PCB Piezotronics EX619A11, Installation And Operating Manual

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9
5 HIGH-TEMPERATURE OPERATION
5.1 Introduction
When subjected to elevated temperature, all piezoelectric sensors/hardline cable systems exhibit decreased insulation
resistance, due in part to the piezoelectric element, but due mostly to the hardline cable necessary to withstand the high
temperatures. This situation can cause serious voltage offset problems in direct-coupled charge amplifiers. To solve
this problem, the user must AC couple (capacitor) the charge amplifier to the sensor/cable system. See 5.3 Solution to
Reduced Resistance , for complete details, or use different amplifiers.
5.2 Reduced Resistance at Charge Amplifier Input
Figure 5.1 illustrates a simplified schematic of a typical direct-coupled charge amplifier where:
R
f
= Feedback resistor (ohms)
R
i
= Input leakage resistance (ohms)
E
o
= Steady-state output voltage (volts)
e
i
= Offset voltage: FET leakage (volts)
C
f
= Feedback capacitor (farads)
Figure 5.1 Typical Charge Amplifier Schematic
The feedback capacitor C
f
comes into play only in the dynamic situation and its influence does not affect the
steady-state situation. The voltage e
i
is a DC offset voltage, usually very tiny (microvolts), that exists at the
input gate of the MOSFET circuit. This minute leakage current exists in all real devices.
As demonstrated in Equation 1, the steady-state (DC) output voltage E
o
is:
Equation 1






 
i
f
i o
R
R
e E 1
This equation shows that if the input (leakage) resistance at the charge amplifier is extremely high
(approaching infinity), the output DC voltage approaches e
i
, usually a very tiny voltage. However, as R
i
decreases, the term