 Charge amplifiers convert the charge output by a piezoelectric sensor into a proportional voltage, which is used as an input variable for analysis systems, and can be digitized in an analog-to-digital (A/D) converter if necessary. A charge amplifier basically consists of an inverting voltage amplifier with a high open-loop gain and capacitive negative feedback. It relies on a metal oxide semiconductor field effect transistor (MOSFET) or a junction field effect transistor (JFET) at its input to achieve high insulation resistance and minimize leakage current. If the open-loop gain is sufficiently high, the cable and sensor capacitance can be neglected, leaving the output voltage dependent on just the charge at the charge amplifier input and the range capacitor.
U0 = –Q/Cr
The amplifier acts as an integrator that constantly compensates the sensor’s electric charge with one of equal magnitude and opposite polarity of the range capacitor. The voltage across this capacitor is proportional to the charge generated by the sensor and to the acting measurand. In effect, the charge amplifier converts an electric input charge Q into a usable proportional output voltage Uo. As most Kistler charge amplifiers allow adjustment of sensor sensitivity and measuring range, the measurement is displayed in the mechanical units of the measurand and the output signal as an integer multiple of the measured variable.
U0: Output voltage
A: Gain
Ct: Sensor capacitance
Cc: Cable capacitance
Cr: Range or negative feedback capacitor
Rt: Time constant resistor (or insulation resistance of range capacitor)
Ri: Input insulation resistance (cable and sensor)
Q: electric charge generated by the piezoelectric element
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