Chapter 2
Connecting the Signals
SCXI-1502 User Manual
2-4
ni.com
Caution
Exceeding the input damage level (±42 VDC powered on or ±25 VDC powered
off between input channels and chassis ground) can damage the SCXI-1502 module, the
SCXIbus, and the DAQ device. NI is
not
liable for any injuries resulting from such signal
connections.
Note
Exceeding the differential or common-mode input channel ranges results in a
distorted signal measurement, and can also increase the settling time requirement of the
connected E/M Series DAQ device.
Ground-Referencing the Signals
The input signals can be either ground-referenced, as shown in Figure 2-1,
or floating, as shown in Figure 2-2. Before you connect the thermocouple
or any other signal, determine whether it is floating or ground-referenced.
If it is a floating signal, you must ground-reference the signal in one of two
ways. You can connect the negative channel input to chassis ground
through a bias resistor as shown in Figure 2-2 or you can use the bias
resistors that are included with some terminal blocks such as the
SCXI-1303. The SCXI-1303 also has a resistor pack for pulling up the
positive inputs for open-thermocouple detection. Consult the installation
guide of your terminal block for details.
Do not ground signals that are already ground-referenced; doing so results
in a ground loop, which can adversely affect the measurement accuracy.
Directly grounding floating signals to the chassis ground without using a
bias resistor is not recommended as this can result in noisy readings.
