I often measured by means of semiconductor detectors
[l]. The sensors produce a current pulse with a total
amount of charge proportional to the absorbed energy.
This charge is then integrated in a charge sensitive amplifier
(CSA), resulting in a voltage step. To increase the
signal-to-noise ratio, the step signal is usually filtered by a
pulse shaping amplifier (PSA) [l], [2]. The resulting output
signal is a voltage pulse with a peaking time (~p) typically
on the order of 1 to several ps. Such a system is realized
as a CMOS integrated circuit with impressive results by
Sansen and Chang [31.
The peak value of the output signal of the PSA contains
the information about the energy of the particle. To
digitize the peak value, for processing reasons, the peak
value should be sampled and held by a peak detect sample
and hold circuit (PDSH). The entire detector readout
system is depicted in Fig. 1.
At this moment, the only possible analog structure to
realize a high performance PDSH without the need of
adjusting component values after fabrication is an amplifier
charging a capacitor through a nonlinear device. The
basic architecture is depicted in Fig. 2. When the input
voltage is higher than the output voltage, the hold capacitor
is charged by the OTA through the conducting diode.
However, when the input voltage is lower than the output
voltage, Cs cannot be discharged since the diode is in that
situation reverse biased.