Driver Amplifiers For Analog-To-Digital Converters Don Tuite Analog/Power Editor FREQUENTLY ASKED QUESTIONS ED Online 20791 What amplifiers are used to drive analog-to-digital converters (ADCs)? Possibilities include single-ended and differential inputs and outputs, plus voltage feedback (VFB) or current feedback (CFB) in the control loop. Specialized amplifiers may provide level shifting, interstage isolation, single-ended to differential conversion, differential to single-ended conversion, plus attenuation or gain. What are the considerations with VFB and CFB amplifiers? RF +VS +DIN –DIN RG RG +IN VOCM VOUT, dm –IN –VS RF 1. Some differential voltage feedback amplifiers include an additional VOCM input that allows the 0326FAQs-FIGURE 1 common-mode voltage of the output signal to be shifted. without requiring tightly matched external components. Thus, differential outputs are very close to the ideal of being identical in amplitude and are exactly 180° out of phase. Also, if it is necessary, preserving the dc content of a signal can be accomplished via the VOCM function. When would I need a single-ended, attenuating, level-translating ADC driver? How does it work? Industrial applications often involve sensors driven by ±10-V signals. That’s a With CFBs, closed-loop gain is largely problem with single-ended input ADCs independent of frequency. Also, CFB fabricated to today’s smaller design amps provide faster slew rate and lower ing and non-inverting inputs, some dif- rules because those ADCs are condistortion and perform well at higher ferential VFB ADC drivers have another strained to a smaller input signal swing. gains than VFB amps. VFBs can offer input, VOCM, that shifts the common- A level-translating ADC driver takes a lower noise and better dc performance mode voltage of the differential output large signal, reduces the amplitude, than CFB amps. Other tradeoffs lie (Fig. 1). Like a VFB op amp, closed-loop and level-shifts the output commonin design constraints. With a VFB op gain is set by means of input and mode voltage so it is compatible with amp, the circuit designer has consider- feedback resistances, but there must low-voltage, single-supply ADCs (Fig. 2). able freedom in choosing the value of be separate, matched resistors for the For example, a 20-V p-p (±10-V) input the feedback resistor, although higher inverting and non-inverting inputs. signal riding on 0 V might be converted resistance values may limit stability. The internal common-mode feedback to a 4-V p-p signal riding on 2.5 V. CFB amplifier datasheets specify the loop produces outputs that are highly There are a number of other ways to feedback resistor values. CFBs therefore balanced over a wide frequency range perform level-translation. It has been lend themselves to applications accomplished using multiple that require higher gain levels. amplifiers, single differential driv+5 V ers like those described above, What are the advantages of or ADC drivers designed for +4.048 V differential ADC drivers? level translation. The approach +2.048 V +VS +10 V +0.048 V These drivers facilitate singleusing a single differential driver SENSE –IN ended-to-differential and is simpler than the multi-ampliVDD – IN+ OUT differential-to-differential confier approach, and the specialversions, common-mode level function level translation driver IN– + GND REF shifting, and amplification of approach is simpler yet. VIN REF2 differential signals. They also –10 V Such amps use internally laser+IN VREF REF1 exhibit lower distortion and trimmed resistors, ensuring high –VS faster settling time than singlegain accuracy, along with high ended drivers. common-mode rejection and low offset. A final advantage is that, How can a differential VFB 2. A level-translating driver, optimized for specific ADCs, provides since the amp and ADC use the ADC driver differ from a a convenient way to accurately match the wide voltage output of same supply voltage as the ADC, single-ended amplifier? sensors excited by ±10 V (common in industrial applications) to a there is no need for multiple In addition to the usual invert- single-ended ADC input with a more restricted voltage range. power supplies. 0326FAQs-FIGURE 2 Sponsored by Analog Devices a d v e r t i s e m e n t If a driver has a 1-GHz, –3-dB bandwidth, can I use it at that frequency to drive converter inputs? If you’re driving a high-resolution ADC, look beyond the –3-dB spec and consider gain flatness and, in particular, harmonic distortion as a function of frequency. Recall that in a VFB amp, the –3-dB bandwidth figure simply reflects the half-power point after the amp’s open-loop gain starts its –6-dB/ octave roll-off. That provides a rough figure for comparing amplifiers. Your concern as a mixed-signal circuit designer must be to minimize the effect of amplifier distortion on the ADC’s effective number of bits (ENOB) performance. ENOB is a function of signal-to-noise ratio (SNR) + distortion (SINAD) across the whole analog signal chain: ENOB = (SINAD – 1.76)/6.02. So, look to the data sheet graphs of harmonic distortion to make your decision. Drivers That Maximize ADC Performance Voltage Feedback Differential ADC Driver Offers Desirable Distortion/Power Consumption Ratio At 70 MHz The ADA4939-1 achieves 82-dB spurious-free dynamic range (SFDR) at 70 MHz while consuming less than 120 mW of power on a single 3.3-V supply. The device can be used in either single-ended-to-differential or differential-todifferential configuration. Ultralow-DistoRtion CurrentFeedback Differential ADC Driver The ADA4927 is a low-noise, ultralow-distortion, highspeed, current-feedback differential amplifier that is an ideal choice for driving high-performance ADCs with resolutions up to 16 bits from dc to 100 MHz. Single-Ended, Attenuating, Level-Translating ADC Driver The AD8275 is a G = 0.2 difference amplifier that can be used to translate ±10-V signals to a +4-V level. The device has fast settle time and low distortion, making it suitable for driving 16-bit SAR ADCs. Why would I want to use an active driver instead of a passive transformer? To view Analog Devices’ complete list of ADC drivers, please visit www.analog.com/ADCdrivers. The main reasons are to get better pass-band flatness and to isolate the signal from the noisy ADC input. Transformers have a rather “lumpy” frequency response. An amplifier should produce less variability, typically ±0.1 dB over the frequency range. If the design calls for wideband gain, an amplifier provides a better match than a transformer to the ADC’s inputs. Still looking at frequency response, some amplifiers provide dc coupling. Transformers can’t deal with slowly varying signals. Because transformers are passive devices and provide no interstage isolation, noise generated on the secondary coil of the transformer from the ADC input will pass through it back to the original signal source. In contrast, amplifiers buffer the signal source with a low output impedance, providing 70 to 80 dB of interstage isolation from the ADC input back to the original signal source. On the other hand, a consideration that favors transformers is that at higher frequencies, they may maintain better SNR and spurious-free dynamic range (SFDR). Nevertheless, within the first or second Nyquist zone, a transformer or an amplifier can be used. Differential ADC Drivers Supply Voltage (V) -3 dB BW (MHz) Slew Rate (V/μs) HD2/ HD3 (-dBc) @ BW (MHz) Voltage Noise (nV/ √Hz) ADA4938-1 5, ±5 1000 4700 82/82 50 ADA4939-1 3, 5 1400 6800 83/97 70 ADA4927-1 5, ±5 2300 5000 87/89 100 Part # IS/ Amp (mA Typ) IS/Amp (mA Typ) Price ($U.S./ 1K) 2.6 37 16-LFCSP 3.84 2.3 36.5 16-LFCSP 3.84 1.4 20 16-LFCSP 3.79 3.63 AD8139 3, 5, ±5 410 800 85/89 1 2.25 24.5 8-LFCSP/ SOIC AD8138 3, 5, ±5 320 1150 94/114 5 5 20 8-SOIC/ MSOP 3.8 3, 5 1900 6000 84/91 70 2.2 39.5 16-LFCSP 3.84 3 to 5.5 2500 9000 82/84 140 2.7 37 16-LFCSP 3.49 12 8-SOIC/ MSOP 1.67 2.2 8-LFCSP/ SOIC 2.42 Package Price ($U.S./ 1K) 8-MSOP 1.6 ADA4937-1 AD8352 AD8132 ADA4941-1 3, 5, ±5 2.7 to 12 350 30 1200 22 83/98 101/98 5 0.1 8 10.2 Difference Amplifier for Driving ADCs Part # AD8275 Supply Voltage (V) 3.3 to 15 -3 dB BW (MHz) 15 Settling Time to 0.001% (ns) 450 Input Voltage (VPP) 20 Output Voltage (VPP) Output Type 4 Singleended Voltage Noise (nV/ √Hz) 40