AN1873| Application Note
Maxim/Dallas > App Notes > TEMPERATURE SENSORS and THERMAL MANAGEMENT Keywords: analog, temperature sensors, temperature, 4-20mA, temp sensor, loop powered
Feb 03, 2003
APPLICATION NOTE 1873
4-20mA Loop Powered Temperature Sensor
A simple circuit that allows a 4-20mA to power an analog temperature sensor. This circuit uses an analog temperature sensor, op amp, transistor, and low dropout linear regulator to provide a 4-20mA output over a 3.75 to 28 volt compliance range. The low quiescent current of the devices used permits them to be powered by the loop with the only consequence being a slight offset error.
Figure 1. The temperature sensor IC1 feeds the op amp/transistor combination, A1 and Q1, with R1 as the load on the amplifier. IC1's output consists of an offset of 744mV at 0 C and a scale factor of 11.9mV/ C. R1 is selected to provide the best possible fit of the temperature range of IC1 to the 4-20mA output. In this example, IC1's output at -25 C is 0.4465 volts, and a 111 resistor for R1 provides a 4mA output at -25 C. At 125 C IC1's output is 2.213 volts, yielding 19.937mA with the 111 resistor used for R1. This current is reflected at the input of the low-dropout linear regulator IC2. Besides regulating the voltage to the sensor and op amp circuit, IC2 provides the compliance at the voltage input required for connection to the 4-20mA loop. IC2 is pin programmable to provide either 3 or 5 volts output, and in this circuit is programmed to provide 3 volts to IC1 and A1. This maximizes the input compliance by allowing input voltages as low as 3.75 volts (as well as resulting in a slight reduction of quies
cent current of IC1 and A1, therefore less error due to quiescent current). Quiescent currents of all components combine and add to the 4mA output which corresponds to negative full
AN1873 Application Note maxim Download PDF
Add this permalink to your bookmarks for future download of AN1873 ApplicationNote
Permalink: http://application.emcelettronica.com/maxim/AN1873
