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3000
2500
I'...
-2000
c:
~
Q)
u
c:
t:
1500
·
~
a:
::J
0.
-=
1000
500
0
0
~
I
I
~
"""
~
,............
~
10
15
Preampl
ifier Sensitivity
(dB)
Fig.5. 3. Preamplifier input resistance as a function of gain
response of the preamplifier for vibration measurements. The input resist-
ance does however depend on the amount of feedback applied to control
the sensitivity of the preamplifier. Typically, it ranges from 2 Gnat 0 dB to
540 Mn at 2 dB as shown in F ig.5.3. The input capacitance remains con-
stant and is approximately 8 pF.
5.2.2. Low
Frequency Characteristics
The low frequency characteristics of the preamplifier without integration
networks connected ("Acceleration" mode) are shown in Fig.5.4.
Without accelerometer. the preamplifier's low frequency response is
determined by the preamplifier's input resistance R
1
and its coupling
capacitances Cc (see Fig.5.2). These components act as a frequency
dependent potential divider netWork which attenuates the input voltage E
0
to produce a voltage
E
1
across
R
1 ,
the input to the preamplifier. The fre-
quency at
.
which
E
1
=
0.707
E
0
is the lower cut off frequency fc
1
of the
preamplifier and is given by
5.1.
23
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