LTC2281CUP-PBF

LTC2281 Dual 10-Bit, 125Msps Low Power 3V ADC FEATURES n n n n n n n n n n n n DESCRIPTION The LTC®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. n 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 × 9mm) QFN Package APPLICATIONS n n n n 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 • • • 2281fb 1 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 ................................................ 0°C to 70°C LTC2281I.............................................. –40°C to 85°C Storage Temperature Range................... –65°C to 150°C TJMAX = 150°C, θJA = 20°C/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 × 9mm) Plastic QFN 64-Lead (9mm × 9mm) Plastic QFN PACKAGE DESCRIPTION 64-Lead (9mm × 9mm) Plastic QFN 64-Lead (9mm × 9mm) Plastic QFN TEMPERATURE RANGE 0°C to 70°C –40°C to 85°C TEMPERATURE RANGE 0°C to 70°C –40°C to 85°C 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 = 25°C. (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 μV/°C ppm/°C ppm/°C 2281fb 2 LTC2281 CONVERTER CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (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 = 25°C. (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 μA μA μA 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 = 25°C. 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 2281fb 3 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 Ω |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 = –200μA IO = 1.6mA High Level Output Voltage Low Level Output Voltage IO = –200μA 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 = –10μA IO = –200μA IO = 10μA 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 = 25°C. (Note 4) MIN ● ● ● TYP MAX UNITS V 2 0.8 –10 3 10 V μA 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 2281fb 4 LTC2281 The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (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 = 25°C. (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. 2281fb 5 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 2281fb 6 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 2281fb 7 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 0.1μF ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 5, 6 with an additional 2.2μF ceramic chip capacitor and to ground with a 1μF ceramic chip capacitor. REFLA (Pins 5, 6): Channel A Low Reference. Short together and bypass to Pins 3, 4 with a 0.1μF ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 3, 4 with an additional 2.2μF ceramic chip capacitor and to ground with a 1μF ceramic chip capacitor. VDD (Pins 7, 10, 18, 63): Analog 3V Supply. Bypass to GND with 0.1μF 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 0.1μF ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 13, 14 with an additional 2.2μF ceramic chip capacitor and to ground with a 1μF ceramic chip capacitor. REFHB (Pins 13, 14): Channel B High Reference. Short together and bypass to Pins 11, 12 with a 0.1μF ceramic chip capacitor as close to the pin as possible. Also bypass to Pins 11, 12 with an additional 2.2μF ceramic chip capacitor and to ground with a 1μF 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 2.2μF 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 0.1μF 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. 2281fb 8 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 2.2μF 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 2.2μF 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 0.1μF REFL CLK MODE SHDN OE 2281 F01 OGND 2.2μF *OF AND CLKOUT ARE SHARED BETWEEN BOTH CHANNELS. 1μF 1μF Figure 1. Functional Block Diagram (Only One Channel is Shown) 2281fb 9 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 2281fb 10 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 (2π • 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 2281fb 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 11 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+ 15Ω CPARASITIC 1pF CSAMPLE 3.5pF CPARASITIC 1pF VDD CLK CSAMPLE 3.5pF VDD 15Ω AIN– 2281 F02 Figure 2. Equivalent Input Circuit 2281fb 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 2.2μF 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 100Ω 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 100Ω 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 2.2μF 0.1μF ANALOG INPUT T1 1:1 25Ω 25Ω T1 = MA/COM ETC1-1T 25Ω RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 25Ω 0.1μF 12pF AIN– 2281 F03 AIN+ LTC2281 Figure 3. Single-Ended to Differential Conversion Using a Transformer 2281fb 13 LTC2281 APPLICATIONS INFORMATION VCM HIGH SPEED DIFFERENTIAL 25Ω AMPLIFIER ANALOG INPUT 2.2μF AIN+ 0.1μF LTC2281 ANALOG INPUT 25Ω 12pF T1 0.1μF 25Ω 12Ω 12Ω 0.1μF 8pF AIN– 2281 F06 VCM 2.2μF AIN+ LTC2281 + CM + – 25Ω 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 0.1μF ANALOG INPUT 1k 25Ω 2.2μF AIN+ LTC2281 VCM 2.2μF 12pF 25Ω 0.1μF AIN– 2281 F05 0.1μF ANALOG INPUT 25Ω T1 0.1μF 25Ω T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 0.1μF 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 25Ω 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 2.2μF 0.1μF ANALOG INPUT 25Ω T1 0.1μF 25Ω 8.2nH AIN– 2281 F08 8.2nH 0.1μF 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 2281fb 14 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 2.2μF 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 1μF ceramic capacitor. For the best channel matching, connect an external reference to SENSEA and SENSEB. 1.5V VCM 2.2μF 12k 0.75V 12k SENSE 1μF 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 1μF Figure 10. 1.5V Range ADC Input Range DIFF AMP 2.2μF 0.1μF 1μF 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). 2281fb Figure 9. Equivalent Reference Circuit 15 LTC2281 APPLICATIONS INFORMATION 4.7μF FERRITE BEAD 0.1μF SINUSOIDAL CLOCK INPUT 0.1μF 1k CLK 50Ω 1k NC7SVU04 LTC2281 100Ω CLEAN SUPPLY 4.7μF FERRITE BEAD 0.1μF 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 0.1μF 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 10Ω to 20Ω 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 2281fb 16 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