Chapter 5
Calibration
©
National Instruments Corporation
5-35
VXI-SC-1102 User Manual
Note:
To make one of your calibration points the zero point, connect the positive
and negative channel leads to one of the chassis ground pins on the front
connector or terminal block.
b.
Measure the input voltage with the DMM. Call the measured
voltage
input1
.
Note:
If you are using a calibrator that supplies accurate voltages, you can
simply use the known applied voltage for
input1
instead of measuring.
c.
Measure the module output at the analog bus receptacle with
the DMM. Call the measured voltage
output1
.
3.
Repeat Step 2, applying the input for the second calibration point.
Call the measured voltages
input2
and
output2
. To select
positive full scale as the calibration point, repeat Step 2 and apply
+9.9 V/G.
4.
You now have two pairs of voltages
(
input1
,
output1
)
and
(
input2
,
output2
)
. Each pair consists of an input voltage and an
output voltage.
5.
Convert the output voltages from volt units to your VXI-MIO
module binary unit. You must take into consideration the polarity
of your VXI-MIO module, its resolution (12 bits or 16 bits), and
gain. For example, if you are using the 12-bit VXI-MIO-64E-1 in
bipolar mode with the gain set to GMIO, your output voltages for
the autozeroing option will be represented in binary units as given
by the following formula:
For other VXI-MIO modules, refer to the user manual to determine
the appropriate formula.
6.
You now have a new set of pairs referred to as voltage binary pairs
(
input1
,
bin_output1
) and (
input2
,
bin_output2
). Pass these
pairs to the
SCXI_Cal_Constants
function or VI as described in
your software user documentation.
Note:
When you use 0 V and positive full-scale for your two calibration points,
you eliminate the error at 0 V and at positive full-scale voltage. However,
because of nonlinearity, the error at the negative full-scale voltage will be
two times the nonlinearity error. This is also true for the positive full-scale
voltage if you use the negative full-scale voltage and 0 V as your two
calibration points.
Binary
Output
5 V
------------------
2
12
•
G
MIO
•
=
01Title.book : 09Ch5.frm Page 35 Friday, March 21, 1997 3:40 PM
