Resistive Temperature Detector(RTD) for Industrial Applications
Created: Sep 03, 2014
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Resistance Temperature Detector (RTD) and Thermocouple are the major instruments used for temperature sensing. However, the RTD is more common to industrial applications compared to thermocouple because it provides good accuracy, excellent stability and repeatability. This RTD reference design demonstrates Microchip's solution to accurately measure temperature using a 22bit ADC, MCP3551. This solution eliminates the need for the traditional analog instrumentation system calibration or gain and offset trimming techniques. In addition, limiting the RTD biasing current using a single resistor can minimize the effect of self-heat.
This reference design uses the technique described in application note AN1154. It allows users to evaluate Microchip's solution to accurately measure temperature using an RTD. The RTD resistance availability typically ranges from 100 to 5000. When biasing an RTD to measure temperature, self-heat due to power dissipation has to be considered. In order to produce an output measurable voltage across the RTD for wide temperature range, the biasing current has to be relatively high. This causes higher power dissipation through heat and skews of the temperature reading.
This solution uses a ratiometric technique between the RTD resistance and the ADC resolution to achieve high accuracy throughout the entire sensor range. This solution uses an on-board (local) surface mount RTD and an external RTD. The local and external RTDs are selected using JP2. The MCP9804 temperature sensor is also used to measure local temperature.