946 Controller Instruction Manual
pg. 28
13. Combination Setup
Two analog output combination channels are available on the 946. Up to 3 vacuum
pressure sensors can be assigned to each combination channel. Refer to section 8.4 for
a more detailed discussion of the settings for the combination channels.
14. Set DAC Parameter
Scaling of the Log/Linear analog output for each individual channel, as well as the combination
analog output can be changed by adjusting the DAC parameter. To view or modify the DAC
parameter, press
System
Setup
and move the cursor to Set DAC Parameter. Select Edit and press
Enter
and the DAC parameters used in scaling the logarithmic/linear analog output are displayed as
shown in Figure 6-5.
Both values for slope A and offset B must be selected when a logarithmic linear equation is used.
The slope A is the voltage per decade, and the offset B is the voltage when the pressure is 1 Torr.
The valid range for A is from 0.5 to 5, while the valid range for B is from –20 to +20 V. The default
settings are 0.6 and 7.2 for A and B, respectively, which provides for a 0.6 to 9.6 Volt output over
the maximum pressure range covered by the Controller. If a single channel module is present in
( HC or CC), only one equation is displayed (Channel A example in Figure 6.6).
Linearized analog output can be used when high analog output resolution is required over a
narrow pressure range. When a linear equation is used, the parameter B is always set to zero as
it indicates zero voltage output at high vacuum. The A value can be calculated using the equation:
Here, P
max
is the maximum pressure (or the flow rate for an MFC) when analog output voltage is
10 V. For a Capacitance Manometer, the full range of the Manometer is normally selected for
P
max
. For example, if a 1000 Torr manometer is connected (P
max
=1000), 1×10
-2
should be
selected for A, while for a 20 SCCM MFC (P
max
=20), 5×10
-1
should be used. For other types of
gauges (such as CC, HC, PR, CP), linear analog output can be used to magnify the analog out
over a special range. For example, if the pressure range from 10 to 10
-1
Torr needs higher
resolution, (P
max
=10), a value of 1 can be selected for A which results in 10 V analog output at 10
Torr.
Set DAC Parameter (P unit is torr in Eq)
B
A
Channel A1
7.20E+0
6.00E-1
Channel A2
Channel B1
Channel B2
Channel C1
Channel C2
Combined
V=AlogP+B
V=AlogP+B
V=AP
V=AP
V=AlogP+B
V=AlogP+B
Equation
1.00E+2
7.20E+0
6.00E-1
7.20E+0
6.00E-1
1.00E+3
7.20E+0
6.00E-1
Figure 6-5 Setting DAC Logarithmic and Linear Analog Output
10
10
To keep the analog output unaffected by the pressure unit change, the pressure unit in these equations
is fixed to Torr.