Chapter 4
Theory of Operation
©
National Instruments Corporation
4-25
Gain/Input Range
In normal NI PXI-4220 operation you do not need to set the gain.
NI-DAQmx automatically sets the appropriate gain based on the range
of your task or global channel, or the input limits set in LabVIEW.
The NI PXI-4220 has multiple gain stages to provide optimal overall signal
gains appropriate for fully utilizing the range of the NI PXI-4220 DAQ
circuitry. The first gain stage (the instrumentation amplifier stage) provides
gains of either 1 or 20. The second gain stage provides many discrete
settings between 1 and 50. Together these two gain stages combine for
49 overall gain settings between 1 and 1000. This allows the driver to select
a gain setting for your input range that fully utilizes the input range of the
ADC. The third stage, the NI programmable gain instrumentation amplifier
(NI PGIA) provides four input ranges of ±10 V, ±5 V, ±0.5 V, and ±0.05 V
for the ADC.
For overall module gain settings equal to or greater than 20, the gain of the
first stage is set to 20 so that the noise and offset drift of later stages is small
in comparison to this stage. The instrumentation amplifier stage uses
operational amplifiers with very low temperature drift and noise
characteristics. If overall module gain is less than twenty, the first stage is
set to 1 and the appropriate second stage gain is applied.
For common strain-gauge configurations where the gauge factor is 2.0,
the maximum input signal (in microvolts) is:
Once you determine the input signal voltage, you can use the following
equation to determine the appropriate gain:
If you are using a bridge-based sensor, use the manufacturer-specified
sensitivity (usually expressed in the units of millivolts per volt) to
determine the maximum input signal. The maximum input signal is:
quarter bridge
max strain
(
)
excitation voltage
(
)
×
(0.5
×
µ
V/V/
µε
)
=
half bridge
max strain
(
)
excitation voltage
(
)
×
(1.0
×
µ
V/V/
µε
)
=
full bridge
max strain
(
)
excitation voltage
(
)
×
(2.0
×
µ
V/V/
µε
)
=
gain
NI PXI-4220 output voltage range
10 V
(
)
(
)
maximum input signal voltage
(
)
÷
≤
max input signal voltage
(
)
sensor sensitivity
(
)
excitation voltage
(
)
×
maximum input
(
)
×
sensor full-scale input
(
)
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=
