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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

Hello,

 

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,

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