1.>This is Decoupling Capacitor, the Purpose is to make OP of the PSRR Better.
2.> you can simulate the OP PSRR, with this Decoupling Capacitor , the PSRR better.
3.> Generally, Always at Decoupling Capacitor at "BIAS node"
the AVDD decoupling Capacitor should be workable if the AVDD has a step response Noise.
If the capacitor value too small ?
generally,
the capacitor value need greater than 1pF .