INA851: adding differential dc bias to a fully differential instrumention amplifier

HI Electronx,

I am not aware of an accurate way to add differential signals with precision, while using the INA851 instrumentation amplifier stand-alone.

Typically, fully-differential amplifiers (FDAs)  are used to add differential signals.

The INA851 provides access to the FDA_IN- and FDA_IN+, fully-differential output stage inverting and non-inverting terminals. These inputs are primarily intended to add external capacitors for stability and/or noise filtering. It is technically possible to place external 5kΩ resistors between FDA_IN-  and FDA_IN+ inputs and inject the differential DC bias signal, however, please keep in mind, this will cause gain accuracy errors due to the mismatch in absolute value and mismatch of drift of the external 5kΩ resistors, versus the absolute value and drift of the INA851 FDA output stage internal 5kΩ resistors. The INA851 R1-R6 output stage internal gain resistors are accurately ratiometrically matched among each other, to provide very low gain error and very low gain error drift; but their drift and absolute value will not be matched to the absolute value and drift of the 5k external resistors, causing gain error and gain drift error.  Although the internal resistors of the output stage are precisely matched among each other, their absolute value could vary up to ±15% over process variation and temperature:

- An accurate way to add differential signals would be to use a fully-differential amplifier (FDA).  In the example below, the INA851 is at G=10V/V gaining the VIN1 differential signal, and the THP210 (FDA) is used to add a differential offset VIN2 differential signal, after the differential signal VIN1 is gained up by the INA851.

Thank you and Best Regards,

Luis