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Single Supply OpAmp with fully differential input on floating groud

Question asked by Damien on May 28, 2014
Latest reply on May 28, 2014 by Damien

Hello there,

 

I've been trying to do a very high accuracy acquisition (>0.05%) system based on instrumentation amplifier and 16bit ADC to measure mostly DC voltage signals.

 

I had a previous design that had the instrumentation (AD8426) op amp ref tied to the ground on y single supply. The circuit is not grounded and virtually floating by galvanic isolation. 

It worked good with good accuracy and less than 1LSB noise, however the input range needed at least to be over 140nV to be within specified accuracy.

 

 

To fix that problem, I've reviewed the design and integrated an offset to the op amp ref to be able to measure negative values. As i didn't want to have dual supply to avoid adding too much components, I've tied the input IN- to the opamp ref offset so that the differential input is offset-ed within the range of the opamp. Since the ground is floating, actually what happens is that the opamp ref value become the ground of the measured device while the floating ground of the circuit is at - opamp ref value.

 

It seem to work, however i'm experiencing higher noise and less stability. Have anyone any suggestions on how to improve this design without going to dual supply configuration ?

 

Thanks

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