LTC2240IUP-12#PBF

LTC2240-12 12-Bit, 170Msps ADC FEATURES DESCRIPTION n The LTC®2240-12 is a 170Msps, sampling 12-bit A/D converter designed for digitizing high frequency, wide dynamic range signals. The LTC2240-12 is perfect for demanding communications applications with AC performance that includes 65.6dB SNR and 80dB SFDR. Ultralow jitter of 95fsRMS allows IF undersampling with excellent noise performance. n n n n n n n n n n n n n Sample Rate: 170Msps 65.6dB SNR 80dB SFDR 1.2GHz Full Power Bandwidth S/H Single 2.5V Supply Low Power Dissipation: 445mW LVDS, CMOS, or Demultiplexed CMOS Outputs Selectable Input Ranges: ±0.5V or ±1V No Missing Codes Optional Clock Duty Cycle Stabilizer Shutdown and Nap Modes Data Ready Output Clock Pin Compatible Family 250Msps: LTC2242-12 (12-Bit), LTC2242-10 (10-Bit) 210Msps: LTC2241-12 (12-Bit), LTC2241-10 (10-Bit) 170Msps: LTC2240-12 (12-Bit), LTC2240-10 (10-Bit) 185Msps: LTC2220-1 (12-Bit)* 170Msps: LTC2220 (12-Bit), LTC2230 (10-Bit)* 135Msps: LTC2221 (12-Bit), LTC2231 (10-Bit)* 64-Pin 9mm × 9mm QFN Package APPLICATIONS n n n n DC specs include ±0.6LSB INL (typ), ±0.4LSB DNL (typ) and no missing codes over temperature. The digital outputs can be either differential LVDS, or single-ended CMOS. There are three format options for the CMOS outputs: a single bus running at the full data rate or two demultiplexed buses running at half data rate with either interleaved or simultaneous update. A separate output power supply allows the CMOS output swing to range from 0.5V to 2.625V. The ENC+ and ENC – inputs may be driven differentially or single ended with a sine wave, PECL, LVDS, TTL, or CMOS inputs. An optional clock duty cycle stabilizer allows high performance over a wide range of clock duty cycles. L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. *LTC2220-1, LTC2220, LTC2221, LTC2230, LTC2231 are 3.3V parts. Wireless and Wired Broadband Communication Cable Head-End Systems Power Amplifier Linearization Communications Test Equipment TYPICAL APPLICATION 2.5V SFDR vs Input Frequency VDD REFH REFL 0.5V TO 2.625V FLEXIBLE REFERENCE 85 80 OVDD ANALOG INPUT INPUT S/H – 12-BIT PIPELINED ADC CORE CORRECTION LOGIC D11 • • • D0 OUTPUT DRIVERS CMOS OR LVDS SFDR (dBFS) 75 + 70 65 1V RANGE 60 2V RANGE 55 OGND CLOCK/DUTY CYCLE CONTROL 45 40 224012 TA01 ENCODE INPUT 50 0 100 200 300 400 500 600 700 800 900 1000 INPUT FREQUENCY (MHz) 224012 G11 224012fd 1 LTC2240-12 ABSOLUTE MAXIMUM RATINGS OVDD = VDD (Notes 1, 2) Supply Voltage (VDD) ...............................................2.8V 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 LTC2240C-12 ........................................... 0°C to 70°C LTC2240I-12 ........................................–40°C to 85°C Storage Temperature Range................... –65°C to 150°C PIN CONFIGURATION 64 GND 63 VDD 62 VDD 61 GND 60 VCM 59 SENSE 58 MODE 57 LVDS 56 OF+/OFA 55 OF–/DA11 54 D11+/DA10 53 D11–/DA9 52 D10+/DA8 51 D10–/DA7 50 OGND 49 OVDD TOP VIEW AIN+ 1 AIN+ 2 AIN– 3 AIN– 4 REFHA 5 REFHA 6 REFLB 7 REFLB 8 REFHB 9 REFHB 10 REFLA 11 REFLA 12 VDD 13 VDD 14 VDD 15 GND 16 48 D9+/DA6 47 D9–/DA5 46 D8+/DA4 45 D8–/DA3 44 D7+/DA2 43 D7–/DA1 42 OVDD 41 OGND 40 D6+/DA0 39 D6–/CLKOUTA 38 D5+/CLKOUTB 37 D5–/OFB 36 CLKOUT+/DB11 35 CLKOUT–/DB10 34 OVDD 33 OGND ENC+ 17 ENC– 18 SHDN 19 OE 20 DO–/DB0 21 +/DB1 22 DO D1–/DB2 23 D1+/DB3 24 OGND 25 OVDD 26 D2–/DB4 27 D2+/DB5 28 D3–/DB6 29 D3+/DB7 30 D4–/DB8 31 D4+/DB9 32 65 UP PACKAGE 64-LEAD (9mm × 9mm) PLASTIC QFN EXPOSED PAD (PIN 65) IS GND, MUST BE SOLDERED TO PCB TJMAX = 150°C, θJA = 20°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2240CUP-12#PBF LTC2240CUP-12#TRPBF LTC2240UP-12 64-Lead (9mm × 9mm) Plastic QFN 0°C to 70°C LTC2240IUP-12#PBF LTC2240IUP-12#TRPBF LTC2240UP-12 64-Lead (9mm × 9mm) Plastic QFN –40°C to 85°C LEAD BASED FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC2240CUP-12 LTC2240CUP-12#TR LTC2240UP-12 64-Lead (9mm × 9mm) Plastic QFN 0°C to 70°C LTC2240IUP-12 LTC2240IUP-12#TR LTC2240UP-12 64-Lead (9mm × 9mm) Plastic QFN –40°C to 85°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *Temperature grades are 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/ 224012fd 2 LTC2240-12 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 Resolution (No Missing Codes) MIN l 12 TYP MAX UNITS Bits Integral Linearity Error Differential Analog Input (Note 5) l –2.1 ±0.6 2.1 LSB Differential Linearity Error Differential Analog Input l –1 ±0.4 1 LSB Offset Error (Note 6) l –15 ±5 15 mV External Reference l –3.5 ±0.7 3.5 Gain Error Offset Drift %FS ±10 μV/C Full-Scale Drift Internal Reference External Reference ±60 ±45 ppm/C ppm/C Transition Noise SENSE = 1V 0.74 LSBRMS 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–) 2.375V < VDD < 2.625V (Note 7) l VIN, CM Analog Input Common Mode (AIN+ + AIN–)/2 Differential Input (Note 7) l 1.2 1.3 V IIN Analog Input Leakage Current 0 < AIN+, AIN– < VDD l –1 1 μA ISENSE SENSE Input Leakage 0V < SENSE < 1V l –1 1 μA IMODE MODE Pin Pull-Down Current to GND 7 μA ILVDS LVDS Pin Pull-Down Current to GND 7 μA tAP Sample and Hold Acquisition Delay Time 0.4 ns tJITTER Sample and Hold Acquisition Delay Time Jitter 95 fsRMS 1200 MHz Full Power Bandwidth MIN TYP MAX UNITS ±0.5 to ±1 Figure 8 Test Circuit 1.25 V 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 SNR Signal-to-Noise Ratio (Note 10) CONDITIONS MIN TYP 65.6 dB 64.2 65.5 dB 140MHz Input 65.4 dB 240MHz Input 65.2 dB 10MHz Input 80 dB 10MHz Input 70MHz Input SFDR Spurious Free Dynamic Range 2nd or 3rd Harmonic (Note 11) Spurious Free Dynamic Range 4th Harmonic or Higher (Note 11) S/(N+D) IMD Signal-to-Noise Plus Distortion Ratio (Note 12) Intermodulation Distortion l l UNITS 75 dB 140MHz Input 74 dB 240MHz Input 72 dB 70MHz Input 65 MAX 10MHz Input 87 dB 87 dB 140MHz Input 87 dB 240MHz Input 87 dB 10MHz Input 65.5 dB 70MHz Input l l 76 65.3 dB 140MHz Input 65.2 dB 240MHz Input 64.2 dB 70MHz Input fIN1 = 135MHz, fIN2 = 140MHz 62.7 81 dBc 224012fd 3 LTC2240-12 INTERNAL REFERENCE CHARACTERISTICS (Note 4) PARAMETER CONDITIONS MIN TYP MAX VCM Output Voltage IOUT = 0 1.225 1.25 1.275 VCM Output Tempco ±35 UNITS V ppm/°C VCM Line Regulation 2.375V < VDD < 2.625V 3 mV/V VCM Output Resistance –1mA < IOUT < 1mA 2 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 ENCODE INPUTS (ENC +, ENC –) VID Differential Input Voltage (Note 7) l VICM Common Mode Input Voltage Internally Set Externally Set (Note 7) l RIN Input Resistance CIN Input Capacitance 0.2 1.2 (Note 7) V 1.5 1.5 2.0 V V 4.8 kΩ 2 pF LOGIC INPUTS (OE, SHDN) VIH High Level Input Voltage VDD = 2.5V l VIL Low Level Input Voltage VDD = 2.5V l IIN Input Current VIN = 0V to VDD l CIN Input Capacitance (Note 7) 3 pF 1.7 V –10 0.7 V 10 μA LOGIC OUTPUTS (CMOS MODE) OVDD = 2.5V COZ Hi-Z Output Capacitance OE = High (Note 7) 3 pF ISOURCE Output Source Current VOUT = 0V 37 mA ISINK Output Sink Current VOUT = 2.5V 23 mA VOH High Level Output Voltage IO = –10μA IO = –500μA 2.495 2.45 V V VOL Low Level Output Voltage 0.005 0.07 V V OVDD = 1.8V VOH High Level Output Voltage IO = –500μA 1.75 V VOL Low Level Output Voltage IO = 500μA 0.07 V 224012fd 4 LTC2240-12 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 mV LOGIC OUTPUTS (LVDS MODE) VOD Differential Output Voltage 100Ω Differential Load l 247 350 454 VOS Output Common Mode Voltage 100Ω Differential Load l 1.125 1.250 1.375 V POWER REQUIREMENTS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. (Note 9) SYMBOL PARAMETER CONDITIONS VDD Analog Supply Voltage (Note 8) PSLEEP Sleep Mode Power SHDN = High, OE = High, No CLK 1 mW PNAP Nap Mode Power SHDN = High, OE = Low, No CLK 28 mW OVDD Output Supply Voltage (Note 8) 2.5 2.625 IVDD Analog Supply Current l 170 185 mA IOVDD Output Supply Current l 58 70 mA PDISS Power Dissipation l 570 638 mW 2.5 2.625 170 185 l MIN TYP MAX UNITS 2.375 2.5 2.625 V LVDS OUTPUT MOD l 2.375 V CMOS OUTPUT MODE OVDD Output Supply Voltage (Note 8) l IVDD Analog Supply Current (Note 7) l PDISS Power Dissipation 0.5 445 V mA 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 fS Sampling Frequency (Note 8) l MIN 1 tL ENC Low Time (Note 7) Duty Cycle Stabilizer Off Duty Cycle Stabilizer On l l 2.79 1.5 tH ENC High Time (Note 7) Duty Cycle Stabilizer Off Duty Cycle Stabilizer On l l 2.79 1.5 tAP Sample-and-Hold Aperture Delay tOE Output Enable Delay (Note 7) l ENC to DATA Delay (Note 7) l ENC to CLKOUT Delay (Note 7) l DATA to CLKOUT Skew (tC – tD) (Note 7) l TYP MAX UNITS 170 MHz 2.94 2.94 500 500 ns ns 2.94 2.94 500 500 ns ns 0.4 ns 5 10 ns 1 1.7 2.8 ns 1 1.7 2.8 ns –0.6 0 0.6 ns LVDS OUTPUT MODE tD tC Rise Time 0.5 ns Fall Time 0.5 ns Pipeline Latency 5 Cycles CMOS OUTPUT MODE tD ENC to DATA Delay (Note 7) l 1 1.7 2.8 ns tC ENC to CLKOUT Delay (Note 7) l 1 1.7 2.8 ns 224012fd 5 LTC2240-12 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 Pipeline Latency PARAMETER CONDITIONS DATA to CLKOUT Skew (tC – tD) (Note 7) l MIN TYP MAX –0.6 0 0.6 UNITS ns Full Rate CMOS 5 Cycles Demuxed Interleaved 5 Cycles 5 and 6 Cycles Demuxed Simultaneous ELECTRICAL CHARACTERISTICS 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 = 2.5V, fSAMPLE = 170MHz, LVDS outputs, differential ENC+/ENC– = 2VP-P sine wave, input range = 2VP-P with differential drive, unless otherwise noted. Note 5: Integral nonlinearity is defined as the deviation of a code from a “best straight line” fit to 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 0000 0000 0000 and 1111 1111 1111 in 2’s complement output mode. Note 7: Guaranteed by design, not subject to test. Note 8: Recommended operating conditions. Note 9: VDD = 2.5V, fSAMPLE = 170MHz, differential ENC+/ENC– = 2VP-P sine wave, input range = 1VP-P with differential drive, output CLOAD = 5pF. Note 10: SNR minimum and typical values are for LVDS mode. Typical values for CMOS mode are typically 0.3dB lower. Note 11: SFDR minimum values are for LVDS mode. Typical values are for both LVDS and CMOS modes. Note 12: SINAD minimum and typical values are for LVDS mode. Typical values for CMOS mode are typically 0.3dB lower. TYPICAL PERFORMANCE CHARACTERISTICS 8192 Point FFT, fIN = 5MHz, –1dB, 2V Range, LVDS Mode Differential Nonlinearity Integral Nonlinearity 1.0 0 0.8 0.8 –10 0.6 0.6 –20 0.4 0.4 0.2 0.2 0 –0.2 –30 AMPLITUDE (dB) DNL (LSB) 1.0 INL (LSB) (TA = 25°C unless otherwise noted, Note 4) 0 –0.2 –40 –50 –60 –70 –0.4 –0.4 –0.6 –0.6 –90 –0.8 –0.8 –100 –1.0 0 1024 2048 3072 OUTPUT CODE 4096 224012 G01 –80 –110 –1.0 0 1024 2048 3072 OUTPUT CODE 4096 224012 G02 0 10 20 30 40 50 60 FREQUENCY (MHz) 70 80 224012 G03 224012fd 6 LTC2240-12 TYPICAL PERFORMANCE CHARACTERISTICS 8192 Point FFT, fIN = 140MHz, –1dB, 2V Range, LVDS Mode 8192 Point FFT, fIN = 240MHz, –1dB, 2V Range, LVDS Mode 0 0 –10 –10 –20 –20 –20 –30 –30 –30 –40 –50 –60 –70 AMPLITUDE (dB) 0 –10 AMPLITUDE (dB) AMPLITUDE (dB) 8192 Point FFT, fIN = 70MHz, –1dB, 2V Range, LVDS Mode (TA = 25°C unless otherwise noted, Note 4) –40 –50 –60 –70 –40 –50 –60 –70 –80 –80 –90 –90 –80 –90 –100 –100 –100 –110 –110 0 10 20 30 40 50 60 FREQUENCY (MHz) 70 –110 0 80 10 20 30 40 50 60 FREQUENCY (MHz) 70 8192 Point FFT, fIN = 500MHz, –1dB, 1V Range, LVDS Mode 0 0 –10 –10 –20 –20 –20 –30 –30 –30 –60 –70 AMPLITUDE (dB) 0 –50 –40 –50 –60 –70 –70 –90 –90 –100 –100 –100 –80 –110 30 40 50 60 FREQUENCY (MHz) 70 80 –110 0 10 20 30 40 50 60 FREQUENCY (MHz) 224012 G07 0 10 20 85 66 80 2V RANGE 30 40 50 60 FREQUENCY (MHz) 70 80 224012 G09 SFDR (HD2 and HD3) vs Input Frequency, –1dB, LVDS Mode 67 SFDR (HD4+) vs Input Frequency, –1dB, LVDS Mode 95 90 1V RANGE 85 2V RANGE 75 SFDR (dBFS) 64 63 62 70 65 1V RANGE 60 2V RANGE 55 61 60 80 224012 G08 SNR vs Input Frequency, –1dB, LVDS Mode 65 70 SFDR (dBFS) 20 80 –50 –90 10 70 –60 –80 0 30 40 50 60 FREQUENCY (MHz) –40 –80 –110 20 224012 G06 –10 –40 10 8192 Point 2-Tone FFT, fIN = 135MHz and 140MHz, –1dB, 2V Range, LVDS Mode 8192 Point FFT, fIN = 1GHz, –1dB, 1V Range, LVDS Mode AMPLITUDE (dB) AMPLITUDE (dB) 0 224012 G05 224012 G04 SNR (dBFS) 80 1V RANGE 80 75 70 50 59 65 45 58 40 0 100 200 300 400 500 600 700 800 900 1000 INPUT FREQUENCY (MHz) 224012 G10 0 100 200 300 400 500 600 700 800 900 1000 INPUT FREQUENCY (MHz) 224012 G11 60 0 100 200 300 400 500 600 700 800 9001000 INPUT FREQUENCY (MHz) 224112 G12 224012fd 7 LTC2240-12 TYPICAL PERFORMANCE CHARACTERISTICS SFDR and SNR vs Sample Rate, 2V Range, fIN = 30MHz, –1dB, LVDS Mode SFDR vs Input Level, fIN = 70MHz, 2V Range SNR vs SENSE, fIN = 5MHz, –1dB 100 90 66 90 SFDR (dBc AND dFBS) 80 75 70 SNR 65 65 dBFS 80 SFDR 64 70 dBc SNR (dBFS) 85 SFDR AND SNR (dBFS) (TA = 25°C unless otherwise noted, Note 4) 60 50 40 30 63 62 61 20 60 60 10 55 0 0 –50 20 40 60 80 100 120 140 160 180 200 SAMPLE RATE (Msps) –40 –20 –30 –10 INPUT LEVEL (dBFS) 224012 G13 0.6 0.7 0.8 0.9 1 SENSE PIN (V) 224112 G15 IOVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB 190 60 180 50 170 LVDS OUTPUTS OVDD = 2.5V 40 IOVDD (mA) 2V RANGE IVDD (mA) 59 0.5 224012 G14 IVDD vs Sample Rate, 5MHz Sine Wave Input, –1dB 160 1V RANGE 150 30 20 140 CMOS OUTPUTS OVDD = 1.8V 10 130 220 10 0 0 0 40 80 160 120 SAMPLE RATE (Msps) 200 224012 G16 0 40 80 120 160 SAMPLE RATE (Msps) 200 224012 G17 224012fd 8 LTC2240-12 PIN FUNCTIONS (CMOS Mode) AIN+ (Pins 1, 2): Positive Differential Analog Input. AIN – (Pins 3, 4): Negative Differential Analog Input. REFHA (Pins 5, 6): ADC High Reference. Bypass to Pins 7, 8 with 0.1μF ceramic chip capacitor, to Pins 11, 12 with a 2.2μF ceramic capacitor and to ground with 1μF ceramic capacitor. REFLB (Pins 7, 8): ADC Low Reference. Bypass to Pins 5, 6 with 0.1μF ceramic chip capacitor. Do not connect to Pins 11, 12. REFHB (Pins 9, 10): ADC High Reference. Bypass to Pins 11, 12 with 0.1μF ceramic chip capacitor. Do not connect to Pins 5, 6. REFLA (Pins 11, 12): ADC Low Reference. Bypass to Pins 9, 10 with 0.1μF ceramic chip capacitor, to Pins 5, 6 with a 2.2μF ceramic capacitor and to ground with 1μF ceramic capacitor. VDD (Pins 13, 14, 15, 62, 63): 2.5V Supply. Bypass to GND with 0.1μF ceramic chip capacitors. GND (Pins 16, 61, 64): ADC Power Ground. ENC+ (Pin 17): Encode Input. Conversion starts on the positive edge. ENC – (Pin 18): Encode Complement Input. Conversion starts on the negative edge. Bypass to ground with 0.1μF ceramic for single-ended encode signal. SHDN (Pin 19): Shutdown Mode Selection Pin. Connecting SHDN to GND and OE to GND results in normal operation with the outputs enabled. Connecting SHDN to GND and OE to VDD results in normal operation with the outputs at high impedance. Connecting SHDN to VDD and OE to GND results in nap mode with the outputs at high impedance. Connecting SHDN to VDD and OE to VDD results in sleep mode with the outputs at high impedance. OE (Pin 20): Output Enable Pin. Refer to SHDN pin function. DB0 - DB11 (Pins 21, 22, 23, 24, 27, 28, 29, 30, 31, 32, 35, 36): Digital Outputs, B Bus. DB11 is the MSB. At high impedance in full rate CMOS mode. OGND (Pins 25, 33, 41, 50): Output Driver Ground. OVDD (Pins 26, 34, 42, 49): Positive Supply for the Output Drivers. Bypass to ground with 0.1μF ceramic chip capacitor. OFB (Pin 37): Over/Under Flow Output for B Bus. High when an over or under flow has occurred. At high impedance in full rate CMOS mode. CLKOUTB (Pin 38): Data Valid Output for B Bus. In demux mode with interleaved update, latch B bus data on the falling edge of CLKOUTB. In demux mode with simultaneous update, latch B bus data on the rising edge of CLKOUTB. This pin does not become high impedance in full rate CMOS mode. CLKOUTA (Pin 39): Data Valid Output for A Bus. Latch A bus data on the falling edge of CLKOUTA. DA0 - DA11 (Pins 40, 43, 44, 45, 46, 47, 48, 51, 52, 53, 54, 55): Digital Outputs, A Bus. DA11 is the MSB. OFA (Pin 56): Over/Under Flow Output for A Bus. High when an over or under flow has occurred. LVDS (Pin 57): Output Mode Selection Pin. Connecting LVDS to 0V selects full rate CMOS mode. Connecting LVDS to 1/3VDD selects demux CMOS mode with simultaneous update. Connecting LVDS to 2/3VDD selects demux CMOS mode with interleaved update. Connecting LVDS to VDD selects LVDS mode. MODE (Pin 58): Output Format and Clock Duty Cycle Stabilizer Selection Pin. Connecting MODE to 0V selects offset binary output format and turns the clock duty cycle stabilizer off. Connecting MODE to 1/3VDD selects offset binary output format and turns the clock duty cycle stabilizer on. Connecting MODE to 2/3VDD selects 2’s complement output format and turns the clock duty cycle stabilizer on. Connecting MODE to VDD selects 2’s complement output format and turns the clock duty cycle stabilizer off. SENSE (Pin 59): Reference Programming Pin. Connecting SENSE to VCM selects the internal reference and a ±0.5V input range. Connecting SENSE to VDD selects the internal reference and a ±1V input range. An external reference greater than 0.5V and less than 1V applied to SENSE selects an input range of ±VSENSE. ±1V is the largest valid input range. VCM (Pin 60): 1.25V Output and Input Common Mode Bias. Bypass to ground with 2.2μF ceramic chip capacitor. GND (Exposed Pad) (Pin 65): ADC Power Ground. The exposed pad on the bottom of the package needs to be soldered to ground. 224012fd 9 LTC2240-12 PIN FUNCTIONS (LVDS Mode) AIN+ (Pins 1, 2): Positive Differential Analog Input. AIN– (Pins 3, 4): Negative Differential Analog Input. REFHA (Pins 5, 6): ADC High Reference. Bypass to Pins 7, 8 with 0.1μF ceramic chip capacitor, to Pins 11, 12 with a 2.2μF ceramic capacitor and to ground with 1μF ceramic capacitor. REFLB (Pins 7, 8): ADC Low Reference. Bypass to Pins 5, 6 with 0.1μF ceramic chip capacitor. Do not connect to Pins 11, 12. REFHB (Pins 9, 10): ADC High Reference. Bypass to Pins 11, 12 with 0.1μF ceramic chip capacitor. Do not connect to Pins 5, 6. REFLA (Pins 11, 12): ADC Low Reference. Bypass to Pins 9, 10 with 0.1μF ceramic chip capacitor, to Pins 5, 6 with a 2.2μF ceramic capacitor and to ground with 1μF ceramic capacitor. VDD (Pins 13, 14, 15, 62, 63): 2.5V Supply. Bypass to GND with 0.1μF ceramic chip capacitors. GND (Pins 16, 61, 64): ADC Power Ground. ENC+ (Pin 17): Encode Input. Conversion starts on the positive edge. ENC– (Pin 18): Encode Complement Input. Conversion starts on the negative edge. Bypass to ground with 0.1μF ceramic for single-ended encode signal. SHDN (Pin 19): Shutdown Mode Selection Pin. Connecting SHDN to GND and OE to GND results in normal operation with the outputs enabled. Connecting SHDN to GND and OE to VDD results in normal operation with the outputs at high impedance. Connecting SHDN to VDD and OE to GND results in nap mode with the outputs at high impedance. Connecting SHDN to VDD and OE to VDD results in sleep mode with the outputs at high impedance. OE (Pin 20): Output Enable Pin. Refer to SHDN pin function. D0–/D0+ to D11–/D11+ (Pins 21, 22, 23, 24, 27, 28, 29, 30, 31, 32, 37, 38, 39, 40, 43, 44, 45, 46, 47, 48, 51, 52, 53, 54): LVDS Digital Outputs. All LVDS outputs require differential 100Ω termination resistors at the LVDS receiver. D11–/D11+ is the MSB. OGND (Pins 25, 33, 41, 50): Output Driver Ground. OVDD (Pins 26, 34, 42, 49): Positive Supply for the Output Drivers. Bypass to ground with 0.1μF ceramic chip capacitor. CLKOUT–/CLKOUT+ (Pins 35 to 36): LVDS Data Valid Output. Latch data on rising edge of CLKOUT–, falling edge of CLKOUT+. OF–/OF+ (Pins 55 to 56): LVDS Over/Under Flow Output. High when an over or under flow has occurred. LVDS (Pin 57): Output Mode Selection Pin. Connecting LVDS to 0V selects full rate CMOS mode. Connecting LVDS to 1/3VDD selects demux CMOS mode with simultaneous update. Connecting LVDS to 2/3VDD selects demux CMOS mode with interleaved update. Connecting LVDS to VDD selects LVDS mode. MODE (Pin 58): Output Format and Clock Duty Cycle Stabilizer Selection Pin. Connecting MODE to 0V selects offset binary output format and turns the clock duty cycle stabilizer off. Connecting MODE to 1/3VDD selects offset binary output format and turns the clock duty cycle stabilizer on. Connecting MODE to 2/3VDD selects 2’s complement output format and turns the clock duty cycle stabilizer on. Connecting MODE to VDD selects 2’s complement output format and turns the clock duty cycle stabilizer off. SENSE (Pin 59): Reference Programming Pin. Connecting SENSE to VCM selects the internal reference and a ±0.5V input range. Connecting SENSE to VDD selects the internal reference and a ±1V input range. An external reference greater than 0.5V and less than 1V applied to SENSE selects an input range of ±VSENSE. ±1V is the largest valid input range. VCM (Pin 60): 1.25V Output and Input Common Mode Bias. Bypass to ground with 2.2μF ceramic chip capacitor. GND (Exposed Pad) (Pin 65): ADC Power Ground. The exposed pad on the bottom of the package needs to be soldered to ground. 224012fd 10 LTC2240-12 FUNCTIONAL BLOCK DIAGRAM AIN+ AIN– VCM VDD INPUT S/H FIRST PIPELINED ADC STAGE SECOND PIPELINED ADC STAGE THIRD PIPELINED ADC STAGE FOURTH PIPELINED ADC STAGE FIFTH PIPELINED ADC STAGE GND 1.25V REFERENCE 2.2μF SHIFT REGISTER AND CORRECTION RANGE SELECT SENSE REFH REF BUF REFL INTERNAL CLOCK SIGNALS OVDD DIFFERENTIAL INPUT LOW JITTER CLOCK DRIVER DIFF REF AMP REFLB REFHA 2.2μF 0.1μF ENC • • • OUTPUT DRIVERS + –+ – D0 CLKOUT 224012 F01 REFLA REFHB 0.1μF 1μF CONTROL LOGIC + OF –+ – D11 OGND + ENC– M0DE LVDS SHDN OE 1μF Figure 1. Functional Block Diagram 224012fd 11 LTC2240-12 TIMING DIAGRAMS LVDS Output Mode Timing All Outputs Are Differential and Have LVDS Levels tAP ANALOG INPUT N+4 N+2 N N+3 tH N+1 tL ENC– ENC+ tD N–5 D0-D11, OF N–4 N–3 N–2 N–1 tC CLKOUT– CLKOUT+ 224012 TD01 Full-Rate CMOS Output Mode Timing All Outputs Are Single-Ended and Have CMOS Levels tAP ANALOG INPUT N+4 N+2 N N+3 tH N+1 tL ENC– ENC+ tD N–5 DA0-DA11, OFA N–4 N–3 N–2 N–1 tC CLKOUTB CLKOUTA DB0-DB11, OFB HIGH IMPEDANCE 224012 TD02 224012fd 12 LTC2240-12 TIMING DIAGRAMS Demultiplexed CMOS Outputs with Interleaved Update All Outputs Are Single-Ended and Have CMOS Levels tAP ANALOG INPUT N+4 N+2 N N+3 tH N+1 tL ENC– ENC+ tD N–5 DA0-DA11, OFA N–3 N–1 tD N–6 DB0-DB11, OFB N–4 tC N–2 tC CLKOUTB CLKOUTA 224012 TD03 Demultiplexed CMOS Outputs with Simultaneous Update All Outputs Are Single-Ended and Have CMOS Levels tAP ANALOG INPUT N+4 N+2 N N+3 tH N+1 tL ENC– ENC+ tD DA0-DA11, OFA N–6 N–4 N–2 N–5 N–3 N–1 tD DB0-DB11, OFB tC CLKOUTB CLKOUTA 224012 TD04 224012fd 13 LTC2240-12 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. Full Power 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 a rising ENC+ equals the ENC– voltage to the instant that the input signal is held by the sample and hold circuit. ⎛ THD = 20Log ⎜ ⎝ ( V2 + V3 + V4 + ...Vn )/ V1⎞⎟⎠ 2 2 2 2 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 full power bandwidth is that input frequency at which the amplitude of the reconstructed fundamental is reduced by 3dB for a full scale input signal. 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) CONVERTER OPERATION As shown in Figure 1, the LTC2240-12 is a CMOS pipelined multi-step converter. The converter has five pipelined ADC stages; a sampled analog input will result in a digitized value five cycles later (see the Timing Diagram section). For optimal performance the analog inputs should be driven differentially. The encode input is differential for improved common mode noise immunity. The LTC2240-12 has two phases of operation, determined by the state of the differential ENC+/ENC– input pins. For brevity, the text will refer to ENC+ greater than ENC– as ENC high and ENC+ less than ENC– as ENC low. 224012fd 14 LTC2240-12 APPLICATIONS INFORMATION 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 ENC 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 ENC transitions from low to high, the sampled input is held. While ENC 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 ENC. When ENC 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 ENC goes back high, the second stage produces its residue which is acquired by the third stage. An identical process is repeated for the third and fourth stages, resulting in a fourth stage residue that is sent to the fifth 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 LTC2240-12 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 ENC is low, the transistors connect the analog inputs to the sampling capacitors and they charge to, and track the differential input voltage. When ENC transitions from low to high, the sampled input LTC2240-12 VDD AIN+ RON 14Ω 10Ω CPARASITIC 1.8pF VDD AIN– CSAMPLE 2pF RON 14Ω 10Ω CSAMPLE 2pF CPARASITIC 1.8pF VDD 1.5V 6k ENC+ ENC– 6k 1.5V 224012 F02 Figure 2. Equivalent Input Circuit voltage is held on the sampling capacitors. During the hold phase when ENC is high, the sampling capacitors are disconnected from the input and the held voltage is passed to the ADC core for processing. As ENC 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. 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 1.25V. The VCM output pin (Pin 60) 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. 224012fd 15 LTC2240-12 APPLICATIONS INFORMATION Input Drive Impedance As with all high performance, high speed ADCs, the dynamic performance of the LTC2240-12 can be influenced by the input drive circuitry, particularly the second and third harmonics. Source impedance and input reactance can influence SFDR. At the falling edge of ENC, the sampleand-hold circuit will connect the 2pF sampling capacitor to the input pin and start the sampling period. The sampling period ends when ENC 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/(2fS); 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 LTC2240-12 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. 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 capacitively-coupled input circuit. The impedance seen by the analog inputs should be matched. 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 10Ω VCM 2.2μF 0.1μF T1 1:1 ANALOG INPUT 25Ω 25Ω AIN+ 0.1μF AIN+ LTC2240-12 12pF 25Ω AIN– 25Ω AIN– T1 = MA/COM ETC1-1T RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 224012 F03 Figure 3. Single-Ended to Differential Conversion Using a Transformer 50Ω HIGH SPEED DIFFERENTIAL AMPLIFIER ANALOG INPUT + 0.1μF – + VCM 2.2μF 25Ω AIN+ 3pF AIN+ LTC2240-12 12pF CM – AIN– 25Ω AIN– 3pF 224012 F04 Figure 4. Differential Drive with an Amplifier VCM 100Ω 0.1μF 100Ω 2.2μF 25Ω AIN+ AIN+ ANALOG INPUT 12pF 0.1μF 25Ω LTC2240-12 AIN– AIN– 224012 F05 Figure 5. Capacitively-Coupled Drive 224012fd 16 LTC2240-12 APPLICATIONS INFORMATION wideband noise at the converter input. For input frequencies higher than 100MHz, the capacitor may need to be decreased to prevent excessive signal loss. + – The AIN and AIN inputs each have two pins to reduce package inductance. The two AIN+ and the two AIN– pins should be shorted together. For input frequencies above 100MHz 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 1.25V. In Figure 8 the series inductors are impedance matching elements that maximize the ADC bandwidth. 10Ω 2.2μF 0.1μF ANALOG INPUT 25Ω The 1.25V 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 1.25V reference output, VCM. This provides a high frequency low impedance path to ground for internal and external circuitry. The difference amplifier generates the high and low reference for the ADC. High speed switching circuits are connected to these outputs and they must be externally bypassed. Each output has four pins: two each of REFHA and REFHB for the high reference and two each of REFLA 12Ω AIN+ 0.1μF AIN+ T1 0.1μF LTC2240-12 8pF 25Ω 12Ω AIN– AIN– T1 = MA/COM ETC1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 224012 F06 Figure 6. Recommended Front End Circuit for Input Frequencies Between 100MHz and 250MHz Reference Operation Figure 9 shows the LTC2240-12 reference circuitry consisting of a 1.25V 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; typing the SENSE pin to VCM selects the 1V range. VCM 10Ω VCM 2.2μF 0.1μF AIN+ ANALOG INPUT 25Ω 0.1μF AIN+ LTC2240-12 T1 0.1μF AIN– 25Ω AIN– T1 = MA/COM ETC1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 224012 F07 Figure 7. Recommended Front End Circuit for Input Frequencies Between 250MHz and 500MHz 10Ω VCM 2.2μF 0.1μF 2.7nH ANALOG INPUT 25Ω 0.1μF AIN+ AIN+ LTC2240-12 T1 0.1μF 25Ω 2.7nH AIN– AIN– T1 = MA/COM ETC1-1-13 RESISTORS, CAPACITORS ARE 0402 PACKAGE SIZE 224012 F08 Figure 8. Recommended Front End Circuit for Input Frequencies Above 500MHz 224012fd 17 LTC2240-12 APPLICATIONS INFORMATION the SENSE pin is driven externally, it should be bypassed to ground as close to the device as possible with a 1μF ceramic capacitor. LTC2240-12 VCM 1.25V 2Ω 1.25V BANDGAP REFERENCE 2.2μF 1V 0.5V Input Range TIE TO VDD FOR 2V RANGE; TIE TO VCM FOR 1V RANGE; RANGE = 2 • VSENSE FOR 0.5V < VSENSE < 1V 1μF RANGE DETECT AND CONTROL 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 5dB. See the Typical Performance Characteristics section. SENSE REFLB BUFFER INTERNAL ADC HIGH REFERENCE 0.1μF REFHA 2.2μF Driving the Encode Inputs DIFF AMP 1μF REFLA 0.1μF INTERNAL ADC LOW REFERENCE REFHB 224012 F09 Figure 9. Equivalent Reference Circuit 1.25V 8k VCM 2.2μF 0.75V SENSE 12k 1μF LTC2240-12 224012 F10 Figure 10. 1.5V Range ADC and REFLB for the low reference. The multiple output pins are needed to reduce package inductance. Bypass capacitors must be connected as shown in Figure 9. Other voltage ranges in between the pin selectable ranges can be programmed with two external resistors as shown in Figure 10. An external reference can be used by applying its output directly or through a resistor divider to SENSE. It is not recommended to drive the SENSE pin with a logic device. The SENSE pin should be tied to the appropriate level as close to the converter as possible. If The noise performance of the LTC2240-12 can depend on the encode signal quality as much as on the analog input. The ENC+/ENC– inputs are intended to be driven differentially, primarily for noise immunity from common mode noise sources. Each input is biased through a 4.8k resistor to a 1.5V bias. The bias resistors set the DC operating point for transformer coupled drive circuits and can set the logic threshold for single-ended drive circuits. Any noise present on the encode signal will result in additional aperture jitter that will be RMS summed with the inherent ADC aperture jitter. In applications where jitter is critical (high input frequencies) take the following into consideration: 1. Differential drive should be used. 2. Use as large an amplitude as possible; if transformer coupled use a higher turns ratio to increase the amplitude. 3. If the ADC is clocked with a sinusoidal signal, filter the encode signal to reduce wideband noise. 4. Balance the capacitance and series resistance at both encode inputs so that any coupled noise will appear at both inputs as common mode noise. The encode inputs have a common mode range of 1.2V to 2.0V. Each input may be driven from ground to VDD for single-ended drive. 224012fd 18 LTC2240-12 APPLICATIONS INFORMATION VDD LTC2240-12 TO INTERNAL ADC CIRCUITS CLOCK INPUT VDD T1 MA/COM 0.1μF ETC1-1-13 • 1.5V BIAS 4.8k ENC+ • 50Ω 8.2pF 100Ω 50Ω 0.1μF ENC– VDD 1.5V BIAS 4.8k 0.1μF 224012 F11 Figure 11. Transformer Driven ENC+/ENC– 0.1μF ENC+ VTHRESHOLD = 1.5V 1.5V ENC– LTC2240-12 LVDS CLOCK 100Ω 0.1μF ENC+ ENC– LTC2240-12 0.1μF 224012 F12a Figure 12a. Single-Ended ENC Drive, Not Recommended for Low Jitter Maximum and Minimum Encode Rates The maximum encode rate for the LTC2240-12 is 210Msps. For the ADC to operate properly, the encode signal should have a 50% (±5%) duty cycle. Each half cycle must have at least 2.79ns for the ADC internal circuitry to have enough settling time for proper operation. Achieving a precise 50% duty cycle is easy with differential sinusoidal drive using a transformer or using symmetric differential logic such as PECL or LVDS. An optional clock duty cycle stabilizer circuit can be used if the input clock has a non 50% duty cycle. This circuit uses the rising edge of the ENC+ pin to sample the analog input. The falling edge of ENC+ is ignored and the internal falling edge is generated by a phase-locked loop. The input clock 224012 F12b Figure 12b. ENC Drive Using LVDS 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 one hundred clock cycles for the PLL to lock onto the input clock. To use the clock duty cycle stabilizer, the MODE pin should be connected to 1/3VDD or 2/3VDD using external resistors. The lower limit of the LTC2240-12 sample rate is determined by droop of the sample-and-hold circuits. The pipelined architecture of this ADC relies on storing analog signals on small valued capacitors. Junction leakage will discharge the capacitors. The specified minimum operating frequency for the LTC2240-12 is 1Msps. 224012fd 19 LTC2240-12 APPLICATIONS INFORMATION DIGITAL OUTPUTS Digital Output Buffers (CMOS Modes) Table 1 shows the relationship between the analog input voltage, the digital data bits, and the overflow bit. Figure 13a shows an equivalent circuit for a single output buffer in the CMOS output mode. Each buffer is powered by OVDD and OGND, which are isolated from the ADC power and ground. The additional N-channel transistor in the output driver allows operation down to voltages as low as 0.5V. 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. Table 1. Output Codes vs Input Voltage AIN+ – AIN– (2V Range) OF D11 – D0 (Offset Binary) D11 – D0 (2’s Complement) >+1.000000V +0.999512V +0.999024V 1 0 0 1111 1111 1111 1111 1111 1111 1111 1111 1110 0111 1111 1111 0111 1111 1111 0111 1111 1110 +0.000488V 0.000000V –0.000488V –0.000976V 0 0 0 0 1000 0000 0001 1000 0000 0000 0111 1111 1111 0111 1111 1110 0000 0000 0001 0000 0000 0000 1111 1111 1111 1111 1111 1110 –0.999512V –1.000000V