A leakage compensation circuit is connected in negative feedback across the preamplifier and
provides a path to source or sink the detector leakage current so that it does not get integrated on
the feedback capacitor. The voltage bias to the compensation circuit is trimmable to eliminate
effects of transistor mismatch. This is necessary since the offset of the preamplifier will be different
when the reset is active. The specification for the leakage compensation circuit is for currents up to
100nA in both polarities but it should typically be not more than 20nA at nominal operating
temperatures (40C).
In the case of a low bias current to the leakage compensation circuit it may not provide sufficient DC
stability to the preamplifier. For this reason an additional feedback circuit is included which has a
constant current biased transistor in the feedback.
RST
Constant Current
Feedback
Ibias
Leakage Compensation
Circuit
-
+
Trimmed
Miller Comp Control
VPRE
Detector
-
To Shaper
VPRE
+
Overload
Protection
Cf = 1.41pF
Figure 2: Preamplifier Configured for Electrons Mode
An additional circuit limits the response of the circuit to very large signals to aid recovery following
overload conditions. There is also a circuit which is used to reset the preamplifier.
The preamplifier also has programmable compensation capacitors to optimise the rise time for the
detector strip capacitance.
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