OM-USB-TEMP-AI User's Guide
Signal I/O Connections
13
Sensor selection
The type of sensor you select will depend on your application needs. Review the temperature ranges and
accuracies of each sensor type to determine which is best suited for your application.
You can connect up to four temperature sensors to the differential sensor inputs (T0H/T0L to T3H/T3L).
Supported sensor categories include thermocouples, RTDs, thermistors, or semiconductor sensors.
Do not mix sensor categories within channel pairs. You can mix thermocouple types (J, K, R, S, T, N, E, and B)
within channel pairs, however.
Do not connect two different sensor categories to the same channel pair
The OM-USB-TEMP-AI provides a 24 bit A/D converter for each channel pair. Each channel pair can monitor
one sensor category. To monitor a sensor from a different category, connect the sensor to a different pair of
sensor input terminals.
Current excitation output terminals (±I1 and ±I2)
The OM-USB-TEMP-AI has two dedicated pairs of current excitation output terminals (±
I1
and ±
I2
). These
terminals have a built-in precision current source to provide excitation for the resistive sensors used for RTD
and thermistor measurements. Each current excitation terminal is dedicated to one pair of sensor input channels:
I1+ is the current excitation source, and I1- is the current excitation return for channel 0 and channel 1
I2+ is the current excitation source, and I2- is the current excitation return for channel 2 and channel 3
Four-wire, two sensor common terminals (4W01 and 4W23)
The 4W01 and 4W23 terminals are used as the common connection for four-wire configurations with two RTD
or thermistor sensors.
Sensor common terminals (IT01 and IT23)
The IT01 and IT23 terminals are used as the common connection for two-wire configurations with two RTD or
thermistor sensors.
Digital terminals (DIO0 to DIO7)
You can connect up to eight digital I/O lines to the screw terminals labeled
DIO0
to
DIO7
. Each terminal is
software configurable for input or output.
Counter terminal (CTR)
The
CTR
terminal (pin 40) is the input to the 32-bit event counter. The internal counter increments when the
TTL level transitions from low to high. The counter can count events at frequencies of up to 1 MHz.
Caution!
All ground pins on the OM-USB-TEMP-AI (pins 9, 19, 22, 27, 30, 33, 36, 39, 49) are common
and are isolated from earth ground. If a connection is made to earth ground when using digital I/O
and conductive thermocouples, the thermocouples are no longer isolated. In this case,
thermocouples must not be connected to any conductive surfaces that may be referenced to
earth ground.
CJC sensor
The OM-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.
Ground terminals (GND)
The nine ground terminals (
GND
) provide a common ground for the input channels and DIO bits and are
isolated (500 VDC) from the USB GND.
