MKS HPS 945 Series, Operation And Maintenance Manual

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A.2 Digital Pairani Vacuum Sensor System
The Series 945 Pirani System Design Overview
This section is intended to give the reader an overview of the internal workings
of the Series 945 Pirani System. This manual does not provide sufficient detail
for advanced levels of troubleshooting. Refer to the block diagram while
reading this section.
Amplifier
The Sensor uses a wire
as one arm of a balanced
Wheatstone bridge. The
bridge amplifier
maintains the wire at a
constant temperature.
Thus, the bridge driving
voltage from the amplifier
is related to the pressure
in the Sensor.
This voltage is buffered
by an op amp and then
fed to the analog output,
the Controller's digital
display, and the
comparator circuit.
Output
The analog output is
driven by an op amp.
Although it has a fairly low output impedance, it is best to measure the analog
output signal with an instrument having at least 1 MW of input impedance.
Set Point Relays
A reference voltage and op amp form a precision adjustable reference that is
fed to the comparator circuit. A potentiometer can be adjusted to the proper
voltage to trigger the set point relay at the desired pressure.
Comparator
An analog comparator circuit compares the voltage from the buffer
amplifier (pressure signal) to the set point. When the pressure signal falls
below the adjusted reference voltage, a transistor turns on, which
energizes the set point relay. A resistor provides a feedback path around
the op amp, which increases the set point voltage by approximately 10
mV. This hysteresis provides stable operation even though some noise
may be present on the buffered output signal.
Bridge
Amp
Atm Adjust
Zero Adjust
Buffered Analog Output
A/D
Converter
micro-
controller
V
ref
Comparators
Relay
Relay
Display