Measurement Computing USB-5201 - Manual (Page 13)

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Chapter 4
Functional Details
Thermocouple measurements
A thermocouple consists of two dissimilar metals that are joined together at one end. When the junction of the
metals is heated or cooled, a voltage is produced that correlates to temperature.
The USB-5201 hardware level-shifts the thermocouple’s output voltage into the A/D’s common mode input
range by applying +2.5 V to the thermocouple’s low side at the C#L input. Always connect thermocouple
sensors to the USB-5201 in a floating fashion. Do not attempt to connect the thermocouple low side C#L to
GND or to a ground referencing resistor.
Cold junction compensation (CJC)
When you connect the thermocouple sensor leads to the sensor input channel, the dissimilar metals at the USB-
5201 terminal blocks produce an additional thermocouple junction. This junction creates a small voltage error
term which must be removed from the overall sensor measurement using a cold junction compensation
technique. The measured voltage includes both the thermocouple voltage and the cold junction voltage. To
compensate for the additional cold junction voltage, the USB-5201 subtracts the cold junction voltage from the
thermocouple voltage.
The USB-5201 has two high-resolution temperature sensors that are integrated into the design of the USB-5201.
One sensor is located on the right side of the package, and one sensor is located at the left side. The CJC sensors
measure the average temperature at the terminal blocks so that the cold junction voltage can be calculated. A
software algorithm automatically corrects for the additional thermocouples created at the terminal blocks by
subtracting the calculated cold junction voltage from the analog input's thermocouple voltage measurement.
Increasing the thermocouple length
If you need to increase the length of your thermocouple, use the same type of thermocouple wires to minimize
the error introduced by thermal EMFs.
Data linearization
After the CJC correction is performed on the measurement data, an on-board microcontroller automatically
linearizes the thermocouple measurement data using National Institute of Standards and Technology (NIST)
linearization coefficients for the selected thermocouple type.
The measurement data is then output as a 32-bit floating point value in the configured format (voltage or
temperature).
Open-thermocouple detection (OTD)
The USB-5201 is equipped with an open-thermocouple detection for each analog input channel. With OTD, any
open-circuit or short-circuit condition at the thermocouple sensor is detected by the software. An open channel
is detected by driving the input voltage to a negative value outside the range of any thermocouple output. The
software recognizes this as an invalid reading and flags the appropriate channel. The software continues to
sample all channels when OTD is detected.
Input leakage current
With open-thermocouple detection enabled, 105 nA (max.) of input leakage current is injected into the
thermocouple. This current can cause an error voltage to develop across the lead resistance of the thermocouple
that is indistinguishable from the thermocouple voltage you are measuring. You can estimate this error voltage
with the following formula:
error voltage = resistance of the thermocouple x 105 nA
To reduce the error, reduce the length of the thermocouple to lower its resistance, or lower the AWG of the wire
by using a wire with a larger diameter. With open-thermocouple detection disabled, 30 nA (max.) of input
leakage current is injected into the thermocouple.