ABCs of ADCs - Analog-to-Digital Converter Basics (PDF)

A Better Reference CircuitLM4040-4.110KEXTT+5V+5V10uF, 6V0.10.1 +5V82-750 1002N39042K4.7K+1.5K7502K+5V0.11.5K470+1002N3906--V0.110010K -V10 uF, 6VEXTB53This reference circuit is not only much simpler than the previous one, it is much more stable.One key factor is to choose a slow op-amp. To ensure stability, high frequency gain is killedwith a 0.1uF capacitor in each op-amp feedback loop. The transistors are emitter followers,providing current gain, but no voltage gain, which could cause oscillation.The reference circuit of many ADCs have switches connected to a resistive or capacitiveladder. These switches cause external current pulses at the reference pin(s) as they open andclose. A reference drive circuit that is not capable of driving such a load and settling fastenough will result in noise on the reference pin(s), further resulting in noisy conversions. It isnot necessary to eliminate these voltage spikes, but they must settle before the samplingswitch opens.A reference drive circuit that will not provide enough current to drive the ADC reference inputwill not be able to hold the reference stable, resulting in noisy conversions.The circuit shown here is a good design for use as a reference driver for ADCs like theADC1175 and ADC1175-50. Just modify the input dividers for other reference voltages forother ADCs. The potentiometers, of course, could be replaced with a fixed divider, butcomponent tolerances must be taken into account to prevent the possibility of the input signalgoing beyond the resultant valid range for digitization, which would result in output clipping.ABCs of ADCs - Rev 3, June 2006Authored by: Nicholas “Nick” Gray53Copyright © 2003, 2004, 2006 National SemiconductorCorporationAll rights reserved