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I have 2 stage OPamp with DC gain=90dB. After configured as below, the overall open-loop gain become 50dB.
But the dc offset still lower than 0.2mV, why almost no difference?
Does DC gain (open-loop gain) has a direct relation to DC offset?
DC input offset of an OP has to be measured under specific conditions. Observed input difference voltage is also affected by other OP parameters and operation conditions, e.g. common mode rejection and actual common mode voltage, loop gain and output voltage.
As you are biasing input voltage and output load to mid supply, these additional effects are cancelled.
Depends on your specification and operation parameters. Did you check how the "offset" changes when you vary the input voltage? Loading a high output impedance OP with 500 ohm load isn't a good idea, I think. I suppose the voltage swing will be rather limited.