National Instruments 321645c-01 用户手册

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:

Ground analog input channel i, where i can be any valid input channel.

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:

Connect the known voltage V

to channel i.

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:

reading

offset

–

maxReading

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









maxVolt

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









gainAdjust

–

(

)

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

–