DIfferential amplifier filter question

[teslinius]

I use a differential amplifier with op07 to amplify voltage drop of a shunt resistor, i set the gain to 17.4. The problem the output of the opamp is very noisy, supply voltages are clean (< 10mV), but the problem is that it's an electronic load, has common gnd and so the noisy voltage from cheap smps that i'm testing gets in the opamp input, so i need to add a filter, or maybe there is a different way to solve this?,  i'm gonna try rc, since the opamp inputs use very low current (< 1uA) i can use few k value resistors and 100nF cap, not sure if i should place it on the both inputs or on the output or even both. I found a few schematics on the internet that look ok, but the problem is that max voltage will be 150V, and if i place a cap between input and gnd the difference of potentials will be 150V which will require a physically bigger cap, so i should place it between the inputs like in some schematics

[Terry Bites]

Unless the components are very well matched, your CMRR will be degraded. If one or both inputs go outside the supply rails you'll have an electroboom experience. I assume you are measuring current from a high side shunt. If so you'll be much better off using a made for purpose HV diff amp like a AD8479 or INA149 with a post filter. Put some back to back diodes across your shunt to limit the spike amplitudes.

[teslinius]

Thanks, i didn't know you can get already premade differential opamps, i am using 0.1% 25ppm resistors so they should be pretty well matched, so diodes like this, or no probably they should be placed before the resistors?, and a rc filter on the output?

[Terry Bites]

The voltage across the shunt is tiny compared to the Vf of a diode. Put them them at the amp inputs. Add the low value resistors and a filter cap to polish off some noise. Experiment with caps but keep R low to avoid upseting the balance of the amp's input.