Chapter 4
Theory of Operation
4-8
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Excitation
You can set the excitation voltage of the SCXI-1521/B from 0 to 5 V in
1,023 steps. The excitation voltage is buffered on a per channel basis to
prevent excitation load variation errors from interfering with other
channels. You can set the excitation voltage in banks of six channels
grouped as follows: CH <0..5>, CH <6..11>, CH <12..17>, and
CH <18..23>.
Refer to the
section of Chapter 3,
, for more information about programmatically
setting excitation level in MAX. For more information about
programmatically setting excitation level in NI-DAQmx, refer to the
Developing Your Application in NI-DAQmx
.
Offset Null Compensation
The SCXI-1521/B provides hardware offset null compensation that
removes the offset voltage that exists in the strain gauge when it is
unstrained to adjust signal voltages to proper levels. Hardware offset
nulling is performed programmatically using software.
The fine null potentiometer is used for offset adjustments. The
potentiometer signals are added to the analog input path to adjust the signal
voltage to remove the offset, which nulls the strain-gauge channel. The
voltage input to the potentiometer is a voltage proportional to the excitation
voltage setting. Therefore, if the excitation voltage changes by a small
amount due to a changes such as temperature and sensor loading, the
correction signal produced by the fine null potentiometer changes by the
same amount and the offset null is maintained.
The fine null potentiometer is controlled digitally using control codes. The
control codes of the fine null potentiometer are set in software using integer
values. Table 4-1 summarizes the nulling range and scale of the control
codes.
Table 4-1.
Control Codes for Fine Null Potentiometers
Range (Integer Values)
Mid-Scale
Bridge Offset Nulling
0 to 4,095
2,048
±0.45% of V
EX