Chapter 5
Calibration
©
National Instruments Corporation
5-3
SCXI-1102/B/C User Manual
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 between MCH0+ and MCH0–
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 DAQ device
binary unit. You must take into consideration the polarity of your
DAQ device, its resolution (12 bits or 16 bits), and gain. For
example, if you are using the 12-bit AT-MIO-16E-2 in bipolar
mode with the gain set to G
MIO
, your output voltages for the
autozeroing option will be represented in binary units as given by
the following formula:
For other DAQ devices, refer to your DAQ device 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 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.
When you make a measurement using National Instruments software, the
driver automatically uses the calibration constants to correct the measured
voltages.
For an example of how to calibrate your module, refer to Appendix B,
Calibration Sample Program.
Binary
Output
5 V
------------------
2
12
•
G
MIO
•
=