This low power platinum resistance temperature detector (RTD) sensor circuit draws only 35μA total supply current on a minimum 2.6V rail, and is accurate to within ±1°C at room temperature, including all error intrinsic to the Vishay PTS Class F0.3 Variant RTD. It covers the temperature range from –40°C to 85°C in 10mV/°C increments and produces an output of 1V at nominal room temperature of 25°C.
The LTC2063’s extremely low typical offset of 1μV and typical input bias current of 3pA allows for the use of a very low excitation current in the RTD. Thus, self-heating is negligible, improving accuracy.
The LT5400-3, B-grade, is used to provide a ±0.025% matched resistor network that is effectively a precision 131:1 voltage divider. This precision divider forms one half of a bridge circuit, with the 0.1% 110kΩ and RTD in the other branch. Note that the 110kΩ’s precision requirement is to ensure matching with the RTD. The 11kΩ R2 serves to provide a DC offset for the entire bridge so that the output is 1V at room temperature. Since bridge imbalances can lead to error, it is recommended to minimize the length of the leads connecting the RTD to reduce additional lead resistance.
The LT6656-2.048 reference helps create a known excitation current in the RTD at each temperature of operation, and also acts as a supply for the LTC2063, all while using less than 1μA itself. The LT6656 can accept input voltages anywhere between 2.6V and 18V, allowing for flexibility in selection of supply voltage while maintaining a fixed output range. The LT6656 reference can easily source the 35μA required to run the entire circuit, thanks to the LTC2063’s 2μA maximum supply current and ability to handle microvolt signals produced by the RTD under low excitation current.
Care should be taken to minimize thermocouple effects by preventing significant thermal gradients between the two op amp inputs. It is also important to choose feedback and series resistors that are low-tempco to minimize error due to drift over the entire temperature range. The LTC®2063 is a single low power, zero-drift, 20kHz amplifier. The LTC2063 enables high resolution measurement at extremely low power levels.
Tools & Simulations
LTspice® is a powerful, fast and free simulation software, schematic capture and waveform viewer with enhancements and models for improving the simulation of analog circuits.
To launch ready-to-run LTspice demonstration circuits for this part:
Step 1: Download and install LTspice on your computer.
Step 2: Click on the link in the section below to download a demonstration circuit.
Step 3: If LTspice does not automatically open after clicking the link below, you can instead run the simulation by right clicking on the link and selecting “Save Target As.” After saving the file to your computer, start LTspice and open the demonstration circuit by selecting ‘Open’ from the ‘File’ menu.