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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