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Source impedance for driving difference amplifiers

Question asked by zea on Dec 17, 2013
Latest reply on Jan 9, 2014 by KurtBecker



I am designing a conditioning circuit for a MEMS analog microphone and I would like to use an IC for single-ended-to-differential conversion in order to have a balanced line output capable of driving twisted-pair cables. I found that the difference amplifier AD8273 is recommended for such a task in AN-1165 (Fig. 9, pp. 6), and it is directly connected from the output of the MEMS to the inputs of the AD8273 (via the AC-coupling cap. C1).


Since my application demands constrained PCB dimensions (as small as possible) and low power, I would rather use the AD8476, particularly the circuit found in its datasheet Fig. 54, pp. 19. However in such a circuit I would expect that the source impedance, depending on its value, demands the need of a voltage follower right between the source and the input terminals of the AD8476; mentioned in the section "Driving the AD8476", pp. 18, and it should be kept below 0.1 ohm. In other words, an additional but undesirable op-amp should be included in the design.


So my main question would be to what extent the MEMS output impedance (say 200-450 ohms) will unbalance the resistor ratios of the AD8273/AD8476 and affect its performance? Are there any other alternatives for smaller/more compact designs capable of dealing with the MEMS impedance and providing a differential output? I was already thinking of the AD8295.


Many thanks in advance for your kind attention and support, I look forward to hearing from you.


Best regards,

Elías Zea