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Appendix B Analog Input Channel, Gain Settings, and Voltage Calculation
©
National Instruments Corporation B-7 NI-DAQ FRM for PC Compatibles
Offset and Gain Adjustment
Measurement of Offset
To determine the offset parameter used in the AI_VScale and
DAQ_VScale functions, follow this procedure:
1. Ground analog input channel i, where i can be any valid input channel.
2. Call the
AI_Read function with gain set to the gain that will be used
in your real acquisition (g). The reading given by the
AI_Read
function is the value of offset. The offset is only valid for the gain
setting at which it was measured. Remember that the data type of
offset in the
AI_VScale and DAQ_VScale functions is floating point,
so if you use
AI_Read to get the offset, you will have to typecast it
before passing it to the scale function.
Note
Another way to read the offset is to perform multiple readings using a DAQ
function call and average them to be more accurate and reduce the effects
of noise.
Measurement of Gain Adjustment
To determine the gainAdjust parameter used in the AI_VScale and
DAQ_VScale functions, follow this procedure:
1. Connect the known voltage V
in
to channel i.
2. Call the
AI_Read function with gain equal to g. Use the reading
returned by
AI_Read with the offset value determined above to
calculate the real gain.
Note
You can use the DAQ functions to take many readings and average them instead
of using the
AI_Read function.
The real gain is computed as follows:
The gain adjustment is computed as follows:
G
R
reading offset–
maxReading
-------------------------------------------
maxVolt
V
in
------------------------
×=
gainAdjust 1
gG
R
–()
g
----------------------–=