Chapter 4 Connecting and Using Accessories and Transducers
Getting Started with SCXI 4-8 www.ni.com
You can also ground the thermocouple at the source, assuming the ground
difference between the source and your SCXI system is within the
common-mode voltage range of the SCXI module. If the thermocouple
is already ground-referenced, do not connect the negative input to chassis
ground.
Note
The SCXI-1100 has a jumper, W1, that connects a 100 kΩ resistor to ground at the
output of the input multiplexer when measuring floating sources. However, when you are
using thermocouples, leavejumper W1 in the factory-default position (no connect) because
connecting jumper W1 introduces noise and settling time delays.
Using Thermocouples with the SCXI-1102
The most convenient way to ground-reference your floating thermocouples
is to use the SCXI-1303 or TBX-1303, which provides a ground-reference
for all 32 input channels. If you are using the SCXI-1303 or TBX-1303,
install the supplementary 10MΩ resistornetworks into the terminal blocks.
This configuration allows you to connect either floating or
ground-referenced thermocouples to the SCXI-1102. However, if you are
using the SCXI-1300, you must connect the negative lead of all your
floating thermocouples to the negative terminal of the input channel and to
chassis ground.
Note
Chassis ground is also available at the strain-relief bar. Be certain to connect to
chassis ground at only one point.
Using Thermocouples with the SCXI-1112,
SCXI-1120, SCXI-1121, SCXI-1122, and SCXI-1125
The SCXI-1120, SCXI-1121, SCXI-1122, and SCXI-1125 modules have
isolated analog inputs. Therefore, you can connect these modules directly
to both floating and ground-referenced thermocouples. You do not need to
reference a floating thermocouple to chassis ground. Simply wire the
positive and negative lead of the thermocouple to the positive and negative
terminals of the input channel on the terminal block.
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