946 Controller Instruction Manual
pg. 91
8.4 Logarithmic/Linear and Combination Analog Output
Since most of the buffered analog outputs are non-linear, logarithmically linearized or linear analog
outputs are also provided for each individual sensor. However, since the logarithmic/linear analog
outputs are processed by the microprocessor, these are updated only every 50 msec, regardless of the
number of sensors being connected to the controller.
In addition to these Log/Lin analog outputs, 2 combination analog outputs are also available. Up to 3
sensors can be selected for a combination analog output.
8.4.1 Logarithmic/Linear Analog Output
There are two types of analog output that can be selected for each channel: logarithmic
(
B
p
A
V
•
)
log(
) or linear (
)
p
A
V
•
. These analog outputs are determined by the DACs inside
the controller, and can be modified by setting appropriate DAC parameters in the System Setup screen
shown in Figure 6-6.
The default analog output for the controller is logarithmic having a slope of 0.6V per decade and an offset
of 7.2 V (
2
.
7
)
(
6
.
0
p
Log
V
). This provides an analog output ranging from 0.6 to 9.6 V (equivalent to
a pressure range from 1X10
-11
to 1X10
4
Torr).
For detailed setting of the DAC parameters, refer to System Setup, Section 6.4.
8.4.2 Combination Analog Output
In addition to the logarithmic analog output for each individual sensor, two combination analog outputs
are available. These can provide a wider pressure range coverage since the combination output
combines the measurement ranges of multiple gauges.
When Capacitance Manometers are used in combination, no smoothing is provided in the overlap range.
The 95% rule is used in switching the Capacitance Manometers. Once the reported pressure on the lower
range Manometer is greater than 95% of its full scale, the combined analog output will be off the upper
range Manometer.
A Pirani/Convection Pirani and capacitance manometer with a full scale of 500 torr or more, or a 902B
piezo sensor, can be used in combination. When the Convection Pirani reading is greater than 5% of the
full scale of the capacitance manometer the combined analog output will be based off the capacitance
manometer.
When a capacitance manometer and a hot or cold cathode sensor are used in combination, the HC/CC
pressure will be used as the combined analog output. The combined output will switch to the capacitance
manometer only when the HC/CC is turned OFF.
If a Pirani/Convection Pirani and an HC/CC are used in combination, a smoothing formula is used where
sensor ranges overlap (10
-3
to 10
-4
Torr). The combination output is 10V when 946 power is ON but the
combination option is disabled if a fault is detected when three gauges are used in combination. Possible
faults include filament failure or cable disconnected.
To set the combination analog output from the from panel, press the
System
Setup
button, then move the cursor
to change the Set Combination Ch parameter to ON and press
Enter
, a setup screen will appear, as
shown in Figure 8-8.