ENDEVCO 2680
INSTRUCTION MANUAL
IM2680
Page A-1
APPENDIX A:
THEORY OF OPERATION
1.
INTRODUCTION
The Model 2680 series of Airborne Charge Amplifiers are designed for use with Piezoelectric
Transducers, and Piezoelectric Transducers are self-generating devices requiring no electrical
excitation. The electrical charge generated by the Transducer is proportional to the stress on the
Piezoelectric Crystal. In an Accelerometer, the output is proportional to acceleration; in a
Pressure Transducer, the output is proportional to pressure, etc. For any Transducer, the charge
generated is independent of the amount of external capacitance attached to the Transducer.
FIGURE A-1: PIEZOELECTRIC TRANSDUCER EQUIVALENT CIRCUIT
A simplified equivalent circuit of a Piezoelectric or Capacitive Transducer is shown in Figure A-1.
The open-circuit voltage (Eo) is equal to the charge (q) divided by the Transducer Capacitance
(Cp) as noted in the following formula:
Eo = q
Cp
where
Eo is expressed in V
q is expressed in pC
Cp is expressed in pF
If the Open-Circuit Voltage is known, the Transducer charge can be calculated by q = Eo Cp.
As an example, suppose a system consisting of an Endevco Model 2680 Series Charge Amplifier
(which has a 2.5 V pk, or 2500 mV pk full-scale output) and an Endevco Model 2272
Acceleromet
er is used as a system to measure ±50 g full scale pk. The formula used to
determine the desired system sensitivity in mV/g is:
Eo (in mV pk)
A s = _____________
Desired FS (g pk)
= /F (25mV, 50 g)
= 50 mV/g pk
With the Charge Sensitivity (Sq) of the Accelerometer given as 13 pC/g, the Gain Setting of the
Charge Amplifier is:
