# Explanation and solution of an 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?

• What is the Vcc used in this circuit? – rdtsc Jul 30 '15 at 11:50
• Im using a 12 volt wall adapter – Adil Malik Jul 30 '15 at 11:52
• Have you looked at the CA3140 output with a scope (during current limit)? – Spehro Pefhany Jul 30 '15 at 11:58
• Yes i did just now, and it seems clean both with and without the current limit. No Oscillations. – Adil Malik Jul 30 '15 at 12:44

As for this amp's true "rail-to-rail output" capability, it simply says "Output Swing Complements Input Common Mode Range", which is then defined as "Wide Common Mode Input Voltage Range (VlCR) - Can be Swung 0.5V Below Negative Supply Voltage Rail". This is ambiguous... is it a true rail-to-rail opamp or not? You should definitely connect a 'scope to it's output and see what is going on as Spehro hints to. There could be high-frequency oscillations, solvable with a small (pF) capacitance across it's inverting feedback loop.