PCB Piezotronics EX619A11, Инструкция по установке и эксплуатации

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11
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.