We are using ADA4891 for sample and hold circuit , but would like to know if the Amplifier be stable when the capacitor drains of the
charge leaving the input similar to that of Floating state.
We are using a switch before the capacitor connected to ADA4891 which used in unity gain.
With best regards and wishes
Apology for the very late reply.
As the switch acts as an open switch, the capacitor will almost retain its voltage due to the very high capacitor discharging time caused by the very high input impedance of ADA4891 (5Gohms). Therefore, the output of the ADA4891 will be equal to the voltage across the capacitor (gain of 1) if and only if the voltage across the cap is within the Input Voltage Range (shown in the spec table of the ADA4891 datasheet).
If the capacitor is totally drained, then the input will then be connected to the ground thus the output will be the offset of the part. The part is stable unless you remove the capacitor.