User's Manual

Chapter 2 Function Reference — SCXI_Cal_Constants
NI-DAQ FRM for PC Compatibles 2-326
© National Instruments Corporation
milliamperes. If opCode = 3, you should pass your second calibration constant in calConst2
for NI-DAQ to store in calibrationArea.
Note C Programmers—calConst1 and calConst2 are pass-by-reference parameters.
Using This Function
Analog Input Calibration
When you call SCXI_Scale to scale binary analog input data, NI-DAQ uses the binary offset
and gain adjust calibration constants loaded for the given module, channel, and gain setting
to scale the data to voltage or frequency. Refer to the
SCXI_Scale function description for
the equations used.
By default, NI-DAQ loads calibration constants for the SCXI-1122, SCXI-1126, and
SCXI-1141 from the module EEPROM (see the EEPROM Organization section later in
this function for more information). The SCXI-1141 has only gain adjust constants in the
EEPROM and does not have binary zero offset in the EEPROM. All other analog input
modules have no calibration constants by default; NI-DAQ assumes no binary offset and ideal
gain settings for those modules unless you use the following procedure to store calibration
constants for your module.
You can determine calibration constants based specifically on your application setup, which
includes your type of DAQ device, your DAQ device settings, and your cable assembly, all
combined with your SCXI module and its configuration settings.
Note NI-DAQ stores constants in a table for each SCXI module gain setting. If your
module has independent gains on each channel, NI-DAQ stores constants for each
channel at each gain setting. When you use the following procedure, you are also
calibrating for your DAQ device settings, so you must use the same DAQ device
settings whenever you use the new calibration constants. The SCXI-1122,
SCXI-1126, and SCXI-1141 factory-set EEPROM constants apply only to the
SCXI-1122, SCXI-1126, and SCXI-1141 amplifiers, respectively, so you can use
those with any DAQ device setup.
To perform a two-point analog input calibration, perform the following steps:
1. If you are using an AT-MIO-16F-5, AT-MIO-64F-5, or AT-MIO-16X device, you should
calibrate your ADC first using the
MIO_Calibrate function.
2. Make sure the SCXI gain is set to the gain you will be using in your application. If
you are using an SCXI-1100, SCXI-1122, SCXI-1126, or SCXI-1141, you can use the
SCXI_Set_Gain function, because those modules have software-programmable gain.
For other analog input modules, you need to set gain jumpers or DIP switches
appropriately.
3. Use
SCXI_Single_Chan_Setup to program the module for a single-channel operation
(as opposed to a channel scanning operation).