OPA810: Do we still need the second amplifier at GND side

Hi Liping,

You could definitely remove the second amplifier and set that non-inverting input of the FDA directly to ground. The potential problems lie in the scenario that you have poor connections to ground. For example, if there are poor layout practices and the ground at the resistor divider has some slight error voltage and the ground at the non-inverting input is truly 0V or some other voltage, this offset would get amplified by the FDA stage. The error voltage at the resistor ground will be present at the inverting input of the FDA and amplified but is not present at the non-inverting input as it is now at a different potential than the error voltage. The addition of the OPA810 would bring this error voltage at the ground of the resistor to the non-inverting input and now both inputs of the FDA will have this error voltage and eliminates the potential of an offset at the inputs.

The OPA810 acts as a high impedance buffer allowing you to truly measure the voltage difference of the second resistor in the divider you added to the design.

If a high level of precision is not required, you should be able to remove the amplifier and tie that node directly to ground and still get expected performance.

Best regards,

Ignacio