National Instruments RM-4339, Calibration Procedure

Page: 11 / 13

RM-4339 Calibration Procedure | © National Instruments | 11
c. Enable the SCB shunt calibration circuit using the following parameters:
Property Node: DAQmx Channel
AI.Bridge.ShuntCal.Select: B
AI.Bridge.ShuntCal.ShuntCalBResistance: 50k for 120
Ω , 100k for 350 Ω and 1 k Ω
AI.Bridge.ShuntCal.Enable: True
d. Start the task by calling DAQmx Start Task.
e. Average the readings that you acquired and record the values as Result
SCE
.
f. Clear the task by calling DAQmx Clear Task.
8. Calculate the shunt calibration scaling factor as follows:
SCalScaleFactor = SCalExp /( Result
SCE
- Result
SCD
)
where
SCalExp is the expected shunt calibration deflection, given in the following, for each
of the supported quarter-bridge completion values.
SCalExp = -0.00059928 for 120 Ω
SCalExp = -0.00087347 for 350 Ω
SCalExp = -0.00248756 for 1 k Ω
For reference, the calculation for the expected shunt calibration deflection is
given by the following equation:
where
R
eq
is the equivalent parallel resistance of the quarter-bridge and shunt
calibration resistors.
R
qtr
is the resistance of the quarter-bridge completion resistor.
9. Set the calibrator output to the value of the Calibrator Output indicated in Table 7 for the
quarter-bridge configuration that is being verified.
10. Acquire an AI Bridge (V/V) measurement.
a. Create a task.
b. Create and configure an AI Bridge (V/V) channel according to the values found in
Table 6.
c. Disable the SCB shunt calibration circuit using the following parameters:
Property Node: DAQmx Channel
AI.Bridge.ShuntCal.Enable: False
d. Start the task by calling DAQmx Start Task.
e. Average the readings that you acquired, subtract Result
SCD
from the averaged result,
and record the values as Unscaled
OffsetComp
.
SCalExp
R
eq
R
eq
R
qtr
+
-------------------------
0.5
– =