National Instruments VXI-SC-1102 User Manual

Page 34

Chapter 5

Calibration

National Instruments Corporation

5-3

VXI-SC-1102/B/C 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.

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.

Measure the module output at the analog bus receptacle with
the DMM. Call the measured voltage

output1

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.

You now have two pairs of voltages

(

input1

output1

)

and

(

input2

output2

)

. Each pair consists of an input voltage and an

output voltage.

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 G

MIO

, 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 device user manual to
determine the appropriate formula.

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

------------------

•

MIO

•