ENDEVCO
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IM 2680/5 083119
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APPENDIX B:
WHY A CALIBRATION CAPACITOR?
A calibration capacitor allows for the testing of a charge amplifier without an accelerometer attached
to the amplifier’s input. Common electronic laboratory test instruments may be used to simulate the
input charge signal.
A voltage signal from an audio signal generator is fed into the charge amplifier’s input via a 1000pF
series capacitor (see figure 14) the voltage signal is thus converted to a charge signal.
The transfer function is: Q = EC
Where:
Q= Charge in picocoulombs (pC)
E= Voltage in volts (V)
C= Capacitance in picofarads (pF)
Since the capacitor is 1000 pF, the transfer function simplifies to Q (pC) = E (mV). The charge
must be expressed in pC peak if the voltage is expressed in peak units. The calibration reference
frequency is 100 Hz, thus this frequency is the recommended input used for this test.
The uncertainty will be that of the signal source and the capacitor. The Endevco 2947C has an
uncertainty of ±1%. Adequate shielding is required for optimum accuracy; the amplifier, being a
high impedance devic sensitive to ambient noise.