Drawing Number: 21067
Revision: A
PIEZOELECTRIC CHARGE MODE PRESSURE SENSOR GENERAL OPERATION MANUAL
7
6.0
HIGH-TEMPERATURE
OPERATION
6.1
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, such as PCB Models
462, 463, and 464, as well as with the miniature Series
422. To solve this problem, the user must AC couple
(capacitor) the charge amplifier to the sensor/cable
system. See Section 6.3, Solution to Reduced
Resistance, for complete details.
6.2
Reduced Resistance at Charge
Amplifier Input
Figure 6.1 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)
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
i
f
R
R
+
1
increases, such that the output voltage can, with large
ratios of R
f
/ R
i
, become large enough to result in a large
E
o
, perhaps large enough to be outside the normal output
voltage range of the charge amplifier.
Because of the feedback capacitor C
f
, this output voltage
change usually does not occur rapidly but rather, it
manifests itself as a slow drift in the output voltage
level. If R
i
is low enough with respect to R
f
, the voltage
drift may continue until saturation of the charge
amplifier occurs.
6.3
Solution to Reduced Resistance
Since the drift or offset problem is caused by a static or
steady-state imbalance at the input of the charge
amplifier, the solution involves blocking this steady-
state effect while allowing the desired dynamic
phenomena to pass. This may be accomplished by
installing a series capacitor at the input of the charge
amplifier, between the offending sensor (or low-
impedance hardline) and the input.