Chapter 4
Theory of Operation
©
National Instruments Corporation
4-9
In most cases, you do not explicitly set the fine null potentiometer, but
instead allow the NI-DAQ driver software to automatically adjust it for you.
You can do this either through MAX or in your application.
Refer to the
section of Chapter 3,
, for more information about programmatically
performing offset null compensation in MAX. For more information about
programmatically performing offset null compensation in NI-DAQmx,
refer to the
Developing Your Application in NI-DAQmx
section of
Chapter 5,
Shunt Calibration
Shunt calibration is a process used to obtain a gain adjust factor, which is
used to correct for system gain error.
The gain adjust factor is derived using theoretical (simulated) signal levels
that should result from engaging a shunt resistor across one leg of a bridge
sensor and the measured signal levels with the shunt resistor actually
engaged.
Use the following formula to calculate the gain adjust factor:
The gain adjust factor is then multiplied by each future measurement to
obtain accurate measurements that are adjusted for gain errors. Refer to the
, for more information about performing shunt calibration
automatically in global channels using NI-DAQmx in MAX. For more
information about programmatically performing shunt calibration in
NI-DAQmx, refer to the
Developing Your Application in NI-DAQmx
section of Chapter 5,
.
Note
Perform an offset null compensation just before you perform a shunt calibration.
Performing a shunt calibration before an offset null compensation causes improper gain
adjustment because the offset signal voltage is compensated multiple times.
Modes of Operation
The SCXI-1521/B operates in multiplexed mode, passing the conditioned
signals to the digitizing DAQ device.
gain adjust factor
simulated signal level
measured signal level with shunt resistor engaged
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