I am trying to use the OP1177 as a differential amplifier to measure a DC current shunt. It will have a gain of 50, 2k and 100K ohm. Most of the time the op1177 will be reading the shunt from the low side. It also needs to be able to read a high side shunt. The high side maximum is 200Vdc. The data sheet doesn't specify the CMV, probably due to the reliance on the external resisters. The only paths for the CMV are the Rg resister to ground and opamp output via the Rf resister. I don't think the CMV will be a problem due to the fact that the opamp is powered by an isolated power supply. The common Rg is attached to the isolated power supply return. So there is no current path between the input CMV and ground/common. Can the OP1177 withstand +-200vdc without a current path?
Thank you for your interest in ADI products. The OP1177 has an operating input range of +/-13.5V for both the inverting and non-inverting pins for a supply voltage of +/-15V. That being said, I don't think the OP1177 will be able to withstand +/-200V at its input pins. I am interested though in your circuit, do mind sharing a picture of your circuit schematic?
Normally, for best performance, current sensing employs an integrated difference amplifier. Integrated difference amplifier, such as AD629, has superior CMRR compared to discrete difference amps because of the tightly matched internal resistors in IC difference amps. AD629 can handle common mode voltage of +/-270V. It also has a CMRR of 86dB minimum, which is far better than the 66dB of a discrete difference amp with a net matching tolerance of 0.1% in the resistor ratios.
AD629 is unity gain and since you want to amplification, you can employ a gain stage after the AD629. The common mode level at this stage is not as high as the inputs of the AD629, thus you can use any precision amplifier depending on your other requirements.
You may want to read more on Difference and Current Sens Amplifiers on this Mini Tutorial. You might also be interested on this Circuits from the Lab reference circuit.
I hope this helps.