Chapter 2
SC-2070 Board
© National Instruments Corporation
2-17
SC-207X Series User Manual
Using the SC-2070 Board for Thermocouple Measurements
The maximum voltage level generated by thermocouples is typically a few millivolts. Therefore, an
MIO-16 with high gain (MIO-16L) should be used for best resolution. Thermocouples can be
measured in either differential or single-ended configurations. Differential configuration has better
noise immunity, but single-ended configuration has twice as many inputs. The MIO-16 board must
have a ground reference because thermocouples are floating signal sources. Therefore, use bias
resistors if the board is in differential mode. For single-ended configuration, use the referenced single-
ended input configuration.
Cold-junction compensation with the SC-2070 board is accurate only if the temperature sensor reading
is close to the actual temperature of the screw terminals. Therefore, when thermocouples are being
read, keep the SC-2070 board away from drafts or other temperature gradients such as those caused by
heaters, radiators, fans, and very warm equipment.
Input Filtering and Broken Thermocouple Detection (Optional)
To reduce noise, a lowpass filter can be connected. Refer to Building Lowpass Filters earlier in this
chapter for an explanation of how to design these filters.
Broken thermocouple detection circuitry is built by connecting a high-value resistor between the
positive input and +5 V. The value of this resistor is relatively unimportant; a few megohms or more
works fine. With a high-value resistor, you can detect an open or defective thermocouple. If the
thermocouple opens, the voltage measured across the input terminals rises to +5 V, a value much larger
than any legitimate thermocouple voltage. The 100-k
Ω
resistor between the negative input and
AIGND is a bias current return path as described under Connecting Floating Signal Sources earlier in
this chapter. A sample circuit and component placement for a lowpass filter and broken thermocouple
detection are shown in Figure 2-13. Assume the thermocouple is connected to channel 1.
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