In RTD temperature measurement applications requiring a very high level of accuracy, the DI-5B35 4-wire RTD input module offers a significant advantage over 3-wire measurement techniques. The DI-5B35 measures only the voltage dropped across the RTD and almost completely ignores the resistance or length of the RTD lead wires. The DI-5B34 3-wire RTD module provides lead resistance compensation, but requires equal lead resistances, while the DI-5B35 does not require matched lead resistances.

Each DI-5B35 module provides a single channel of RTD input which is filtered, isolated, amplified, linearized, and converted to a high level analog voltage output. This voltage output is logic switch controlled, which allows these modules to share a common analog bus without the requirement of external multiplexers.

The DI-5B35 module is designed with a completely isolated computer side circuit which can be floated to ±50V from power common (pin 16). This complete isolation means that no connection is required between I/O common and power common for proper operation of the output switch.

RTD excitation is provided from the module by a precision current source. The excitation current is available on two leads which are separate from the two input signal measuring leads. The excitation current does not flow in the input signal leads, which allows RTD measurements to be totally independent of lead resistance. The excitation current is very small (0.25mA for 100Ω Platinum and 120Ω Nickel, and 1.0mA for 10Ω Copper) which minimizes self-heating of the RTD.

Signal filtering is accomplished with a six-pole filter which provides 95dB of normal-mode rejection at 60Hz and 90dB at 50Hz. Two poles of this filter are on the field side of the isolation barrier, and the other four are on the computer side. After the initial field-side filtering, the input signal is chopped by a proprietary chopper circuit. Isolation is provided by transformer coupling, again using a proprietary technique to suppress transmission of common mode spikes or surges. The module is powered from +5VDC, ±5%.

A special input circuit on the DI-5B35 provides protection against accidental connection of power-line voltages up to 240VAC.