Ultra-low-power, high-accuracy 4–20mA 2-wire current-loop sensor reference design

October 21, 2013 // By Graham Prophet
The 4–20mA current loop continues to be widely used as an analogue communication interface in industrial applications for transmitting the data from remote sensors to a programmable logic controller (PLC) in a central control centre over a twisted pair cable.

Maxim Integrated has published a reference design for a temperature sensor in which 4mA represents the lowest temperature value, and 20mA represents the highest measured temperature. There are four main advantages of the current loop. First, the accuracy of the signal is not affected by the voltage drop in the loop, as long as the power-supply voltage is greater than the total voltage drop across the loop. Secondly, it uses two wires for power as well as data communication over the entire loop. Thirdly, it is more immune to noise. And lastly, it is offered at a low cost and easy installation. Features of the design include;

- ultra-low power

- -100°C to +100°C interval

- high accuracy

- high precision

- simple power supply and wide input range

- system current consumption < 2.1 mA

- resistance temperature detector (RTD) PT1000

- resolution 10,000 counts or 0.1%

- accuracy 0.25%

The design uses the following Maxim parts; MAX44248 low-noise, zero-drift op amplifier; MAX11200 high-accuracy, low-power, 24-bit delta-sigma ADC; MAXQ615 low-power microcontroller; MAX5216 low-noise, high-accuracy, low-power 16-bit DAC; MAX9620 low-power op amplifier; MAX15007, wide input voltage low-power LDO.

This loop-powered sensor transmitter is targeted for industrial sensors, industrial automation, and process control, but it can be used in any application requiring high-accuracy conversion. Maxim says that simplicity, reliability, good noise immunity, long distance, and low cost suit the Monterey interface to industrial process control and automation of remote objects.

Maxim Integrated; www.maximintegrated.com/app-notes/index.mvp/id/5683