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Signal conditioning - PreAmplifier
Piezoelectric accelerometer
Due to the high impedance and low signal levels at the output of a
piezoelectric accelerometer it is nearly always necessary to use
preamplifiers before entering into common instrumentation.
The functions performed by the preamplifier are:
Impedance Conversion
Amplification
Matching output signal to measuring instrumentation input sensitivity
(Conditioning)
Filtering
Integration to obtain velocity or displacement output signals
Warning of overloads anywhere before the following instrumentation
Normally at least the first two points are found in a preamplifier.
Choice of preamplifier
In principle both voltage and charge preamplifiers can be used to make the
necessary impedance conversion, etc.
However, as indicated on the figure, the sensitivity seen by the amplifier
varies dramatically with cable length when voltage amplifiers are used. This
means that a new calibration (or calculation) has to be made if the cable
used is changed. Furthermore the lower limiting frequency can be affected
by cable length and resistance.
Therefore the majority of preamplifiers used today are charge amplifiers as
they are not affected by cable length or resistance changes within
reasonable limits.
For input stages in built-in preamplifiers this is not quite as clear a choice, but
for the best performance charge amplifiers are still to prefer.
Amplifier theory
For the electronically interested, the different parameters concerning the
function of charge and voltage amplifiers are given in this figure.
A good way to understand the charge amplifier is to see that it is practically a
short-circuit in which the current flowing is integrated. This makes it very
insensitive to any impedance changes at the input.
It is also good to repeat the fundamental relationships:
Charge = Current × Time
or
Coulomb = Ampere × Second
and for a capacitor:
Charge = Voltage × Capacitance
or with practical units:
pC = mV × nF
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