Hi,

I'm trying to find a pot value that will null my op amp's input offset (8mV typical).
First, I shorted Vinp and Vinn together and calculated effective Zin value that will give me Vout=0 which resulted negative value for Zin (-352.5Ohms)
Negative Zin value makes me to think this is not the right way to find the pot value. Is this the right way to do it?
How can I find a pot value that will null the offset?

Thanks!

 

I'm not sure you can do it that way. Some opamps have null adjustment pins (eg 741 & TL081) which work internally on both legs of the differential input circuit.

 

Any impedance from the pot at the point will degrade the common-mode rejection of your differential amp circuit.

One way to adjust the offset without degrading CM rejection is to remove the R4 ground connection and connect it to the output of an added opamp.
That op amp is configured as a follower with its plus input connected to the pot.
That way you can inject a offset correction voltage without affecting the impedance at the input nodes.

The LTspice simulation of such a circuit is below:
The values shown give a pot U3 offset adjustment from -25mV to +25mV.

R5 is needed to minimize the sensitivity of the offset correction voltage to power supply voltage change.
Without R5 the offset voltage would change by 1/2 of any such power supply change.

(Note that 470Ω resistors are too low for most standard op amps to drive so I increased them to a more typical value.)

All in all it would be much simpler just to use a low offset opamp.