Charge amplifiers

A piezoelectric transducer can be considered a small capacitor which is also a coulmb generator. In order to operate into the usual readout instrumentation, such a device requires specialized matching electronics. A cathode follower or high input impedance voltage amplifier limits maximum cable length between transducer and amplifier (long cable result in poor signal-to-noise ratios; frequency response depends on amplifier input impedance, and cable change require system recalibration). However, if the transducer is operated into a charge sensing amplifier, thses limitation no longer exists, and the acquisition of reliable data is much simpler.

Advantages of charge amplifiers

•    The low frequency response of  a charge amplifier is not a function of  a input RC time constant. This means that calculation involving transducer capacitance,cable length, and amplifier input impedance are unnecessary. System frequency response is determined by amplifier frequency alone.
•    Since RC time constant is not involved, charge amplifiers need not have high input impedance, this eliminates problems such as connector contamination and RF pickup.
•    When using charge amplifiers, long input cables can be used without any resultant reduction in the transducer output. in fact, input cables up to a mile or more in length can be used without any measurable reduction of system sensitivity or overall system performance.
•    Change in input capacity do not affect system sensitivity. This means that a given transducer  amplifier system may be calibrated in the laboratory with a convenient length of cable and then installed with cables as long as needed for this specific test. The system calibration remains the same.


In addition, all these advantages are achieved while maintaining a system signal-to-noise ratio as good as, and frequently better than, that achievable with a voltahe amplifier system.

Endevco Corporation , a Meggitt group company, is one of the world's leading designers and manufacturers of dynamic instrumentation for vibration, shock and pressure measurement. The company's comprehensive line of piezoelectric, piezoresistive, ISOTRON® and variable capacitance accelerometers are used to solve measurement problems in a wide variety of industries including aerospace, automotive, defense, medical, industrial and marine. Available products include pressure transducers, microphones, electronic instruments, and calibration