The two-stage I/V converter was followed by A4 which included
stages of gain, filter and buffer. The precision OPA, A1, had a very
low voltage noise, but a current noise up to 10 fA= ffiHffiffiffizffiffi
p
. The filter
limited the bandwidth of the circuit to 1 kHz to reduce the current
noise. The last stage was a buffer amplifier. It generated an output
signal that was split into two paths. One was fed into DAQ system
for current curve display, and the other was fed into GI for
integration.