Generate accurate 20mV Vref
With a shunt resistance of 10m&ohm; and maximum current of 0.4A rms the full scale input voltage will only be 4mV rms (11mVpp). Op amp input offset voltage could a similar amount so you may have difficulty getting it trimmed out, and the gain is very high so drift could also be a problem. I suggest using a higher value shunt to get more input voltage, eg. 100m&ohm; for 40mV rms. Then you can reduce gain so offset voltage and drift are less of a problem, and the reference voltage will be high enough to buffer accurately.  

If RV1 is fixed at 10k then you don't even need a buffer. Just give the voltage divider a Thevenin equivalent resistance of 10k and it will do the same job. 

For example:-







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