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
-
+
Miller
Comp
Control
Trimmed
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.
4
|