NI USB-9237 User Guide and Specifications
14
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Excitation Voltages
Although the sensor industry does not recognize a single standard excitation
voltage level, excitation voltage levels of between 2.5 V and 10 V are
common. You can program the NI USB-9237 to supply 2.5 V, 3.3 V, 5 V,
or 10 V of excitation voltage, and the module can provide up to 150 mW of
excitation power. Unless you supply external excitation voltage, National
Instruments recommends that you set the excitation voltage to a value that
keeps the total power below 150 mW. The NI USB-9237 automatically
reduces internal excitation voltages as needed to stay below 150 mW.
The power consumed by a single bridge is
where
R
is the total resistance of the bridge.
For a full bridge,
R
is equal to the resistance of each element. For a half or
quarter bridge,
R
is equal to two times the resistance of each element.
The 150 mW limit allows you to power full and half bridges as follows:
•
Four 350
Ω
half bridges at 5.0 V
•
Four 350
Ω
full bridges at 3.3 V
•
Four 120
Ω
half bridges at 2.5 V
If you need an excitation voltage greater than 150 mW, use the
four-position external excitation voltage connector to connect an external
excitation source to the NI USB-9237. Refer to Figure 6 for an illustration
and pinout description of the four-position external excitation voltage
connector.
NI USB-9237 Circuitry
The NI USB-9237 is isolated from earth ground. However, the individual
channels are not isolated from each other. The EX+, EX–, and T– signals
are common among all channels. You can connect the NI USB-9237 to a
device that is biased at any voltage within the NI USB-9237 rejection range
of earth ground. Refer to the
section for more information.
You also can connect floating signals to the NI USB-9237. If you connect
floating signals to the NI USB-9237, National Instruments recommends
you connect the EX– signal to the earth ground or shield for better noise
rejection.
V
ex
2
R
------------
