LTC2285IUP#PBF

LTC2285 Dual 14-Bit, 125Msps Low Power 3V ADC DESCRIPTION FEATURES n n n n n n n n n n n n n Integrated Dual 14-Bit ADCs Sample Rate: 125Msps Single 3V Supply (2.85V to 3.4V) Low Power: 790mW 72.4dB 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), LTC2285 (14-Bit) 105Msps: LTC2282 (12-Bit), LTC2284 (14-Bit) 80Msps: LTC2294 (12-Bit), LTC2299 (14-Bit) 65Msps: LTC2293 (12-Bit), LTC2298 (14-Bit) 40Msps: LTC2292 (12-Bit), LTC2297 (14-Bit) 64-Pin (9mm × 9mm) QFN Package The LTC®2285 is a 14-bit 125Msps, low power dual 3V A/D converter designed for digitizing high frequency, wide dynamic range signals. The LTC2285 is perfect for demanding imaging and communications applications with AC performance that includes 72.2dB SNR and 82dB SFDR for signals at the Nyquist frequency. Typical DC specs include ±1.5LSB INL, ±0.6LSB DNL. The transition noise is a low 1.3LSBRMS. 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. APPLICATIONS n n n n Wireless and Wired Broadband Communication Imaging Systems Spectral Analysis Portable Instrumentation TYPICAL APPLICATION + ANALOG INPUT A INPUT S/H – SNR vs Input Frequency, –1dB, 2V Range OVDD 14-BIT PIPELINED ADC CORE OUTPUT DRIVERS D13A 75 •• • 74 D0A 73 OGND CLK A OF CLOCK/DUTY CYCLE CONTROL MUX CLK B CLOCK/DUTY CYCLE CONTROL CLKOUT SNR (dBFS) 72 71 70 69 68 67 66 OVDD + ANALOG INPUT B INPUT S/H – 14-BIT PIPELINED ADC CORE OUTPUT DRIVERS D13B •• • 65 0 50 100 150 200 250 300 350 2285 TA01b INPUT FREQUENCY (MHz) D0B OGND 2285 TA01 2285fb 1 LTC2285 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION OVDD = VDD (Notes 1, 2) 64 GND 63 VDD 62 SENSEA 61 VCMA 60 MODE 59 SHDNA 58 OEA 57 OF 56 DA13 55 DA12 54 DA11 53 DA10 52 DA9 51 DA8 50 OGND 49 OVDD TOP VIEW 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 48 DA7 47 DA6 46 DA5 45 DA4 44 DA3 43 DA2 42 DA1 41 DA0 40 CLKOUT 39 DB13 38 DB12 37 DB11 36 DB10 35 DB9 34 DB8 33 DB7 65 GND 17 VDD 18 SENSEB 19 VCMB 20 MUX 21 SHDNB 22 OEB 23 DB0 24 DB1 25 DB2 26 DB3 27 DB4 28 DB5 29 DB6 30 OGND 31 OVDD 32 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 LTC2285C ................................................ 0°C to 70°C LTC2285I.............................................. –40°C to 85°C Storage Temperature Range................... –65°C to 150°C UP PACKAGE 64-LEAD (9mm × 9mm) PLASTIC QFN TJMAX = 150°C, θJA = 20°C/W EXPOSED PAD (PIN 65) IS GND AND MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2285CUP#PBF LTC2285CUP#TRPBF LTC2285UP 64-Lead (9mm × 9mm) Plastic QFN 0°C to 70°C LTC2285IUP#PBF LTC2285IUP#TRPBF LTC2285UP 64-Lead (9mm × 9mm) Plastic QFN –40°C to 85°C LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2285CUP LTC2285CUP#TR LTC2285UP 64-Lead (9mm × 9mm) Plastic QFN 0°C to 70°C LTC2285IUP LTC2285IUP#TR LTC2285UP 64-Lead (9mm × 9mm) Plastic QFN –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/ CONVERTER CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) PARAMETER CONDITIONS MIN ● Resolution (No Missing Codes) TYP MAX 14 UNITS Bits Integral Linearity Error Differential Analog Input (Note 5) ● Differential Linearity Error Differential Analog Input ● –1 ±0.6 1.1 LSB Offset Error (Note 6) ● –12 ±2 12 mV Gain Error External Reference ● –2.5 ±0.5 2.5 Offset Drift Full-Scale Drift ±1.5 LSB %FS ±10 μV/°C Internal Reference ±30 ppm/°C External Reference ±5 ppm/°C 2285fb 2 LTC2285 CONVERTER CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) PARAMETER CONDITIONS Gain Matching External Reference MIN TYP SENSE = 1V UNITS %FS ±2 mV 1.3 LSBRMS Offset Matching Transition Noise MAX ±0.3 ANALOG INPUT The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) SYMBOL PARAMETER CONDITIONS VIN Analog Input Range (AIN+ –AIN–) MIN 2.85V < VDD < 3.4V (Note 7) ● VIN,CM Analog Input Common Mode (AIN+ +AIN–)/2 Differential Input Drive (Note 7) Single Ended Input Drive (Note 7) ● ● 1 0.5 IIN Analog Input Leakage Current 0V < AIN+, AIN– < VDD ● ISENSE SENSEA, SENSEB Input Leakage 0V < SENSEA, SENSEB < 1V ● IMODE MODE Input Leakage Current 0V < MODE < VDD ● TYP MAX UNITS ±0.5V to ±1V 1.5 1.5 V 1.9 2 V V –1 1 μA –3 3 μA –3 3 μA tAP Sample-and-Hold Acquisition Delay Time tJITTER Sample-and-Hold Acquisition Delay Time Jitter 0.2 psRMS CMRR Analog Input Common Mode Rejection Ratio 80 dB 640 MHz Full Power Bandwidth 0 Figure 8 Test Circuit ns DYNAMIC ACCURACY The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. AIN = –1dBFS. (Note 4) SYMBOL PARAMETER CONDITIONS SNR Signal-to-Noise Ratio 5MHz Input MIN 30MHz Input Spurious Free Dynamic Range 4th Harmonic or Higher dB 71.7 dB 5MHz Input 88 dB 30MHz Input 85 dB 82 dB 68.9 ● 70 140MHz Input 78 dB 5MHz Input 90 dB 30MHz Input 90 dB 90 dB 90 dB 5MHz Input 72.2 dB 30MHz Input 72 dB 71.9 dB 70.2 dB 85 dB –110 dB 70MHz Input ● 77 140MHz Input S/(N+D) Signal-to-Noise Plus Distortion Ratio 70MHz Input 140MHz Input IMD dB dB 70MHz Input SFDR UNITS 72.3 ● 140MHz Input Spurious Free Dynamic Range 2nd or 3rd Harmonic MAX 72.2 70MHz Input SFDR TYP 72.4 Intermodulation Distortion fIN = 40MHz, 41MHz Crosstalk fIN = 100MHz ● 67 2285fb 3 LTC2285 INTERNAL REFERENCE CHARACTERISTICS (Note 4) PARAMETER CONDITIONS MIN TYP MAX UNITS VCM Output Voltage IOUT = 0 1.475 1.500 1.525 V VCM Output Tempco ±25 ppm/°C VCM Line Regulation 2.85V < VDD < 3.4V 3 mV/V VCM Output Resistance |IOUT| < 1mA 4 Ω DIGITAL INPUTS AND DIGITAL OUTPUTS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS LOGIC INPUTS (CLK, OE, SHDN, MUX) VIH High Level Input Voltage VDD = 3V ● VIL Low Level Input Voltage VDD = 3V ● IIN Input Current VIN = 0V to VDD ● CIN Input Capacitance (Note 7) COZ Hi-Z Output Capacitance OE = High (Note 7) 3 pF ISOURCE Output Source Current VOUT = 0V 50 mA ISINK Output Sink Current VOUT = 3V 50 mA VOH High Level Output Voltage IO = –10μA IO = –200μA ● IO = 10μA IO = 1.6mA ● 2 V –10 0.8 V 10 μA 3 pF LOGIC OUTPUTS OVDD = 3V VOL Low Level Output Voltage 2.7 2.995 2.99 0.005 0.09 V V 0.4 V V OVDD = 2.5V VOH High Level Output Voltage IO = –200μA 2.49 V VOL Low Level Output Voltage IO = 1.6mA 0.09 V VOH High Level Output Voltage IO = –200μA 1.79 V VOL Low Level Output Voltage IO = 1.6mA 0.09 V OVDD = 1.8V 2285fb 4 LTC2285 POWER REQUIREMENTS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 8) SYMBOL PARAMETER CONDITIONS MIN TYP MAX VDD Analog Supply Voltage (Note 9) ● 2.85 3 3.4 0.5 UNITS V OVDD Output Supply Voltage (Note 9) ● 3 3.6 V IVDD Supply Current Both ADCs at fS(MAX) ● 263 305 mA PDISS Power Dissipation Both ADCs at fS(MAX) ● 790 915 mW PSHDN Shutdown Power (Each Channel) SHDN = H, OE = H, No CLK 2 mW PNAP Nap Mode Power (Each Channel) SHDN = H, OE = L, No CLK 15 mW TIMING CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 4) SYMBOL PARAMETER CONDITIONS MIN fs Sampling Frequency (Note 9) ● 1 tL CLK Low Time Duty Cycle Stabilizer Off (Note 7) Duty Cycle Stabilizer On (Note 7) ● ● 3.8 3 tH CLK High Time Duty Cycle Stabilizer Off (Note 7) Duty Cycle Stabilizer On (Note 7) ● ● 3.8 3 tAP Sample-and-Hold Aperture Delay TYP MAX UNITS 125 MHz 4 4 500 500 ns ns 4 4 500 500 ns ns 0 ns tD CLK to DATA Delay CL = 5pF (Note 7) ● 1.4 2.7 5.4 ns tC CLK to CLKOUT Delay CL = 5pF (Note 7) ● 1.4 2.7 5.4 ns DATA to CLKOUT Skew (tD – tC) (Note 7) ● –0.6 0 0.6 ns tMD MUX to DATA Delay CL = 5pF (Note 7) ● 1.4 2.7 5.4 ns Data Access Time After OE↓ CL = 5pF (Note 7) ● 4.3 10 ns BUS Relinquish Time (Note 7) ● 3.3 8.5 Pipeline Latency Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: All voltage values are with respect to ground with GND and OGND wired together (unless otherwise noted). Note 3: When these pin voltages are taken below GND or above VDD, they will be clamped by internal diodes. This product can handle input currents of greater than 100mA below GND or above VDD without latchup. Note 4: VDD = 3V, fSAMPLE = 125MHz, input range = 2VP-P with differential drive, unless otherwise noted. 5 ns Cycles Note 5: Integral nonlinearity is defined as the deviation of a code from a straight line passing through the actual endpoints of the transfer curve. The deviation is measured from the center of the quantization band. Note 6: Offset error is the offset voltage measured from –0.5 LSB when the output code flickers between 00 0000 0000 0000 and 11 1111 1111 1111. Note 7: Guaranteed by design, not subject to test. Note 8: VDD = 3V, fSAMPLE = 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. 2285fb 5 LTC2285 TYPICAL PERFORMANCE CHARACTERISTICS Crosstalk vs Input Frequency Typical INL, 2V Range, 125Msps –100 0.8 1.5 –105 0.6 –110 –115 –120 DNL ERROR (LSB) INL ERROR (LSB) 1.0 CROSSTALK (dB) Typical DNL, 2V Range, 125Msps 1.0 2.0 0.5 0 –0.5 –1.0 –125 20 40 60 80 INPUT FREQUENCY (MHz) 100 2285 G01 12288 0 16384 0 –10 –20 –20 –20 –30 –30 –30 –60 –70 –80 AMPLITUDE (dB) 0 –10 –50 –40 –50 –60 –70 –80 –60 –70 –80 –90 –100 –100 –100 –110 –110 –110 –120 –120 20 30 40 FREQUENCY (MHz) 50 60 0 10 2285 G04 20 30 40 FREQUENCY (MHz) 50 –120 60 0 20000 –10 18000 –20 –20 –30 –30 –70 –80 –50 –60 –70 –100 –100 –120 0 60 2285 G07 3380 4000 –110 50 10516 6000 –120 20 30 40 FREQUENCY (MHz) 11299 10000 –110 10 17646 18027 12000 2000 0 60 2285 G06 8000 –80 –90 50 14000 –40 –90 20 30 40 FREQUENCY (MHz) 16000 COUNT AMPLITUDE (dB) 0 –60 10 Grounded Input Histogram, 125Msps –10 –50 0 2285 G05 8192 Point 2-Tone FFT, fIN = 28.2MHz and 26.8MHz, –1dB, 2V Range, 125Msps –40 16384 2285 G03 –50 –90 10 12288 –40 –90 0 8192 CODE 8192 Point FFT, fIN = 70MHz, –1dB, 2V Range, 125Msps 0 –40 4096 2285 G02 –10 AMPLITUDE (dB) AMPLITUDE (dB) 8192 CODE 8192 Point FFT, fIN = 30MHz, –1dB, 2V Range, 125Msps 8192 Point FFT, fIN = 140MHz, –1dB, 2V Range, 125Msps AMPLITUDE (dB) –0.4 –1.0 4096 0 8192 Point FFT, fIN = 5MHz, –1dB, 2V Range, 125Msps 0 –0.2 –0.8 –2.0 0 0.2 –0.6 –1.5 –130 0.4 0 10 20 30 40 FREQUENCY (MHz) 50 60 2285 G08 3316 3 54 581 8183 8185 637 68 1 8187 8189 CODE 8191 8193 2285 G09 2285fb 6 LTC2285 TYPICAL PERFORMANCE CHARACTERISTICS SNR vs Input Frequency, –1dB, 2V Range, 125Msps SFDR vs Input Frequency, –1dB, 2V Range, 125Msps 75 SNR and SFDR vs Sample Rate, 2V Range, fIN = 5MHz, –1dB 100 74 90 SFDR 95 90 SFDR (dBRS) 71 70 69 85 80 68 75 67 80 SNR 70 60 70 66 65 65 0 50 0 100 150 200 250 300 350 2285 G10 INPUT FREQUENCY (MHz) 0 SFDR (dBc AND dBFS) dBc 40 30 20 10 –40 –30 –20 INPUT LEVEL (dBFS) –50 –10 0 2285 G13 280 dBFS 90 50 40 60 80 100 120 140 160 2285 G12 SAMPLE RATE (Msps) 290 100 60 20 IVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB 110 dBFS 70 270 80 260 70 60 dBc 50 40 250 2V RANGE 240 1V RANGE 230 220 30 20 210 10 200 0 –80 –70 –60 –50 –40 –30 –20 –10 0 2285 G14 INPUT LEVEL (dBFS) IOVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB, 0VDD = 1.8V 190 0 20 60 80 100 40 SAMPLE RATE (Msps) 120 140 2285 G15 SNR vs SENSE, fIN = 5MHz, –1dB 74 16 73 14 72 12 71 10 SNR (dBFS) IOVDD (mA) 0 –70 –60 50 100 150 200 250 300 350 2285 G11 INPUT FREQUENCY (MHz) SFDR vs Input Level, fIN = 70MHz, 2V Range, 125Msps SNR vs Input Level, fIN = 70MHz, 2V Range, 125Msps 80 50 IVDD (mA) SNR (dBFS) 72 SNR (dBc AND dBFS) SNR AND SFDR (dBFS) 73 8 6 70 69 68 67 4 66 2 65 0 64 0 20 80 100 60 SAMPLE RATE (Msps) 40 120 140 2285 G16 0.4 0.5 0.6 0.7 0.8 0.9 SENSE PIN (V) 1.0 1.1 2285 G17 2285fb 7 LTC2285 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. 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-DA13; Channel B comes out on DB0-DB13. If MUX is Low, the output busses are swapped and Channel A comes out on DB0-DB13; Channel B comes out on DA0-DA13. 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. DB0 – DB13 (Pins 24 to 30, 33 to 39): Channel B Digital Outputs. DB13 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 – DA13 (Pins 41 to 48, 51 to 56): Channel A Digital Outputs. DA13 is the MSB. GND (Pins 17, 64): ADC Power Ground. OF (Pin 57): Overflow/Underflow Output. High when an overflow or underflow has occurred on either channel A or channel B. 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 OEA (Pin 58): Channel A Output Enable Pin. Refer to SHDNA pin function. 8 2285fb LTC2285 PIN FUNCTIONS 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. 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. 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 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+ AIN– VCM INPUT S/H FIRST PIPELINED ADC STAGE SECOND PIPELINED ADC STAGE THIRD PIPELINED ADC STAGE FOURTH PIPELINED ADC STAGE FIFTH PIPELINED ADC STAGE 1.5V REFERENCE SIXTH PIPELINED ADC STAGE SHIFT REGISTER AND CORRECTION 2.2μF RANGE SELECT REFH SENSE REFL INTERNAL CLOCK SIGNALS OVDD REF BUF OF* D13 CLOCK/DUTY CYCLE CONTROL DIFF REF AMP CONTROL LOGIC • • • OUTPUT DRIVERS D0 CLKOUT* REFH 0.1μF 2285 F01 REFL OGND CLK MODE SHDN OE 2.2μF *OF AND CLKOUT ARE SHARED BETWEEN BOTH CHANNELS. 1μF 1μF Figure 1. Functional Block Diagram (Only One Channel is Shown) 2285fb 9 LTC2285 TIMING DIAGRAMS Dual Digital Output Bus Timing (Only One Channel is Shown) tAP ANALOG INPUT N+4 N+2 N N+1 tH N+3 N+5 tL CLKA = CLKB tD N–4 N–5 D0-D13, OF N–3 N–2 N–1 N 2284 TD01 tC CLKOUT Multiplexed Digital Output Bus Timing tAPA ANALOG INPUT A A+4 A+2 A A+1 A+3 tAPB ANALOG INPUT B B+4 B+2 B B+1 tH tL A–5 B–5 B+3 CLKA = CLKB = MUX D0A-D13A A–4 tD D0B-D13B B–5 tC B–4 A–3 B–3 A–2 B–2 B–3 A–3 B–2 A–2 A–1 tMD A–5 B–4 A–4 B–1 2284 TD02 CLKOUT 2285fb 10 LTC2285 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. 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. 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. Input Bandwidth Total Harmonic Distortion Aperture Delay Time 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: The time from when CLK reaches midsupply to the instant that the input signal is held by the sample and hold circuit. THD = 20log
(V22 + V32 + V42 + ...Vn2 )/V1   where V1 is the RMS amplitude of the fundamental frequency and V2 through Vn are the amplitudes of the second through nth harmonics. The THD calculated in this data sheet uses all the harmonics up to the fifth. Intermodulation Distortion If the ADC input signal consists of more than one spectral component, the ADC transfer function nonlinearity can produce intermodulation distortion (IMD) in addition to THD. IMD is the change in one sinusoidal input caused by the presence of another sinusoidal input at a different frequency. If two pure sine waves of frequencies fa and fb are applied to the ADC input, nonlinearities in the ADC transfer function can create distortion products at the sum and difference frequencies of mfa ± nfb, where m and n = 0, 1, 2, 3, etc. The 3rd order intermodulation products are 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 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 LTC2285 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 2285fb 11 LTC2285 APPLICATIONS INFORMATION with slightly worse harmonic distortion. The CLK input is single-ended. The LTC2285 has two phases of operation, determined by the state of the CLK input pin. 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 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. 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. 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 Figure 2 shows an equivalent circuit for the LTC2285 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. SAMPLE/HOLD OPERATION AND INPUT DRIVE Sample/Hold Operation 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 LTC2285 VDD AIN+ CSAMPLE 3.5pF 15Ω CPARASITIC 1pF VDD AIN– CSAMPLE 3.5pF 15Ω CPARASITIC 1pF VDD CLK 2285 F02 Figure 2. Equivalent Input Circuit 2285fb 12 LTC2285 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 LTC2285 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 LTC2285 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 AIN+ LTC2285 0.1μF 12pF 25Ω AIN– T1 = MA/COM ETC1-1T 25Ω RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 2285 F03 Figure 3. Single-Ended to Differential Conversion Using a Transformer 2285fb 13 LTC2285 APPLICATIONS INFORMATION VCM HIGH SPEED DIFFERENTIAL 25Ω AMPLIFIER ANALOG INPUT + 2.2μF 2.2μF 0.1μF + AIN LTC2285 12Ω ANALOG INPUT 25Ω + CM – VCM – 25Ω 0.1μF AIN– LTC2285 0.1μF T1 12pF AIN+ 8pF 25Ω 12Ω AIN– T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 2285 F04 2285 F06 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 2.2μF 1k 25Ω AIN+ VCM LTC2285 2.2μF 0.1μF 12pF 25Ω AIN– 0.1μF AIN+ ANALOG INPUT 25Ω T1 2285 F05 0.1μF 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. 25Ω T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 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. LTC2285 0.1μF AIN– 2285 F07 Figure 7. Recommended Front End Circuit for Input Frequencies Between 170MHz and 300MHz VCM 2.2μF 0.1μF 8.2nH ANALOG INPUT 25Ω AIN+ LTC2285 0.1μF T1 0.1μF 25Ω 8.2nH – AIN T1 = MA/COM, ETC 1-1-13 RESISTORS, CAPACITORS, INDUCTORS ARE 0402 PACKAGE SIZE 2285 F08 Figure 8. Recommended Front End Circuit for Input Frequencies Above 300MHz 2285fb 14 LTC2285 APPLICATIONS INFORMATION Reference Operation Figure 9 shows the LTC2285 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. LTC2285 1.5V VCM 4Ω 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 BANDGAP REFERENCE 1.5V 2.2μF 1V 0.5V VCM 2.2μF 12k TIE TO VDD FOR 2V RANGE; TIE TO VCM FOR 1V RANGE; RANGE = 2 • VSENSE FOR 0.5V < VSENSE < 1V RANGE DETECT AND CONTROL 0.75V 12k 1μF SENSE 2285 F10 BUFFER Figure 10. 1.5V Range ADC INTERNAL ADC HIGH REFERENCE 1μF REFH 2.2μF LTC2285 SENSE 0.1μF Input Range The input range can be set based on the application. The 2V input range will provide the best signal-to-noise performance while maintaining excellent SFDR. The 1V input range will have better SFDR performance, but the SNR will degrade by 5.7dB. See the Typical Performance Characteristics section. DIFF AMP 1μF REFL INTERNAL ADC LOW REFERENCE 2285 F09 Figure 9. Equivalent Reference Circuit 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). 2285fb 15 LTC2285 APPLICATIONS INFORMATION CLEAN SUPPLY 4.7μF CLEAN SUPPLY 4.7μF FERRITE BEAD FERRITE BEAD 0.1μF 0.1μF SINUSOIDAL CLOCK INPUT CLK 1k 0.1μF CLK 50Ω 1k LTC2285 LTC2285 100Ω NC7SVU04 2285 F12 2285 F11 Figure 11. Sinusoidal Single-Ended CLK Drive The noise performance of the LTC2285 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. IF LVDS USE FIN1002 OR FIN1018. FOR PECL, USE AZ1000ELT21 OR SIMILAR Figure 12. CLK Drive Using an LVDS or PECL to CMOS Converter ETC1-1T CLK LTC2285 5pF-30pF DIFFERENTIAL CLOCK INPUT 2285 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 LTC2285 is 125Msps. The lower limit of the LTC2285 sample rate is determined by droop of the sample-and-hold circuits. The pipelined architecture of this ADC relies on storing analog signals on 2285fb 16 LTC2285 APPLICATIONS INFORMATION small valued capacitors. Junction leakage will discharge the capacitors. The specified minimum operating frequency for the LTC2285 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 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 50Ω 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 LTC2285 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. LTC2283 OVDD VDD 0.5V TO 3.6V VDD 0.1μF OVDD DATA FROM LATCH PREDRIVER LOGIC 43Ω TYPICAL DATA OUTPUT OE DIGITAL OUTPUTS OGND 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 occurred on either channel A or channel B. 2283 F14 Figure 14. Digital Output Buffer Data Format Table 1. Output Codes vs Input Voltage AIN+ – AIN– (2V Range) OF D13 – D0 (Offset Binary) D13 – D0 (2’s Complement) >+1.000000V +0.999878V +0.999756V 1 0 0 11 1111 1111 1111 11 1111 1111 1111 11 1111 1111 1110 01 1111 1111 1111 01 1111 1111 1111 01 1111 1111 1110 +0.000122V 0.000000V –0.000122V –0.000244V 0 0 0 0 10 0000 0000 0001 10 0000 0000 0000 01 1111 1111 1111 01 1111 1111 1110 00 0000 0000 0001 00 0000 0000 0000 11 1111 1111 1111 11 1111 1111 1110 –0.999878V –1.000000V