Using an analog temperature sensor, an op amp, a transistor, and a low-dropout linear regulator, this circuit provides a 4-to-20-mA output over a 3.75- to 28-V compliance range (see the figure). Because the devices used maintain a low quiescent current, they can be powered by the loop with a slight offset error being the only consequence.
The temperature sensor IC1 feeds the op-amp/transistor combination, A1 and Q1, with R1 acting as the load on the amplifier. The temperature sensor's output characteristic is described by an offset of 744 mV at 0°C and a scale factor of 11.9 mV/°C.
R1 is selected to achieve the best possible fit between IC1's temperature range and the 4-to-20-mA output. In this example, IC1's output at −25°C is 0.4465 V. Also, a 111-Ω resistor for R1 will supply a 4-mA output at −25°C. At 125°C, IC1's output is 2.213 V, yielding a 19.937-mA output with the 111-Ω resistor previously selected for R1. This current is reflected at the input of the low-dropout linear regulator IC2.
IC2 regulates the voltage to the sensor and op-amp circuit. Also, at the input voltage, it supplies the compliance necessary for connection to the 4-to-20-mA loop. Another feature of IC2 is that it's pin-programmable to generate either a 3- or 5-V output. In this circuit, IC1 and A1 operate at 3 V. This maximizes the input compliance by permitting input voltages as low as 3.75 V. This also results in a slight reduction of quiescent current of IC1 and A1, decreasing the error related to their quiescent currents.
The quiescent currents of all components combine and add to the 4-mA output that corresponds to negative full-scale. Consider this 45-µA current in light of the output-current scale factor, which is proportional to:
119 mV/°C
R1
This yields a current of 106.66 µA/°C. The 45-µA quiescent current represents an offset of approximately 0.43°C. Since the current is an offset, it's possible to compensate for it elsewhere (such as in software, when the temperature data is digitized).