Measurement Computing USB-TEMP-AI - Manual (Page 12)

USB-TEMP-AI User's Guide Signal I/O Connections
12
Power output (+5V)
The +5V output terminal is isolated (500 VDC) from the USB +5V.
Caution! The +5V terminal is an output terminal. Do not connect to an external power supply or you may
damage the USB-TEMP-AI and possibly the computer.
CJC sensor
The USB-TEMP-AI has one built-in high-resolution temperature sensor. The CJC sensor measures the ambient
temperature at the terminal block so that the cold junction voltage can be calculated.
Thermocouple connections
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-TEMP-AI makes fully differential thermocouple measurements without requiring ground-referencing
resistors. A 32-bit floating point value in either a voltage or temperature format is returned by software. An
open thermocouple detection (OTD) feature is available for each thermocouple input. This feature automatically
detects an open or broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, T, N, E, and B) on one or more sensor input channels
to connect the thermocouple.
Wiring configuration
Connect the thermocouple to the USB-TEMP-AI using a differential configuration, as shown in Figure 3.
Figure 3. Typical thermocouple connection
The USB-TEMP-AI GND pins are isolated from earth ground. You can connect thermocouple sensors to
voltages referenced to earth ground as long as the isolation between the GND pins and earth ground is
maintained.
When thermocouples are attached to conductive surfaces, the voltage differential between multiple
thermocouples must remain within ±1.4 V. For best results, we recommend the use of insulated or ungrounded
thermocouples when possible.
Maximum input voltage between analog input and ground
The absolute maximum input voltage between an analog input and the isolated GND pins is ±25 VDC when the
USB-TEMP-AI is powered on, and ±40 VDC when the USB-TEMP-AI is powered off.
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.