12-bit, 4-20mA loop-powered thermocouple measurement system using ARM Cortex-M3_Temperature sensor circuit

　　Advantages and features:

　　T-type thermocouple measurement system, capable of loop power supply for 4-20mA output

　　Typical temperature range is -200C to +400C

　　Single-chip solution using Cortex processing core

　　Cold junction compensation

　　Products used in this circuit

　　ADP1720

　　ADUCM360

　　application:

　　Field instrumentation and smart transmitters

　　Temperature Controller

　　Sensors and sensor interfaces

　　Circuit functions and advantages

　　This circuit uses the ADuCM360 precision analog microcontroller in a precision thermocouple temperature monitoring application and controls the output current from 4mA to 20mA accordingly. ADuCM360 integrates dual-channel 24-bit Δ-type analog-to-digital converter (ADC), dual-channel programmable current source, 12-bit analog-to-analog converter (DAC), 1.2V built-in reference voltage source and ARM Cortex-M3 core, 126KB Flash memory, 8KB SRAM and various digital peripherals such as UART, timers, SPI and I2C interfaces.

　　In this circuit, the ADuCM360 is connected to a T-type thermocouple and a 100 platinum resistance temperature detector (RTD). RTD is used for cold junction compensation. A low-power Cortex-M3 core converts ADC readings into actual temperature values. The supported T-type temperature range is −200°C to +350°C, and the output current range corresponding to this temperature range is 4mA to 20mA.

　　This circuit provides a complete solution for thermocouple measurements, requires minimal external components, and can be loop powered for loop voltages up to 28V.

　　Advantages and features

　　4-20mA loop powered transmitter

　　Voltage or current drive

　　Optimized pressure sensor

　　Products used in this circuit

　　AD8226

　　ADA4091-2

　　ADR02

　　application:

　　Field instrumentation and smart transmitters

　　Circuit functions and advantages

　　The circuit shown in Figure 1 is a robust and flexible loop-powered current transducer that converts the differential voltage output of a pressure sensor into a 4 mA to 20 mA current output. The design is optimized for a variety of bridge-type voltage or current-driven pressure sensors, using only four active components and achieving a total unadjusted error of less than 1%. The loop supply voltage range is 12V to 36V. The inputs of this circuit are ESD protected and provide voltage protection above the supply rails, making it ideal for industrial applications.

Figure 1. Robust loop-powered pressure sensor signal conditioning circuit with 4 mA to 20 mA output (shown in sensor voltage drive mode), schematic: all connections and decoupling not shown