Common-Mode Voltage Considerations - National Instruments SC Express PXIe-4331 User Manual

Chapter 2 Using the Module
The two-point linear conversion uses the following equations:
If offset nulling (bridge balancing) is used to compensate for offset, then the zero point of the
sensor can be assumed to output exactly 0 V/V, simplifying these equations:
When the calibration certificate of the sensor provides a table of more than two calibration
points or a polynomial expression, table or polynomial scaling can produce more accurate
results by compensating for non-linearity in the response of the sensor. Table scaling requires
providing NI-DAQmx with a set of electrical values and corresponding physical values.
Polynomial scaling requires providing NI-DAQmx with the forward and reverse coefficients
of a polynomial representing the response of the sensor. If you only know one set of
coefficients, you can use the DAQmx Compute Reverse Polynomial Coefficients VI/function
to determine the other set.

Common-Mode Voltage Considerations

The NI PXIe-4330/4331 supports common-mode voltages between ±2 V. The majority of
full-bridge sensors are balanced between the upper and lower arms of the bridge, resulting in
a common-mode voltage equal to one-half the excitation voltage when connected to a
single-ended excitation voltage or 0 V when connected to a balanced differential excitation
voltage. Since the NI PXIe-4330/4331 has a balanced differential excitation voltage source,
the common-mode voltage for balanced sensors is equal to 0 V. However, some full-bridge
sensors are unbalanced between the upper and lower arms. Common-mode imbalances are
often given as a voltage relative to a single-ended voltage excitation source. To convert a
common-mode voltage that is specified relative to a single-ended excitation to a
common-mode voltage driven by a balanced differential excitation use the following
equation:
Where r is the ratio of the common-mode voltage of the sensor divided by the excitation, as
listed in sensor specifications.
NI PXIe-4330/4331 User Manual
physical physical
–
1
m
= ------------------------------------------------------------- -
electrical electrical
–
1
×
b physical m
= –
1
physical reading
=
physical
m
= --------------------------- -
electrical
physical reading
=
V
ex
× (
V 2r 1
-------
=
cm
2
2-20
2
2
electrical
1
×
m V b
+
r
2
2
×
m V
r
)
–
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