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SUBSYSTEM BOARD 5683

Monterey (MAXREFDES15#): Ultra-Low Power, High-Accuracy 4–

20mA 2-Wire Current-Loop Sensor

By:

Muhammad

Qazi

Carmelo

Morello

Oct 18, 2013

Abstract: The Monterey (MAXREFDES15#) subsystem reference design is a high-accuracy industrial loop powered sensor transmitter that

connects to any standard PT1000 resistance sensor and converts the linearized temperature to a 4–20mA current signal, which is immune

to noise and remains constant over long distances.

A similar version of this article appears on EDN, March 13, 2014.

Introduction

The 4–20mA current loop is widely used as an analog

communication interface in industrial applications for transmitting

the data from remote sensors to a programmable logic controller

(PLC) in a central control center over a twisted pair cable. Here,

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.

The Monterey design is based on a complete Maxim solution which combines ultra-low power, high accuracy, and high precision.

1. MAX44248 low-noise,. zero-drift op amplifier

2. MAX11200 high-accuracy, low-power, 24-bit delta-sigma ADC

3. MAXQ615 low-power microcontroller

4. MAX5216 low-noise, high-accuracy, low-power 16-bit DAC

5. MAX9620 low-power op amplifier

6. 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.

Simplicity, reliability, good noise immunity, long distance, and low cost make the Monterey (MAXREFDES15#) (Figure 1) interface well

suited for industrial process control and automation of remote objects.

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