I am using an OP77F as a differential amplifier to amplify the voltage across a shunt resister. The shunt resister is 0-50mV for how many thousands of amps runing through it. The shunt resister is on the low side, on the common cable of the DC lines.
The circuit i am using is figure 27 on the datasheet. http://www.analog.com/static/imported-files/data_sheets/OP77.pdf
I have a gain of 50, i have tried 200 and 10K, 2K and 100K, 20K and 1Meg, 1K and 50K. in all cases there is a negative voltage on the output that ranges between -50mv to -120mv depending on gain resister group at zero input. This negative offset shows up through out the input range. so when i put in 20mV i get .9V instead of 1volt, the offset voltage for this gain resisters in -102mV. No offset maladjusted is used, and this is build on a push board. I noticed the offset error gets lower as the gain setting resisters get lower in value.
Is there something i need to be careful about to about this problem?