Over a 200ms period of which this circuit regulates a 500mA current, the current drift downwards for about 0.4%.
At first I thought it was due to R202 temperature coefficient, but from my calculation the drift related to R202 should only be in the range of 0.05%.
R202 is a 2ohm, 2W, 2 point, 100ppm/°C resistor and from the touch it barely heats at all.
Any other idea where could be the source of the drift ? Solder of the resistor pads ?
The input at R198 is stable.
UPDATE 1:
To my surprise, this morning I restarted the circuit, it hasn't moved a bit overnight, and measured the drift, before touching anything. It was down to 0.1%, the night before it was about 0.4%.
Now as time pass, the drift is increasing little by little and I guess in 1h it will be back to 0.4%.
For my tests, I do a 200ms pulse every 1s and is constantly running. The circuit does heats up a little bit.
UPDATE 2:
Here is a graph with 3 measurements done at different times. As one can see, the first two have about 0.15% drift, the last one has 0.7% drift.
The drift seems to vary over time, and so far I've failed to understand the reason nor to find a way to influence the drift.
The circuit hasn't change between those measurements, I've played around with power supply, voltage rails, I tried to heat the circuit up, but no changes.
UPDATE 3:
Removing C121 and C122, affect the stability, but the drift remains the same.
Best Answer
Apparent large DC drift can be, and in this case, almost surely is, caused by oscillation.
Remove C123, it is negatively affecting stability. Swap the values of R200 and R201 and it should be stable.
The purpose of R201 is to isolate the gate capacitance from the output impedance of the op-amp so it should be in the 100 ohm or so range for a normal op-amp and maybe 1K for a very low power op-amp.
R200 and C122 should form a lower frequency pole than the gate capacitance and R201.