Application Note 994: 2002 Reference Voltage Multiple ADCs This p

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CONVERSION/SAMPLING CIRCUITS HIGH-SPEED SIGNAL PROCESSING VOLTAGE REFERENCES
Application Note 994: 2002
Reference Voltage Multiple ADCs This paper discusses successful implementation reference circuits tailored space-saving needs ultrasound imaging applications, which commonly encounter demand reference source power multiple analog-to-digital converters (ADCs). Finally, circuits were test their results discussed following note.
achievable accuracy systems with multiple analog-to-digital converters (ADCs) depends directly reference voltage(s) applied ADCs. Medical ultrasound imaging systems, example, commonly include large number ADCs receiver's beamformer electronics, usually organized groups etc. Maximum beam accuracy requires that minimize errors this path. Poor accuracy reference voltage individual ADCs degrades overall system, does distributed load, which consists many individual resistive capacitive loads. Several approaches provide reference voltage such arrays:
Individual on-chip references: Though offers convenient connection locally each ADC, this option features relatively poor matching between converters. single external reference voltage applied reference inputs array: Such configuration lets engineer external reference voltage arbitrary accuracy, incurs error small variations among resistor ladders (one ladder internal each ADC). external reference driving ADCs' reference ladder taps directly: This option delivers maximum gain accuracy directly controlling reference voltage applied each ladder. However, requires driving (relatively) resistance ladders, some ADCs allow access that internal bias point.
Accuracy many applications, gain noise level have major effect accuracy. gain represented slope transfer function, which relates analog inputs allowable range digital output codes. quantify gain measure full-scale (FS) input range, which directly controlled reference voltage level. medical ultrasound imaging systems, variation full-scale range ADCs cause errors beam formation. This also varies ADCs' clipping point-an effect that important certain signal demodulation schemes.
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ADC's noise level determines useable dynamic range; this dynamic range should generally large possible. reference noise component noise additive multiplicative. Additive noise easily filtered local bypass capacitors individual ADCs, which most designs already present optimize ADC's dynamic performance. Multiplicative noise, other hand, more insidious. ultrasound applications, reference noise audio frequency spectrum modulate large "stationary" signals spectrum. Such signals produced stationary tissue ultrasound target. Audio modulation produces sidebands signal that demodulated Doppler detector, producing audio tones detected Doppler output signal. estimate amount audio noise tolerable ultrasound application, assume nearly full-scale signal applied 10-bit like MAX1448. dynamic range that device (almost 60dB) implies noise floor -60dBFS. That noise level normalized 1Hz-bandwidth. Nyquist bandwidth 80MHz sampling rate would 40MHz. correction factor (40MHz) 76dB, which places ADC's noise floor -60dBFS 76dBFS -136dBFS. Because conservative design requires reference voltage noise least 20dB lower (-156dBFS), +2.0V reference requires extremely noise level 33nVp-p (approximately 8nVRMS/ Hz). array with multiple ADCs require reference voltage more accurate than that internal each converter (the reference internal MAX144x converters, example, accurate). following circuits submitted reference designs such arrays. They feature single common low-frequency noise filter, they achieve high-frequency noise suppression with local decoupling capacitors individual ADCs. Single External Reference Multiple-converter systems based MAX144x family well suited with common reference voltage. REFIN those converters connected directly external reference source, eliminating need circuit modification. Moreover, high input impedance REFIN (even multiple REFIN terminals connected parallel) draws only small load current. precision source like MAX6062 (IC1) generates external level +2.048V (Figure exhibits noise voltage density 150nV/ output passes through 1-pole lowpass filter (with 10Hz cutoff frequency) (IC2), such MAX4250, which buffers reference before output applied second 10Hz lowpass filter. (MAX4250) provides offset voltage (for high gain accuracy) noise level. passive 10Hz filter following buffer attenuates noise produced voltage reference buffer stages. This filtered noise density, which decreases higher frequencies, meets noise levels specified precise operation.
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Figure ultrasound applications, single low-noise reference circuit drives 1000 ADCs.
converters MAX144x family specified typical gain error ±4.4% (better than ±0.5dB). This performance better than gain tolerance other building blocks signal path ultrasound receiver. Note that proper power-up/-down sequencing ensured because active parts driven from same supply voltage. This approach yields excellent gain matching very noise level with minimal circuitry, should suffice many applications that require multiple gain-matched ADCs. Generating Precision External Reference applications requiring more stringent gain matching (Figure MAX144x family again well suited. Connecting each REFIN analog ground disables internal reference each device, allowing internal reference ladders driven directly external reference sources. These voltages have arbitrarily tight tolerance, ADCs track them typically within 0.1%. ADCs this family also feature resistance across ladder
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reference connection, which allows load easily driven even many ADCs operated parallel.
Figure Also ultrasound applications, precision low-noise reference circuit drives ADCs.
level +2.500V generated precision source (IC1), such MAX6066, followed 10Hz lowpass filter precision voltage divider. buffered outputs this divider +2.0V, +1.5V, +1.0V, with accuracy that depends tolerance divider resistors. Those three voltages buffered quad (MAX4254), which selected noise offset. individual voltage followers connected 10Hz lowpass filters, which filter both reference-voltage buffer-amplifier noise level 3nV/ +2.0V +1.0V reference voltages differential full-scale range associated ADCs 2Vp-p. +2.0V +1.0V buffers drive ADCs' internal ladder resistances between them: divided number ADCs circuit. example, ADCs
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will draw from those supplies-a load current well within capability (MAX4252). gain accuracy this configuration almost arbitrarily good, depending accuracy grade (here: MAX6066) tolerance resistors voltage divider. gain matching each such configuration typically 0.1%. With noise level below 3nV/ 100Hz, this circuit provides exemplary performance. Figure common power supply active components removes concern regarding power supply sequencing when powering down. With outputs amps matching better than 0.1%, these buffers subsequent lowpass filters replicated support many ADCs. applications that require more than matched ADCs, voltage reference divider string common converters highly recommended. Summary Systems requiring large numbers data converters with good channel-to-channel matching require careful design voltage reference system. common, high precision, noise reference driving ADCs valuable approach achieving high accuracy matching. flexible reference inputs exceptional dynamic performance 10-bit ADCs MAX144x family make them compelling candidates such applications. Reference Maxim Data Sheets MAX1444 Rev. 7/00. MAX1448EVKIT Rev. 9/00. MAX4249-MAX4257 Rev. 4/98. MAX6061-MAX6067 Rev. 4/00.
similar version this article appeared January 2002 issue magazine.
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Application Note 994: 2002 Reference Voltage Multiple ADCs This p

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