MAXREFDES1161: Small, high-accuracy, 2-wire, 4mmA to 20mA current loop transmitter

The MAXREFDES1161 is a small, high-precision, 2-wire, 4mA to 20mA transmitter that integrates all functions and protection circuitry on a small, 1.0in x 0.6in, dual-row SMD circuit board that can be easily mounted on breadboards and Commercial expansion circuit board. The power supply circuit is implemented using MAX12900, which is an ultra-low-power, highly integrated analog front-end (AFE) suitable for 4mA to 20mA sensor transmitters and integrates 10 circuit units in a small package. The wide-voltage input LDO generates a low-noise power supply for the device and can also power the low-power controller found in most smart sensors. Current loop control is achieved using two general-purpose amplifiers and a low-drift amplifier, coupled with a discrete transistor and a current monitor. Two pulse width modulation (PWM) conditioner circuits receive 8-bit PWM signals from the microcontroller and output stable PWM amplitudes over a wide supply voltage and temperature range. Two comparators are used for diagnostics or as general purpose comparators. A low offset voltage reference provides loop current bias, and upper sequencing control ensures smooth system startup. The MAXREFDES1161 uses the MAX12900 to implement a complete industrial loop-powered current transmitter that converts the microcontroller's 16-bit PWM signal into current without using a DAC, greatly reducing the total system cost without reducing accuracy. The 4mA to 20mA current loop transmitter achieves 1μA resolution and provides diagnostic functions such as loop power monitoring and current reading. The reference design comes with protection features such as loop power reverse protection and overvoltage protection, and can withstand up to 40V loop power input at 4mA current output. The MAXREFDES1161 is used to evaluate the performance of the MAX12900 and can be directly transplanted to the final product, simplifying current loop transmitter design and shortening the development cycle.

Key Features

• Reliable operation over a wide input voltage range

• Small size

• Complete solution without external components

• Reverse voltage protection

• Overcurrent protection

• Provides additional comparators

• Dual PWM input

• Supports external low-power microcontroller power supply

• Power-on sequence control

This circuit board is not for sale.

Design, build and test

This document describes the MAXREFDES1161 hardware design and provides detailed technical guidance for designing a loop-powered 4mA to 20mA transmitter using Maxim's MAX12900.

design resources