6
4.2. DETECTOR BIAS
The amount of bias required to operate the detector
is specified in the data furnished with the detector.
The bias that is accepted into the preamplifier
through the SHV Bias connector is furnished
through the load resistance (about 100 M
S
) to the
Input SHV connector of the preamplifier. If the
leakage current through the detector is appreciable,
a notable voltage drop will occur across the series
load resistance in the preamplifier, and this must be
added to the detector requirement when the bias
supply level is adjusted.
With the protection circuit in, the input cable can be
removed and reconnected without catastrophic
damage to the preamplifier even with bias applied
to the circuit; but the user must be very cautious to
prevent touching the interior of the connector with
anything other than a good insulator because
potentially lethal high voltage can be present on the
center pin of the Input connector under these
conditions.
With the protection circuit out, the operating bias
level must be reduced gradually to zero before
the detector is either connected to or disconnected
from the Input connector on the 142IH.
4.3. ENERGY OUTPUT
The charge-sensitive loop is essentially an
operational amplifier with a capacitive feedback
through a 1-pF capacitor. The conversion gain is
approximately 45 mV/MeV (Si equivalent). A dc
feedback is applied through a 100-M
S
resistor.
The energy signal from the preamplifier is a fast
risetime voltage step with an exponential return to
the baseline with a time constant of 100 µs. The
polarity of signals through the E output is inverted
from the signal polarity at the output of the detector.
When a positive bias polarity is used for the
detector, its output pulses are negative and so the
E output of the preamplifier is positive.
The risetime of the charge-sensitive loop increases
as the external capacity increases. External
capacity is a function of the detector and its cabled
connection to the preamplifier Input.
4.4. TIMING OUTPUT
The T output connector provides an alternate path
for the same output pulses that are furnished
through the E output connector. The intent is to
provide both connections for convenience when the
142IH Preamplifier is used to drive two systems -
one for energy spectroscopy and the other for
timing spectroscopy.
4.5. INPUT PROTECTION
A provision is built into the preamplifier to protect
the input FET from damage when there are high-
voltage transients at its input. These transients can
result from any one or more of many causes
including detector breakdown, moisture
condensation on the Input connector, short circuits
or uncharged capacitance connected across the
input while bias is applied, or disconnection of a
bias voltage without first reducing it gradually to
zero.
The protection circuit is installed in the preamplifier
when the unit is shipped from the factory. Although
it offers protection to the FET, it also causes a slight
degradation of the noise performance of the
preamplifier and this increases as detector capacity
increases.
With the protection circuit in, the emitter lead of
Q11 is attached to the junction between resistor R5
and the FET input. Transistor Q11 is connected as
a diode, with both the base and the collector tied to
ground through R8. This prevents the voltage in the
input circuit from rising beyond the safe limit for the
FET input. To take the protection circuit out, simply
remove the emitter lead of Q11 from its circuit
connection and install a wire jumper across R5.
In order to take full advantage of the risetime
capabilities of the 142IH for timing experiments
(typical risetimes of 20 to 300 ns for input
capacitances from 1 to 500 pF), the total cable
length should be kept as short as possible. Due to
vagaries in the system installation - ground loops,
stray inductances, etc. - and since the maximum
cable length is a factor in the input capacitance of
the preamplifier, it is not possible to give absolute
numbers. In general, it is desirable to keep the
length of the input cable less than two feet so that
the risetime performance with low capacity
detectors will not be degraded significantly.
Содержание 142IH
Страница 6: ...vi...
