USB-2416 User's Guide
Functional Details
15
Thermocouple inputs
You can make up to 16 high-resolution differential thermocouple measurements with the USB-2416. Built-in
cold-junction sensors are provided for each of the screw-terminal blocks, and any supported TC type can be
attached to any of the 16 thermocouple channels.
Do not connect thermocouples as single-ended—doing so can cause false readings.
You do not need to use ground-referencing resistors because the analog front-end circuit level-shifts the
thermocouple's output into the common-mode input range of the A/D.
When configuring thermocouple sensors, keep any stray capacitance as small as possible relative to GND to
avoid settling time and accuracy errors.
The USB-2416 also provides an open thermocouple detection (OTD) feature for each of the analog input
channels configured for thermocouple measurement. This feature is automatically enabled as part of the overall
acquisition process, and detects if an open-circuit condition exists at the thermocouple sensor.
The USB-2416 provides electrostatic discharge (ESD) protection for each of the thermocouple inputs. However,
before handling thermocouple sensors, follow standard ESD practices and discharge any accumulated ESD
charge.
The USB-2416 includes an ESD protection device at each of the analog input terminal block pins. If an ESD or
transient voltage is coupled into the device, the input protection device(s) clamps the voltage to protect the
thermocouple input circuitry from possible damage.
Once the thermocouple sensor is connected to the USB-2416, the configuration options have been selected, and
the recommended 45 minute warm up has elapsed, the USB-2416 is ready to make high-resolution differential
voltage measurements.
Cold junction compensation (CJC) is needed to compensate for the additional thermal EMF's generated by
connecting the thermocouple leads to the USB-2416 terminal blocks.
CJC is performed using a high-resolution temperature sensor connected close to the USB-2416's terminal
blocks. The USB-2416 includes four separate CJC sensors—two on each side of the PCB. Software corrects for
the additional thermocouples created at the terminal.
Once the A/D and CJC data is collected, the application software uses this data to linearize to an accurate
temperature reading. The thermocouple linearization uses the latest NIST linearization coefficients for each of
the eight thermocouple types supported by the USB-2416.
Figure 9 below shows a typical thermocouple connection.
Figure 9. Differential thermocouple connection example
