Single chip, 2-wire, 4–20mA current loop RTD temperature transmitter design

A single chip, cost effective solution is designed for a typical RTD (resistance temperature detector) temperature transmitter for a 2-wire 4-20mA current loop system. The design leverages available integrated analog components on the MSP430F2274 microcontroller. The voltage controlled current source of the transmitter design is enabled by the general purpose operational amplifiers on the device. Biasing of the current source was carried out via a PWM-DAC implementation which allowed for a 12-bit resolution of the output signal. The signal conditioning of the sensed RTD value was carried out via a differential amplifier of a bridge circuit, also incorporating on-chip analog components. The single chip solution provides further flexibility due to the additional processing enabled by the microcontroller. This allows for linearization through software. The observed measured values for both the output of the signal conditioning circuit and that of the transmitter design correlate well to SPICE simulation results.