INAMP CMRR

[Harvs]

I was making a high side current measurement circuit using a INA126PA INAMP, as that's what I had on hand. 

Anyway, I'm scratching my head over the CMRR, so please tell me what I've got wrong here.

- Power rails to the INAMP are +/- 15V.
- The output ref pin is at the center of the +/- 15V rails.
- Gain is set to 100x with an external resistor.
- With both inputs shorted to the ref pin (i.e. zero CM voltage), the output is about 3mV, which will be the input offset voltage x100.
- With the inputs shorted together, and raised 5V above ref, the output is 310mV.
- With the inputs taken to -5V from the ref pin, the output is -300mV.
- Other than the surprisingly high CM output, the circuit works as expected (i.e. the correct gain of 100x is seen on a differential input voltage with all tested CM voltages.)

So according to my calcs, this doesn't add up.  Minimum CMR from the datasheet is 74db.

10^(74/20) = 5012 = diff gain / cm gain
therefore, CMgain = 100/5012 ~= 0.02
So the output should be < 0.02 * 5 = 100mV

[tszaboo]

Series resistance on the REF pin can degrade the CMRR by magnitudes. Are zou driving it with a low impedance source?

[Harvs]

  moment...

Yes that's pretty much it.  I've got an offset trim circuit feeding the ref pin similar to the one described in the datasheet.  But instead of hooking the ref pin to the output of the buffer, I stupidly hooked it to the wiper of the 10k pot.  Problem solved, output due to CM is now in the 1 or 2 mV.

[tszaboo]

That is all right... I've seen it in production