MKS HPS 919 Series, Operation And Maintenance Manual

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Electrometer
The electrometer is a logarithmic current to voltage converter used to
measure the sensor ion current. This electrometer has a dynamic range wide
enough to cover the entire pressure range with no electronic range changing.
Signal Processing
Taking the output from the log electrometer, inverting it, and summing it
with a reference voltage generates the logarithmic output. The output is 1
V per decade of pressure. The reference is 1 x 10
-11
Torr for an output of 0
V, but it should be noted that the Controller can not actually measure this
low of a pressure.
The analog signal from the electrometer is fed to a 12 bit dual slope
integrating analog-to-digital (A/D) converter. The A/D converter integrated
circuit is driven by a crystal controlled clock. The A/D converter is also
used to measure the set point voltages, overpressure shutdown setting,
and the sensitivity setting, dependent upon which front panel display
switch is pushed.
Display
The data bits from the A/D converter go to an EPROM that controls the
segment drivers for the LED displays. The data from the EPROM is multi-
plexed between the two mantissa and the two exponent digits. The most
significant digit of the exponent can only be either blank or 1, which is
commonly referred to as a one and a half digit exponent.
Set Points
The output from the electrometer also goes to the set point comparators.
These compare the pressure voltage to the set point voltages as set by the
user and activate the process control relays when the pressure is below the
set point values. When the pressure rises above the values, the set points
are deactivated.
Collector
Filament
Grid
Log Electrometer
Analog Output
A/D Converter
Display
Set Point Relay
Comparator
SP1
SP2
Degas
Supply
Grid
Supply
Logic
Circults
Filament
Supply
Emission
Current Control
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