AC coupling of the sensor bridge to match the InAmp

[Junus2012]

Hello,
I am using a special type of differential amplifier, it is called a differential difference amplifier (DDF), basically it is a non-inverting fully differential amplifier

This type of amplifier has a common problem, that is the input common mode voltage should be equal to the output common mode voltage that is set by the CMFB circuit to fit to the ADC.

However, the input signals that cames from the sensor bridge has may have different common mode voltage from the VCM of the amplifier, hence my amplifier is failing to operate.

I don't want to use a typical fully differential amplifier before my DDF to fix the problem of the common mode voltage, rather I want to use AC capacitor coupling that block the DC voltage of the sensor output, and biasing the inputs of the DDF using a voltage divider circuit with the same voltage of the CMFB VCM voltage

Thank you in advance
Regards

 

[danadakk]

The FDDA has a Vocm pin typically, so you have the source for error
correction. Question is how to bias up the inputs which have DC fdbk loop
applied to them. For this I will pass the buck to our more site knowledgeable
assistants.

This ap not no help but quite in-depth -

https://www.ti.com/lit/an/sloa054e/sloa054e.pdf?ts=1641014311753&ref_url=https%253A%252F%252Fwww.google.com%252F

But a question comes to mind, since you have a differential ADC being used
why not just do the FDDA output determination with the adc ? I know of ARM
chips with 20 bit DelSig differential inputs, and input buffer amps and microvolt
capability that should be able to handle this. Just curious.

Regards, Dana.