Component Selection for Peak Detectors
The holding-capacitor (C1) in a peak detector circuit normally should be kept as small as possible, so that it can be rapidly charged to allow its voltage to easily follow changes in the peak input.
Another important requirement is that the capacitor should accurately hold its charge for a given time, usually until it is discharged by means of a FET switch.
Consequently, all capacitor discharge currents must be kept to a minimum while the charge is being held.
The discharge currents can be minimized by using a BIFET op-amp for A2 and by keeping resistor R2 as large as possible.
Diode D2 should also be a low-leakage device and the capacitor itself should have a low-leakage dielectric.
However, there is always some discharge current which introduces a voltage error.
So, the minimum acceptable error is used to calculate the capacitor size.