Current sensing and conditioning problem

hi pic,
OK.
Are you saying that the 'drift/offset' increases with time on the higher current test results.?? or is it present from 'cold' at first power ON.?

Looking at you image of the PCB, I would say its going to be very difficult to make any changes to the current return paths on the pcb.

Have you checked that the CC source is providing a constant 200mA thru the 1.2R resistor also
have you measured the 'offset' directly across the 1.2R, using a calibrated multimeter.??

ericgibbs said:

hi pic,
OK.
Are you saying that the 'drift/offset' increases with time on the higher current test results.?? or is it present from 'cold' at first power ON.?

Click to expand...

Yes.it is present from 'cold' power ON!

ericgibbs said:

Looking at you image of the PCB, I would say its going to be very difficult to make any changes to the current return paths on the pcb.

Click to expand...

PCB can be re-designed. But if each ground path is to be routed as shown in your schematic, I think considering the number of channels, it will be better to go for differential amplifier. What about that?

ericgibbs said:

Have you checked that the CC source is providing a constant 200mA thru the 1.2R resistor also
have you measured the 'offset' directly across the 1.2R, using a calibrated multimeter.??

Click to expand...

When I maintain a 200mA continuous load ,it slowly changes. This application can tolerate such drifts because the test results has wide acceptable ranges. But showing a different value even at switch on at low currents during demonstration is a bad idea!
After the first phase is over I may modify this with precision shunt resistors.

I cannot say that I am using a high quality calibrated multimeter, but reasonably accurate. But I can see comparative differences in readings anyway. In the extreme low current ranges the op-amp output seems slightly attenuated comparing to high voltage levels.
Thanks for the effort.