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Single-ended source impedance at input of AD8476

Question asked by salparadise on May 7, 2015
Latest reply on May 8, 2015 by ScottH



I'm considering using the AD8476 for converting a bipolar +/-9V single-ended signal from a Rs=50Ohm source into fully-differential signal going into a second variable gain differential amplifier, and then into a long twisted-pair cable, and finally into an ADC with differential input and Ri=800k.


On page 18 of the AD8476 datasheet it is mentioned that the source impedance should be kept below 0.1Ohm, to avoid unbalancing the resistor ratios.

Would having a Rs=50Ohm significantly degrade the CMRR?

I was thinking of not matching the input impedance of AD8476 stage since my signals are signals are low-frequency (<1MHz). Would you see an inconvenient with this? Should I still add a 50Ohm resistor at the grounded single-ended input to try to balance input resistances ?


If I want to add a filtering stage, would it be preferably to have it before or after the AD8476 stage? Maybe, if this was an active filter before the AD8476 I could try to decrease the source impedance seen by it. Any advises on topology and opamp?


Many thanks in advance,