National Instruments 321645c-01

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 Vin 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:

GRreading offset–

maxReading

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





maxVolt

Vin

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





×=

gainAdjust 1gG

–()

----------------------–=