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Chapter 4
Theory of Operation
NI PXI-4220 User Manual
4-22
ni.com
The output buffers have negative feedback connections at pins
RS
X
+ (pin 3) and RS
X
– (pin 8). You should run separate wires from the
bridge to these pins so that the amplifiers obtain feedback directly from the
bridge, thereby forcing bridge voltage to equal the desired setting.
Pin P
X
+ (pin 2) is always positive with respect to chassis ground, and pin
P
X
– (pin 7) is always negative with respect to chassis ground. The inverting
amplifier –X1 forces the voltage at pin P
X
– (pin 7) to equal the negative of
the voltage at pin P
X
+ (pin 2). For example, if you set the module output
for +5 V, pin P
X
+ (pin 2) is at +2.5 V with respect to ground, and pin
P
X
– (pin 7) is at –2.5 V with respect to chassis ground. The excitation
setting originates from an internal digital-to-analog converter (DAC).
You can set the excitation voltage between 0 V and 10 V. You can power a
350
Ω
full-bridge at 10 V without exceeding the maximum power rating of
the excitation source. The excitation outputs are protected with surge
suppressors that prevent either of the excitation output pins from exceeding
6 V with respect to chassis ground.
Note
Chassis ground is at the same potential as earth ground when the PXI chassis is
plugged into a standard 3-prong AC outlet. If pin P
X
– (pin 7) is connected to earth ground,
the excitation source does not function properly.
Refer to the
Configurable Settings in MAX
section of Chapter 3,
Configuring and Testing
, 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
section of Chapter 5,
Developing Your Application
.
Remote Sense
The excitation output buffers have negative feedback connections at the
remote-sense pins, RS
X
+ (pin 3) and RS
X
– (pin 8). You should run
separate wires from the bridge to these pins so that the amplifiers obtain
feedback directly from the bridge, forcing the excitation voltage applied to
the bridge to equal the voltage at pins RS
X
+ (pin 3) and RS
X
– (pin 8). This
removes unwanted DC offset in the excitation and compensates for changes
in lead resistance caused by temperature variation in the lead wires.
Note
NI recommends connecting the remote sense pins directly to the sensor to obtain
optimum excitation voltage regulation and measurement accuracy.
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