Three-Wire Configuration; Four-Wire Configuration - Omega OM-USB-TEMP-AI User Manual

OM-USB-TEMP-AI User's Guide Signal I/O Connections
Figure 5. Two-wire, two RTD or thermistor sensors measurement configuration
When you select a two-wire, two sensor configuration with InstaCal, connections to T#H (first sensor) and
T#H/T#L (second sensor) are made internally.
When configured for two-wire mode, both sensors must be connected to obtain proper measurements.

Three-wire configuration

A three-wire configuration compensates for lead-wire resistance by using a single voltage sense connection.
With a three-wire configuration, you can connect only one sensor per channel pair. A three-wire measurement
configuration is shown in Figure 6.
Figure 6. Three-wire RTD or thermistor sensor measurement configuration
When you select a three-wire sensor configuration with InstaCal, the OM-USB-TEMP-AI measures the lead
resistance on the first channel (T#H/T#L) and measures the sensor itself using the second channel (T#H/T#L).
This configuration compensates for any lead-wire resistance and temperature change in lead-wire resistance.
Connections to T#H for the first channel and T#H/T#L of the second channel are made internally.
Three-wire compensation
For accurate three wire compensation, the individual lead resistances connected to the ±I# pins must be of equal
resistance value.

Four-wire configuration

With a four-wire configuration, connect two sets of sense/excitation wires at each end of the RTD or thermistor
sensor. This configuration completely compensates for any lead-wire resistance and temperature change in lead-
wire resistance.
Connect your sensor with a four-wire configuration when your application requires very high accuracy
measurements. Examples of a four-wire single-sensor measurement configuration are shown in Figure 7 and
Figure 8.
You can configure the OM-USB-TEMP-AI with either a single sensor per channel or two sensors per channel
pair.
Four-wire, single-sensor
A four-wire, single-sensor connected to the first channel of a channel pair is shown in Figure 7.
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