MANUAL: 55308
REV: A
ECO: 49582
10
With the series blocking capacitor C
s
in place as
shown, the dynamic charge (Q) generated by the
sensor element is distributed across the two
capacitors, C
t
and C
s
, in proportion to the size
(capacitance) of each. If C
s
, for example, is equal
to 100 times C
t
, 99% of the charge appears at the
input of the charge amplifier, while 1% is across the
shunt capacitor C
t
. This results in a 1% decrease in
apparent sensitivity of the system.
This therefore demonstrates the importance of
selecting the series blocking capacitor at least two
orders of magnitude higher than the total shunt
capacitance C
t
across the input of the charge
amplifier.
It is also important that this capacitor be of high
quality, with a leakage resistance of greater that
10
12
ohms, to avoid the DC offset discussed
previously in 5.1, Introduction.
5.4 Low-Frequency Response
Limitations
In a normal charge amplifier, the low-frequency
response is set by the RC time constant, as
established by the product of C
f
and R
f
. The system
acts like a high-pass first order RC filter with a -3 dB
frequency established by the relationship:
Equation 2
f
f
o
C
R
.16
f
where:
f
o
=
-3 dB Frequency (Hz)
R
f
=
Feedback resistor (ohms)
C
t
=
Feedback capacitor (farads)
However, after the addition of the series blocking
capacitor C
s
, the system becomes the equivalent of
two high-pass filters in series, one as previously
mentioned and one comprised of series capacitor C
s
and total equivalent shunt resistance R
i
. This new
cutoff frequency is:
Equation 3
s
i
o
C
R
.16
f
To avoid compromise of the low-frequency
response established by the charge amplifier
parameters and illustrated by Equation 2, the
product of R
i
C
s
should be several orders of
magnitude higher than R
f
C
f
.
The approximate final system discharge time
constant becomes:
Equation 4a
seconds
C
R
1
C
R
1
1
TC
f
f
s
i
If the input coupling time constant (R
i
C
s
) is very
much greater than the discharge time constant of
the charge amplifier (R
f
C
f
), Equation 4a then
becomes:
Equation 4b
Seconds
0
C
R
1
s
i
Equation 5
TC = R
f
C
f
With the product R
i
C
s
chosen to be much greater
than R
f
C
f
, the system discharge time constant is
simply R
f
C
f
(seconds). The feedback parameters of
the charge amplifier establish the low frequency
characteristics of the system, unaffected by the
degraded input resistance parameters of the test
sensor and/or cable.
5.5 Other Precautions
Always remember to keep the OPR-GND switch on
the charge amplifier in the GND position while
connecting or disconnecting sensors, cable, or
capacitor to the input connector. Stray or
accumulated electrostatic charges may build to the
point that they may saturate or even damage the
input circuitry of the charge amplifier.
Operate the charge amplifier in the SHORT time
constant while the sensor is subject to elevated or
changing temperatures.
If it is not necessary to procure data during the
transition from room temperature to operating
temperature, place the OPR-GND switch in the
