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Explanation and solution for a 10mVof an error in Differential Amplifier Current Soruce circuit

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Explanation and solution for a 10mV error in Differential Amplifier Current Soruce circuit

So ive built up the following schematic as a simple current limit circuit. When the current limit is not in place, ie the PNP is conducting "fully", I null any offset errors in the Differential Amp (CA3140EZ Op Amp) which nicely matches the actual voltage drop across the Shunt Resistor and output of the Diff Amp (Pin 6). Everything works perfectly until the current limit kicks in; when this happens the Diff amp always outputs 10-15mA less than the actual Voltage drop across the Shunt. If i now lift the current limit again by increasing ISet the two outputs match perfectly again.

I have spent a lot of time finding the cause of this but to no avail. Can someone please explain to me what is happening and how i can remove this error?

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