = 0V to VDD (Note 7) LOGIC INPUTS (CLK, OE, SHDN, MUX)
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
MIN

TYP
MAX
UNITS V
2 0.8 -10 3 10
V A pF
3 50 50

pF mA mA V V 0.4 V V V V V V
2.7
2.995 2.99 0.005 0.09 2.49 0.09 1.79 0.09
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LTC2281
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 8)
SYMBOL VDD OVDD IVDD PDISS PSHDN PNAP PARAMETER Analog Supply Voltage Output Supply Voltage Supply Current Power Dissipation Shutdown Power (Each Channel) Nap Mode Power (Each Channel) CONDITIONS (Note 9) (Note 9) Both ADCs at fS(MAX) Both ADCs at fS(MAX) SHDN = H, OE = H, No CLK SHDN = H, OE = L, No CLK

POWER REQUIREMENTS
MIN 2.85 0.5
TYP 3 3 263 790 2 15
MAX 3.4 3.6 305 915
UNITS V V mA mW mW mW
TIMING CHARACTERISTICS
SYMBOL fs tL tH tAP tD tC tMD PARAMETER Sampling Frequency CLK Low Time CLK High Time Sample-and-Hold Aperture Delay CLK to DATA Delay CLK to CLKOUT Delay DATA to CLKOUT Skew MUX to DATA Delay Data Access Time After OE BUS Relinquish Time Pipeline Latency
The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. (Note 4)
CONDITIONS (Note 9) Duty Cycle Stabilizer Off (Note 7) Duty Cycle Stabilizer On (Note 7) Duty Cycle Stabilizer Off (Note 7) Duty Cycle Stabilizer On (Note 7) CL = 5pF (Note 7) CL = 5pF (Note 7) (tD - tC) (Note 7) CL = 5pF (Note 7) CL = 5pF (Note 7) (Note 7)

MIN 1 3.8 3 3.8 3 1.4 1.4 -0.6 1.4
TYP 4 4 4 4 0 2.7 2.7 0 2.7 4.3 3.3 5
MAX 125 500 500 500 500 5.4 5.4 0.6 5.4 10 8.5
UNITS MHz ns ns ns ns ns ns ns ns ns ns ns Cycles
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 = 125MHz, input range = 2VP-P with differential drive, unless otherwise noted.
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 and 11 1111 1111. Note 7: Guaranteed by design, not subject to test. Note 8: VDD = 3V, fSAMPLE = 125MHz, input range = 1VP-P with differential drive. The supply current and power dissipation are the sum total for both channels with both channels active. Note 9: Recommended operating conditions.
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LTC2281 TYPICAL PERFORMANCE CHARACTERISTICS
Crosstalk vs Input Frequency
-100 -105 -110 -115 -120 -125 -130 0 20 40 60 80 INPUT FREQUENCY (MHz) 100
2281 G01
Typical INL, 2V Range, 125Msps
1.0 0.8 0.6 DNL ERROR (LSB) INL ERROR (LSB) 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 0 256 512 CODE 768 1024
2281 G02
Typical DNL, 2V Range, 125Msps
1.0 0.8 0.6 0.4 0.2 0 -0.2 -0.4 -0.6 -0.8 -1.0 0 256 512 CODE 768 1024
2281 G03
CROSSTALK (dB)
8192 Point FFT, fIN = 5MHz, -1dB, 2V Range, 125Msps
0 -10 -20 -30 AMPLITUDE (dB) AMPLITUDE (dB) -40 -50 -60 -70 -80 -90 -100 -110 -120 0 10 20 30 40 FREQUENCY (MHz) 50 60
2281 G04
8192 Point FFT, fIN = 30MHz, -1dB, 2V Range, 125Msps
0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100 -110 -120 0 10 20 30 40 FREQUENCY (MHz) 50 60
2281 G05
8192 Point FFT, fIN = 70MHz, -1dB, 2V Range, 125Msps
0 -10 -20 -30 AMPLITUDE (dB) -40 -50 -60 -70 -80 -90 -100 -110 -120 0 10 20 30 40 FREQUENCY (MHz) 50 60
2281 G06
8192 Point FFT, fIN = 140MHz, -1dB, 2V Range, 125Msps
0 -10 -20 -30 AMPLITUDE (dB) -40 -50 -60 -70 -80 -90 -100 -110 -120 0 10 20 30 40 FREQUENCY (MHz) 50 60
2281 G07
8192 Point 2-Tone FFT, fIN = 28.2MHz and 26.8MHz, -1dB, 2V Range, 125Msps
0 -10 -20 -30 AMPLITUDE (dB) -40 -60 -70 -80 -90 -100 -110 -120 0 0 10 20 30 40 FREQUENCY (MHz) 50 60
2281 G08
Grounded Input Histogram, 125Msps
70000 60000 50000 COUNT 40000 30000 20000 10000 0 510 511 CODE 0 512
2281 G09
65528
-50
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LTC2281 TYPICAL PERFORMANCE CHARACTERISTICS
SNR vs Input Frequency, -1dB, 2V Range, 125Msps
65 64 63 62 61 60 59 58 57 56 55 0 50 100 150 200 250 300 350 2281 G10 INPUT FREQUENCY (MHz) 65 0 50 100 150 200 250 300 350 2281 G11 INPUT FREQUENCY (MHz) 50 0 20 40 60 80 100 120 140 160 SAMPLE RATE (Msps) 2281 G12 SFDR (dBFS) SNR (dBFS) 90 85 80 75 70 SNR AND SFDR (dBFS) 80 SFDR 95
SFDR vs Input Frequency, -1dB, 2V Range, 125Msps
90
SNR and SFDR vs Sample Rate, 2V Range, fIN = 5MHz, -1dB
70
SNR 60
SNR vs Input Level, fIN = 70MHz, 2V Range, 125Msps
80 70 dBFS SFDR (dBc AND dBFS) SNR (dBc AND dBFS) 60 50 40 dBc 30 20 10 0 -50 -40 -30 -10 -20 INPUT LEVEL (dBFS) 0
2281 G13
SFDR vs Input Level, fIN = 70MHz, 2V Range, 125Msps
100 90 80 70 60 50 40 30 20 10 0 -50 -40 -20 -10 -30 INPUT LEVEL (dBFS) 0
2281 G14
IVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB
290 280 270
dBFS
dBc IVDD (mA)
260 250 240 230 220 210 200 190 0 20 60 80 100 40 SAMPLE RATE (Msps) 120 140
2281 G15
2V RANGE 1V RANGE
IOVDD vs Sample Rate, 5MHz Sine Wave Input, -1dB, 0VDD = 1.8V
16 14 12 SNR (dBFS) 0 20 40 80 100 60 SAMPLE RATE (Msps) 120 140 IOVDD (mA) 10 8 6 4 2 0
2281 G16
SNR vs SENSE, fIN = 5MHz, -1dB
61.8 61.6 61.4 61.2 61.0 60.8 60.6 60.4 60.2 0.4 0.5 0.6 0.7 0.8 0.9 SENSE PIN (V) 1 1.1
2281 G17
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LTC2281 PIN FUNCTIONS
AINA+ (Pin 1): Channel A Positive Differential Analog Input. AINA- (Pin 2): Channel A Negative Differential Analog Input. REFHA (Pins 3, 4): Channel A 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. REFLA (Pins 5, 6): Channel A 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, 10, 18, 63): Analog 3V Supply. Bypass to GND with 0.1F ceramic chip capacitors. CLKA (Pin 8): Channel A Clock Input. The input sample starts on the positive edge. CLKB (Pin 9): Channel B Clock Input. The input sample starts on the positive edge. REFLB (Pins 11, 12): Channel B Low Reference. Short together and bypass to Pins 13, 14 with a 0.1F ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 13, 14 with an additional 2.2F ceramic chip capacitor and to ground with a 1F ceramic chip capacitor. REFHB (Pins 13, 14): Channel B High Reference. Short together and bypass to Pins 11, 12 with a 0.1F ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 11, 12 with an additional 2.2F ceramic chip capacitor and to ground with a 1F ceramic chip capacitor. AINB- (Pin 15): Channel B Negative Differential Analog Input. AINB+ (Pin 16): Channel B Positive Differential Analog Input. GND (Pins 17, 64): ADC Power Ground. SENSEB (Pin 19): Channel B Reference Programming Pin. Connecting SENSEB to VCMB 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 SENSEB selects an input range of VSENSEB. 1V is the largest valid input range. VCMB (Pin 20): Channel B 1.5V Output and Input Common Mode Bias. Bypass to ground with 2.2F ceramic chip capacitor. Do not connect to VCMA. MUX (Pin 21): Digital Output Multiplexer Control. If MUX is High, Channel A comes out on DA0-DA9; Channel B comes out on DB0-DB9. If MUX is Low, the output busses are swapped and Channel A comes out on DB0-DB9; Channel B comes out on DA0-DA9. To multiplex both channels onto a single output bus, connect MUX, CLKA and CLKB together. (This is not recommended at clock frequencies above 80Msps.) SHDNB (Pin 22): Channel B Shutdown Mode Selection Pin. Connecting SHDNB to GND and OEB to GND results in normal operation with the outputs enabled. Connecting SHDNB to GND and OEB to VDD results in normal operation with the outputs at high impedance. Connecting SHDNB to VDD and OEB to GND results in nap mode with the outputs at high impedance. Connecting SHDNB to VDD and OEB to VDD results in sleep mode with the outputs at high impedance. OEB (Pin 23): Channel B Output Enable Pin. Refer to SHDNB pin function. NC (Pins 24 to 27, 41 to 44): Do not connect these pins. DB0 - DB9 (Pins 28 to 30, 33 to 39): Channel B Digital Outputs. DB9 is the MSB. OGND (Pins 31, 50): Output Driver Ground. OVDD (Pins 32, 49): Positive Supply for the Output Drivers. Bypass to ground with 0.1F ceramic chip capacitor. CLKOUT (Pin 40): Data Ready Clock Output. Latch data on the falling edge of CLKOUT. CLKOUT is derived from CLKB. Tie CLKA to CLKB for simultaneous operation. DA0 - DA9 (Pins 45 to 48, 51 to 56): Channel A Digital Outputs. DA9 is the MSB. OF (Pin 57): Overflow/Underflow Output. High when an overflow or underflow has occurred on either channel A or channel B.
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LTC2281 PIN FUNCTIONS
OEA (Pin 58): Channel A Output Enable Pin. Refer to SHDNA pin function. SHDNA (Pin 59): Channel A Shutdown Mode Selection Pin. Connecting SHDNA to GND and OEA to GND results in normal operation with the outputs enabled. Connecting SHDNA to GND and OEA to VDD results in normal operation with the outputs at high impedance. Connecting SHDNA to VDD and OEA to GND results in nap mode with the outputs at high impedance. Connecting SHDNA to VDD and OEA to VDD results in sleep mode with the outputs at high impedance. MODE (Pin 60): Output Format and Clock Duty Cycle Stabilizer Selection Pin. Note that MODE controls both channels. 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. VCMA (Pin 61): Channel A 1.5V Output and Input Common Mode Bias. Bypass to ground with 2.2F ceramic chip capacitor. Do not connect to VCMB. SENSEA (Pin 62): Channel A Reference Programming Pin. Connecting SENSEA to VCMA 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 SENSEA selects an input range of VSENSEA. 1V is the largest valid input range. GND (Exposed Pad) (Pin 65): ADC Power Ground. The Exposed Pad on the bottom of the package needs to be soldered to ground.
FUNCTIONAL BLOCK DIAGRAM
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
SHIFT REGISTER AND CORRECTION
RANGE SELECT
REFH SENSE REF BUF
REFL
INTERNAL CLOCK SIGNALS OVDD OF* D9
DIFF REF AMP
CLOCK/DUTY CYCLE CONTROL
CONTROL LOGIC
OUTPUT DRIVERS
* * * D0 CLKOUT*
REFH
0.1F
REFL CLK MODE SHDN OE
2281 F01
OGND
2.2F *OF AND CLKOUT ARE SHARED BETWEEN BOTH CHANNELS. 1F 1F
Figure 1. Functional Block Diagram (Only One Channel is Shown)
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LTC2281 TIMING DIAGRAMS
Dual Digital Output Bus Timing (Only One Channel is Shown)
tAP ANALOG INPUT N N+1 tH CLKA = CLKB tD D0-D9, OF N-5 tC CLKOUT N-4 N-3 N-2 N-1 N
2281 TD01
N+2 N+3
N+4 N+5
tL
Multiplexed Digital Output Bus Timing
tAPA ANALOG INPUT A A A+1 tAPB ANALOG INPUT B B B+1 tH CLKA = CLKB = MUX tL B+2 B+3 B+4 A+2 A+3 A+4
D0A-D9A
A-5 tD
B-5
A-4
B-4 tMD
A-3
B-3
A-2
B-2
A-1
D0B-D9B
B-5 tC
A-5
B-4
A-4
B-3
A-3
B-2
A-2
B-1
2281 TD02
CLKOUT
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LTC2281 APPLICATIONS INFORMATION
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 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 midsupply 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) Crosstalk Crosstalk is the coupling from one channel (being driven by a full-scale signal) onto the other channel (being driven by a -1dBFS signal). CONVERTER OPERATION As shown in Figure 1, the LTC2281 is a dual 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
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2fa + fb, 2fb + fa, 2fa - fb and 2fb - fa. The intermodulation distortion is defined as the ratio of the RMS value of either input tone to the RMS value of the largest 3rd order intermodulation product. 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.
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
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LTC2281 APPLICATIONS INFORMATION
with slightly worse harmonic distortion. The CLK input is single-ended. The LTC2281 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 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 LTC2281 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 disconnected
LTC2281 VDD AIN+ 15 CPARASITIC 1pF CSAMPLE 3.5pF CPARASITIC 1pF VDD CLK CSAMPLE 3.5pF
VDD 15
AIN-
2281 F02
Figure 2. Equivalent Input Circuit
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12
LTC2281 APPLICATIONS INFORMATION
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 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. Input Drive Impedance As with all high performance, high speed ADCs, the dynamic performance of the LTC2281 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 3.5pF 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 LTC2281 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.
VCM 2.2F 0.1F ANALOG INPUT T1 1:1 25 25 T1 = MA/COM ETC1-1T 25 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 25 0.1F 12pF AIN-
2281 F03
AIN+
LTC2281
Figure 3. Single-Ended to Differential Conversion Using a Transformer
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13
LTC2281 APPLICATIONS INFORMATION
VCM HIGH SPEED DIFFERENTIAL 25 AMPLIFIER ANALOG INPUT 2.2F AIN+ 0.1F LTC2281 ANALOG INPUT 25 12pF T1 0.1F 25 12 12 0.1F 8pF AIN-
2281 F06
VCM 2.2F AIN+ LTC2281
+
CM
+ -
25 AIN-
2281 F04
-
T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE
Figure 4. Differential Drive with an Amplifier Figure 6. Recommended Front End Circuit for Input Frequencies Between 70MHz and 170MHz
VCM 1k 0.1F ANALOG INPUT 1k 25 2.2F AIN+ LTC2281
VCM 2.2F
12pF 25 0.1F AIN-
2281 F05
0.1F ANALOG INPUT 25 T1 0.1F 25 T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 0.1F
AIN+
LTC2281
AIN-
2281 F07
Figure 5. Single-Ended Drive
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. 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. 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.
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 8.2nH AIN-
2281 F08
8.2nH 0.1F
AIN+
LTC2281
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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LTC2281 APPLICATIONS INFORMATION
Reference Operation Figure 9 shows the LTC2281 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.
LTC2281 1.5V VCM 2.2F 1V RANGE DETECT AND CONTROL SENSE BUFFER INTERNAL ADC HIGH REFERENCE REFH
2281 F10
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 pins are needed to reduce package inductance. Bypass capacitors must be connected as shown in Figure 9. Each ADC channel has an independent reference with its own bypass capacitors. The two channels can be used with the same or different input ranges. Other voltage ranges 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. For the best channel matching, connect an external reference to SENSEA and SENSEB.
1.5V VCM 2.2F 12k 0.75V 12k SENSE 1F LTC2281
4W
1.5V BANDGAP REFERENCE 0.5V
TIE TO VDD FOR 2V RANGE; TIE TO VCM FOR 1V RANGE; RANGE = 2 * VSENSE FOR 0.5V < VSENSE < 1V 1F
Figure 10. 1.5V Range ADC
Input Range
DIFF AMP
2.2F
0.1F
1F REFL INTERNAL ADC LOW REFERENCE
2281 F09
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 0.9dB. See the Typical Performance Characteristics section. Driving the Clock Input The CLK inputs can be driven directly with a CMOS or TTL level signal. A sinusoidal clock can also be used along with a low jitter squaring circuit before the CLK pin (Figure 11).
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Figure 9. Equivalent Reference Circuit
15
LTC2281 APPLICATIONS INFORMATION
4.7F FERRITE BEAD 0.1F SINUSOIDAL CLOCK INPUT 0.1F 1k CLK 50 1k NC7SVU04 LTC2281 100 CLEAN SUPPLY 4.7F FERRITE BEAD 0.1F CLEAN SUPPLY
CLK
LTC2281
2281 F11
2281 F12
IF LVDS USE FIN1002 OR FIN1018. FOR PECL, USE AZ1000ELT21 OR SIMILAR
Figure 11. Sinusoidal Single-Ended CLK Drive
Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter
The noise performance of the LTC2281 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. It is recommended that CLKA and CLKB are shorted together and driven by the same clock source. If a small time delay is desired between when the two channels sample the analog inputs, CLKA and CLKB can be driven by two different signals. If this delay exceeds 1ns, the performance of the part may degrade. CLKA and CLKB should not be driven by asynchronous signals. 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 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.
ETC1-1T 5pF-30pF DIFFERENTIAL CLOCK INPUT
CLK
LTC2281
2281 F13
0.1F
FERRITE BEAD VCM
Figure 13. LVDS or PECL CLK Drive Using a Transformer
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 LTC2281 is 125Msps. The lower limit of the LTC2281 sample rate is determined by droop of the sample-and-hold circuits. The pipelined architecture of this ADC relies on storing analog signals on
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LTC2281 APPLICATIONS INFORMATION
small valued capacitors. Junction leakage will discharge the capacitors. The specified minimum operating frequency for the LTC2281 is 1Msps. Clock Duty Cycle Stabilizer An optional clock duty cycle stabilizer circuit ensures high performance even if the input clock has a non 50% duty cycle. Using the clock duty cycle stabilizer is recommended for most applications. To use the clock duty cycle stabilizer, the MODE pin should be connected to 1/3VDD or 2/3VDD using external resistors. 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. For applications where the sample rate needs to be changed quickly, the clock duty cycle stabilizer can be disabled. If the duty cycle stabilizer is disabled, care should be taken to make the sampling clock have a 50% (5%) duty cycle. DIGITAL OUTPUTS Table 1 shows the relationship between the analog input voltage, the digital data bits, and the overflow bit. Note that OF is high when an overflow or underflow has occured on either channel A or channel B.
Table 1. Output Codes vs Input Voltage
AIN+ - AIN- (2V Range) >+1.000000V +0.998047V +0.996094V +0.001953V 0.000000V -0.001953V -0.003906V -0.998047V -1.000000V <-1.000000V OF 1 0 0 0 0 0 0 0 0 1 D9 - D0 (Offset Binary) 11 1111 1111 11 1111 1111 11 1111 1110 10 0000 0001 10 0000 0000 01 1111 1111 01 1111 1110 00 0000 0001 00 0000 0000 00 0000 0000 D9 - D0 (2's Complement) 01 1111 1111 01 1111 1111 01 1111 1110 00 0000 0001 00 0000 0000 11 1111 1111 11 1111 1110 10 0000 0001 10 0000 0000 10 0000 0000
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. As with all high speed/high resolution converters, the digital output loading can affect the performance. The digital outputs of the LTC2281 should drive a minimal capacitive load to avoid possible interaction between the digital outputs and sensitive input circuitry. For full speed operation the capacitive load should be kept under 10pF . Lower OVDD voltages will also help reduce interference from the digital outputs.
LTC2281 OVDD VDD VDD 0.5V TO 3.6V 0.1F OVDD DATA FROM LATCH OE OGND PREDRIVER LOGIC 43 TYPICAL DATA OUTPUT
2281 F14
Figure 14. Digital Output Buffer
Data Format Using the MODE pin, the LTC2281 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.
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LTC2281 APPLICATIONS INFORMATION
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 STABILIZER Off On On Off
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 30mW. 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. Channels A and B have independent SHDN pins (SHDNA, SHDNB). Channel A is controlled by SHDNA and OEA, and Channel B is controlled by SHDNB and OEB. The nap, sleep and output enable modes of the two channels are completely independent, so it is possible to have one channel operating while the other channel is in nap or sleep mode. Digital Output Multiplexer The digital outputs of the LTC2281 can be multiplexed onto a single data bus if the sample rate is 80Msps or less. The MUX pin is a digital input that swaps the two data busses. If MUX is High, Channel A comes out on DA0-DA9; Channel B comes out on DB0-DB9. If MUX is Low, the output busses are swapped and Channel A comes out on DB0-DB9; Channel B comes out on DA0-DA9. To multiplex both channels onto a single output bus, connect MUX, CLKA and CLKB together (see the Timing Diagram for the multiplexed mode). The multiplexed data is available on either data bus--the unused data bus can be disabled with its OE pin. Grounding and Bypassing The LTC2281 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
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Overflow Bit When OF outputs a logic high the converter is either overranged or underranged on channel A or channel B. Note that both channels share a common OF pin, which is not the case for slower pin compatible parts such as the LTC2280 or LTC2289. OF is disabled when channel A is in sleep or nap mode. Output Clock The ADC has a delayed version of the CLKB input available as a digital output, CLKOUT. The falling edge of the CLKOUT pin can be used to latch the digital output data. CLKOUT is disabled when channel B is in sleep or nap mode. 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. Channels A and B have independent output enable pins (OEA, OEB).
18
LTC2281 APPLICATIONS INFORMATION
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 LTC2281 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 LTC2281 is transferred from the die through the bottom-side 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 is especially demanding 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, 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 the circuit is sensitive to closein 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 re-timing 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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LTC2281
*
64 63 62 61 60 59 58 57 56 55 54 53 52 51 50 49 GND VDD SENSEA VCMA MODE SHDNA OEA OF DA9 DA8 DA7 DA6 DA5 DA4 OGND OVDD 5 6 17 16 15 14 2 A2 B2 18 R9 * C9 1F C10 2.2F C13 1F U1 LTC2281 11 OVDD QDVDD 12 13 25 C11 0.1F C8 0.1F VDD 19
APPLICATIONS INFORMATION
17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32
GND VDD SENSEB VCMB MUX SHDNB OEB NC NC NC NC DB0 DB1 DB2 OGND OVDD
20
Evaluation Circuit Schematic of the LTC2281
OVDD J1 EDGE-CON-100 JP1 MODE VDD 1 5 3 5 9 7 C1 0.1F 22 GND GND GND GND 11 22 24 VDD OVDD QDVDD C6 * C4 0.1F OVDD C5 0.1F U9 1 24 23 FXLH42245MPX VCCA VCCB VCCB 21 3 B0 A0 20 4 B1 A1 26 28 30 32 34 36 R5 * R6 24.9 R7 24.9 12 13 25 18 20 16 GND 8 10 C2 2.2F 15 17 19 21 23 25 27 29 31 33 35 1/3VDD 6 7 8 8 10 12 14 7 9 11 13 2/3VDD 4 6 R1 1k R2 1k R3 1k C44 0.1F 2 4 6 1 3 5 VDD 2 3 4 QDVDD OVDD U2 1 24 23 FXLH42245MPX R42 VCCA VCCB VCCB 1k 21 B0 A0 20 B1 A1 19 A2 B2 18 B3 A3 EXPOSED 17 A4 B4 PAD 16 B5 A5 15 B6 A6 14 B7 A7 2 T/R OE 38 40 42 44 46 48 50 52 54 56 58 60 62 64 66 68 70 72 74 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 VDD C18 1F C20 2.2F C23 1F 3 4 5 6 7 8 9 10 C28 2.2F 22 VDD R39 1k OVDD VDD T2 * VCMB 1 2 R20 24.9 JP3 SENSEB C31 * VDD VCMB E2 EXT REF B C35 0.1F R25 105k OVDD E3 VDD 3V C38 0.01F C40 0.1F E4 GND QDVDD C41 0.1F C47 0.1F C48 0.1F EXT REF 5 6 OVDD 3 VCM 4 3 4 5 C37 10F 6.3V R25 105k QDVDD 6 7 8 9 10 22 1 2 VDD R22 24.9 R24 * C34 0.1F R18 * C27 0.1F C25 0.1F C21 0.1F B3 A3 EXPOSED 7 A4 B4 PAD 8 B5 A5 9 B6 A6 10 B7 A7 22 T/R OE GND GND GND GND 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 AINA+ AINA- REFHA REFHA REFLA REFLA VDD CLKA CLKB VDD REFLB REFLB REFHB REFHB AINB- AINB+ DA3 DA2 DA1 DA0 NC NC NC NC CLKOUT DB9 DB8 DB7 DB6 DB5 DB4 DB3 48 47 46 45 44 43 42 41 40 39 38 37 36 35 34 33 U10 1 24 23 FXLH42245MPX VCCA VCCB VCCB 21 B0 A0 20 B1 A1 19 A2 B2 18 B3 A3 EXPOSED 17 A4 B4 PAD 16 B5 A5 15 B6 A6 14 B7 A7 2 T/R OE GND GND GND GND 11 12 OVDD 13 25 QDVDD 76 78 80 82 75 77 79 81
JP2 SENSEA
VDD
1
VDD
2
3
VCM
4
E1 EXT REF A
EXT REF 5 6
VCMA
J2 ANALOG R4 INPUT A OPT
C3 0.1F
T1 *
5
1
2
4
*3
C7 0.1F
R8 51
L1 BEAD
C12 4.7F 6.3V
VCMA
VDD
C14 0.1F
R10 1k
C15 0.1F
J3 CLOCK INPUT
C19 0.1F
5
2
4
C17 0.1F
R14 49.9
R15 3 1k U3 NC7SVU04
R32 OPT
J4 R17 ANALOG OPT INPUT B
C29 0.1F
4
5
*
R23 51
*3
VSS SCL 92 94 96 98 100 91 93 95 97 99 ENABLE QDVDD 5 4 11 12 13 25 U4 NC7SV86P5X 3 R34 4.7k VSS VCCIN 1 2 C24 0.1F R35 100k C46 0.1F R36 4.99k SDA VCCIN
C33 0.1F
84 86 88 90
83 85 87 89
VCMB
R33 QDVDD 4.7k
VDD
+
E5 PWR GND
C45 100F 6.3V OPT
C36 4.7F
U12 VDD LT1761ES5-BYP 1 5 OUT IN 3 C39 BYP 1F 4 ADJ GND 2
U11 1 24 23 FXLH42245MPX VCCA VCCB VCCB 21 B0 A0 20 B1 A1 19 B2 A2 18 B3 A3 EXPOSED 17 A4 B4 PAD 16 B5 A5 15 B6 A6 14 B7 A7 2 T/R OE GND GND GND GND
C52 0.1F
C53 0.1F
C54 0.1F
C55 0.1F
U13 VDD LT1761ES5-BYP 1 5 OUT IN 3 C51 BYP 1F 4 ADJ GND 2
C49 0.01F
R40 105k
C50 10F 6.3V R41 100k
U5 24LC025
R37 4.99k
SCL 1 A0 2 A1 3 A2 4 A3 8 VCC 7 WP 6 SCL 5 SDA R38 4.99k SDA
2281 AI01
*VERSION TABLE C6, C31 12pF 12pF 12pF 8pF 8pF 8pF T1, T2 MABAES0060 MABAES0060 MABAES0060 MABA-007159-000000 MABA-007159-000000 MABA-007159-000000 INPUT FREQUENCY 1MHz < AIN < 70MHz 1MHz < AIN < 70MHz 1MHz < AIN < 70MHz 70MHz < AIN < 140MHz 70MHz < AIN < 140MHz 70MHz < AIN < 140MHz
ASSEMBLY TYPE DC1098A-A DC1098A-B DC1098A-C DC1098A-D DC1098A-E DC1098A-F
U1 LTC2281IUP LTC2283IUP LTC2285IUP LTC2281IUP LTC2283IUP LTC2285IUP
BITS 10 12 14 10 12 14
Msps 125 125 125 125 125 125
R5, R9, R18, R24 24.9 24.9 24.9 12.4 12.4 12.4
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LTC2281 APPLICATIONS INFORMATION
Silkscreen Top
Top Side
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21
LTC2281 APPLICATIONS INFORMATION
Inner Layer 2 GND Inner Layer 3 Power
Bottom Side
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LTC2281 PACKAGE DESCRIPTION
UP Package 64-Lead Plastic QFN (9mm x 9mm)
(Reference LTC DWG # 05-08-1705)
0.70 0.05
7.15 0.05
7.50 REF 8.10 0.05 9.50 0.05 (4 SIDES)
7.15 0.05
PACKAGE OUTLINE 0.25 0.05 0.50 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS APPLY SOLDER MASK TO AREAS THAT ARE NOT SOLDERED 0.75 0.05 R = 0.115 TYP
9 .00 0.10 (4 SIDES)
R = 0.10 TYP
63 64 0.40 0.10 1 2 PIN 1 CHAMFER C = 0.35
PIN 1 TOP MARK (SEE NOTE 5)
7.15 0.10 7.50 REF (4-SIDES)
7.15 0.10
(UP64) QFN 0406 REV C
0.200 REF 0.00 - 0.05 NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION WNJR-5 2. ALL DIMENSIONS ARE IN MILLIMETERS 3. 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, IF PRESENT 4. EXPOSED PAD SHALL BE SOLDER PLATED 5. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 6. DRAWING NOT TO SCALE
0.25 0.05 0.50 BSC BOTTOM VIEW--EXPOSED PAD
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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.
23
LTC2281 TYPICAL APPLICATION
PART NUMBER LTC1748 LTC1750 LTC1993-2 LTC1994 LTC2208 LTC2220 LTC2224 LTC2242-12 LTC2254 LTC2255 LTC2280 LTC2282 LTC2284 LTC2286 LTC2287 LTC2288 LTC2289 LTC2290 LTC2291 LTC2292 LTC2293 LTC2294 LTC2295 LTC2296 LTC2297 LTC2298 LTC2299 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, 130Msps, 3.3V ADC, LVDS Outputs 12-Bit, 170Msps, 3.3V ADC, LVDS Outputs 12-Bit, 135Msps, 3.3V ADC, High IF Sampling 12-Bit, 250Msps, 2.5V ADC, LVDS Outputs 14-Bit, 105Msps, 3V ADC, Lowest Power 14-Bit, 125Msps ADC, 3V ADC, Lowest Power 10-Bit, Dual, 105Msps, 3V ADC, Low Crosstalk 12-Bit, Dual, 105Msps, 3V ADC, Low Crosstalk 14-Bit, Dual, 105Msps, 3V ADC, Low Crosstalk 10-Bit, Dual, 25Msps, 3V ADC, Low Crosstalk 10-Bit, Dual, 40Msps, 3V ADC, Low Crosstalk 10-Bit, Dual, 65Msps, 3V ADC, Low Crosstalk 10-Bit, Dual, 80Msps, 3V ADC, Low Crosstalk 12-Bit, Dual, 10Msps, 3V ADC, Low Crosstalk 12-Bit, Dual, 25Msps, 3V ADC, Low Crosstalk 12-Bit, Dual, 40Msps, 3V ADC, Low Crosstalk 12-Bit, Dual, 65Msps, 3V ADC, Low Crosstalk 12-Bit, Dual, 80Msps, 3V ADC, Low Crosstalk 14-Bit, Dual, 10Msps, 3V ADC, Low Crosstalk 14-Bit, Dual, 25Msps, 3V ADC, Low Crosstalk 14-Bit, Dual, 40Msps, 3V ADC, Low Crosstalk 14-Bit, Dual, 65Msps, 3V ADC, Low Crosstalk 14-Bit, Dual, 80Msps, 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: -94dB at 1MHz 1250mW, 77.1dB SNR, 100dB SFDR, 64-Pin QFN Package 890mW, 67.7dB SNR, 84dB SFDR, 64-Pin QFN Package 630mW, 67.6dB SNR, 84dB SFDR, 48-Pin QFN Package 740mW, 65.4dB SNR, 84dB SFDR, 64-Pin QFN Package 320mW, 72.4dB SNR, 88dB SFDR, 32-Pin QFN Package 395mW, 72.5dB SNR, 88dB SFDR, 32-Pin QFN Package 320mW, 61.6dB SNR, 85dB SFDR, 64-Pin QFN Package 540mW, 70.1dB SNR, 88dB SFDR, 64-Pin QFN Package 540mW, 72.4dB SNR, 88dB SFDR, 64-Pin QFN Package 150mW, 61.8dB SNR, 85dB SFDR, 64-Pin QFN Package 235mW, 61.8dB SNR, 85dB SFDR, 64-Pin QFN Package 400mW, 61.8dB SNR, 85dB SFDR, 64-Pin QFN Package 422mW, 61.6dB SNR, 90dB SFDR, 64-Pin QFN Package 120mW, 71.3dB SNR, 90dB SFDR, 64-Pin QFN Package 150mW, 71.4dB SNR, 90dB SFDR, 64-Pin QFN Package 235mW, 71.4dB SNR, 90dB SFDR, 64-Pin QFN Package 400mW, 71.3dB SNR, 90dB SFDR, 64-Pin QFN Package 422mW, 70.6dB SNR, 90dB SFDR, 64-Pin QFN Package 120mW, 74.4dB SNR, 90dB SFDR, 64-Pin QFN Package 150mW, 74.5dB SNR, 90dB SFDR, 64-Pin QFN Package 235mW, 74.4dB SNR, 90dB SFDR, 64-Pin QFN Package 400mW, 74.3dB SNR, 90dB SFDR, 64-Pin QFN Package 444mW, 73dB SNR, 90dB SFDR, 64-Pin QFN Package DC to 3GHz, 21dBm IIP3, Integrated LO Buffer 450MHz to 1dB BW, 47dB OIP3, Digital Gain Control 10.5dB to 33dB 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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24 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 1207 REV B * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2006

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