Measurement Specialties USB-5201 - Manual (Page 16)

USB-5201 User's Guide Sensor Connections
16
CJC sensors
The USB-5201 has two built in high-resolution temperature sensors. One sensor is located on the right side of
the package, and one sensor is located on the left side.
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-5201 makes fully-differential thermocouple measurements without the need of ground-referencing
resistors. A 32-bit floating point value in either a voltage or temperature format is returned by software. An
open thermocouple detection feature is available for each analog input which automatically detects an open or
broken thermocouple.
Use InstaCal to select the thermocouple type (J, K, R, S, N. E, and B) and one or more sensor input channels to
connect the thermocouple.
Wiring configuration
Connect the thermocouple to the USB-5201 using a differential configuration, as shown in Figure 3.
R
S
V
D
N
C
C
#
H
C
#
L
N
C
R
S
V
D
C
#
H
C
#
L
G
N
D
R
S
V
D
Figure 3. Typical thermocouple connection
Connect thermocouples to the USB-5201 such that they are floating with respect to GND (pins 9, 19, 28, 38).
The USB-5201
GND pins are isolated from earth ground, so you can connect thermocouple sensors to voltages
referenced to earth ground as long as the isolation between the GND pins (9, 19, 28, 38) 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, you should use 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-5201 is powered on, and ±40 VDC when the USB-5201 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.