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Calibration Procedures Chapter 5
SC-2040 User Manual 5-4 © National Instruments Corporation
4. Set the DAQ board to a high gain and measure its offset by shorting its input.
You must subtract this measured offset from all subsequent board measurements to ensure
accuracy.
For the offset of the DAQ board not to affect the calibration, you must subtract the offset
from all board offset measurements. Thus, you must first measure the offset to an accuracy
better than that to which the board is to be calibrated. Furthermore, setting the DAQ board to
a high gain (100, for example) makes it possible to resolve offset changes that would be
indiscernible at lower gains. Because the DAQ board offset will not be independent of gain,
you should set the gain first, and then measure the offset. To measure the offset, disconnect
the board from the DAQ board, short circuit the board inputs to ground, and take some data.
The measured value is the offset. For a reliable value, use software to average a few hundred
readings. After you have measured and recorded the offset, remove the short circuits and
reconnect the board.
5. Measure the output of the first channel with its gain set to one and adjust its output offset pot
until the output is close to zero.
At this gain, most of the board offset is due to output offset. The input offset adjustment has
minimal effect. Set the DAQ board to read the channel. Acquire the data, averaging as in
step 4. Adjust the output offset pot until the difference between the measured offset and the
DAQ board offset is close to zero. There is no need to adjust it perfectly because the input
offset adjustment in the next step might make a slight change in the measured gain-of-one
offset.
6. Measure the output of the same channel with its gain set to 500 and adjust its input offset pot
until the output is close to zero.
At this gain, the input offset adjustment has the dominant effect. Acquire the data and
average as before. Adjust the input offset pot until the difference between the measured
offset and the DAQ board offset is close to zero. Again, it is not necessary to adjust the input
offset perfectly because there will be a slight interdependence between the two offsets.
7. Repeat step 5, adjusting the offset as carefully as desired. Then repeat step 6, adjusting the
offset as carefully as desired. Switch back to a gain of one to ensure that the low-gain offset
is still calibrated. If necessary, repeat steps 5 and 6 until the offset is calibrated at both gains.
8. Repeat steps 5 through 7 for the additional channels you want to calibrate. There is no
calibration interdependence among the channels.
9. Replace the rack-mount chassis cover, if used.