I have used op amp peak detector & hold circuit for finding the peak of voltage obtained from scintillation detector and pre-amplifier.
I observed that the observed peak and hold output was 700 mV, instead of the expected 1.5V and that when the pre-amplifier was connected to the peak and hold circuit, the portion of the pre-amplifier output waveform above 700mV was clipped. For debugging the issue, the following points were checked:
- The Op-Amp circuit is not loading the source by drawing excess current.
- Op-Amp slew rate is 125 V/us and that the rate of charging the capacitor with 1uF cap and 100 mA short circuit current is 100 V/us. The detector pulse rises from 0 to 1.5V in 100 ns, so rate required is 15V/us
- A 20kHz triangular wave with 1.7V was provided to the peak-hold circuit. The peak-hold output was 1V and the input triangular wave was clipped to 1V pk-pk. SPICE simulations show similar, but less severe results. The effect is least if the input is a square wave or the frequency is least.
- The circuit works for DC input (provide 3.3V and disconnect it, the capacitor voltage started falling from 3.3V)
All the above observations have been done with nMOSFET in open-state.
Can any one tell me where am I going wrong in the design?