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AD7705 DIFFERENTIAL INPUT RANGE

Question asked by RM24 on Sep 12, 2015
Latest reply on Sep 16, 2015 by jcolao

Hello

I'm working on a design where I will have a varying ac-signal of 1v. This signal is galvanically isolated from the rest of my system (or rather, my entire circuit is working on an isolated supply). My question relates to the differential input range on the AD7705 which I intend to use on this design.

 

My understanding is this- considering that it is a true differential input, I should be able to directly couple the input signal to AIN(+) and AIN(-). However, I have a doubt with the range. The datasheet on page 22 says this:

 

"The analog inputs on the AD7705/AD7706 can accept either

unipolar or bipolar input voltage ranges. Bipolar input ranges

do not imply that these parts can handle negative voltages on

their analog inputs; the analog inputs cannot go more negative

than −100 mV to ensure correct operation of these parts. The

input channels are fully differential. As a result, on the AD7705,

the voltage to which the unipolar and bipolar signals on the

AIN(+) input are referenced is the voltage on the respective

AIN(−) input."

 

My absolute range is definitely within the common mode range as also within REF(+) and REF(-). So would this work? Or would the part be damaged when AIN(+) swings -1v with respect to AIN(-)? In that case, would the correct solution be biasing the input to 1.25v assuming a 2.5v reference (AIN(+) still goes negative with respect to AIN(-), but is above REF(-)) ? Or I have another solution which uses a 100mV peak to peak signal, which I assume should also work.

 

Thanks

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