Precision RTD Instrumentation for Temperature Sensing

AN1154 Precision RTD Instrumentation for Temperature Sensing SOLUTION Author: Ezana Haile Microchip Technology Inc. This solution uses a common reference voltage to bias the RTD and the ADC which provides a ratio-metric relation between the ADC resolution and the RTD tem- INTRODUCTION , isperature resolution. Only one biasing resistor, R A Precision RTD (Resistive Temperature Detector) needed to set the measurement resolution ratio instrumentation is key for high performance thermal (Equation 1). management applications. This application note shows how to use a high resolution Delta-Sigma Analog-to- EQUATION 1: RTD RESISTANCE Digital converter, and two resistors to measure RTD ?? Code resistance ratiometrically. A ±0.1°C accuracy and ----------------------------------R R= ?? RTD A n1- ±0.01°C measurement resolution can be achieved ?? 2 - Code across the RTD temperature range of -200°C to Where: +800°C with a single point calibration. Code = ADC output code A high resolution Delta-Sigma ADC can serve well for = Biasing resistorR A high performance thermal management applications n = ADC number of bitssuch as industrial or medical instrumentation. Tradi- (22 bits with sign, MCP3551) tionally, RTDs are biased with a constant current source. The voltage drop across the RTD is condi- tioned using an Instrumentation Amplifier which For instance, a 2V ADC reference voltage (V ) REF requires multiple resistors, capacitors and few opera- results in a 1 µV/LSb (Least Significant Bit) resolution. tion amplifiers and/or a stand-alone instrumentation Setting R = R = 6.8 k? provides 111.6 µV/°C A B amplifier. This analog instrumentation technique temperature coefficient (PT100 RTD with 0.385?/°C requires a low noise and stable system to calibrate and temperature coefficient). This provides 0.008°C/LSb accurately measure temperature. It also requires an temperature measurement resolution for the entire operator for optimization on the production floor. With range of 20? to 320? or -200°C to +800°C. A single the Delta-Sigma ADC solution, the RTD is directly point calibration with a 0.1% 100? resistor provides connected to the ADC (Microchip's MCP3551 family of ±0.1°C accuracy as shown in Figure 1. 22 bit Delta-Sigma ADCs) and a single low tolerance This approach provides a plug-and-play solution with resistor is used to bias the RTD from the ADC minimum adjustment. However, the system accuracy reference voltage (Figure 1) and accurately measure depends on several factors such as the RTD type, temperature ratiometrically. A low drop out linear biasing circuit tolerance and stability, error due to regulator (LDO) is used to provide a reference voltage. power dissipation or self-heat, and RTD non-linear characteristics. V DD V LDO 0.1 LDO ) C*C* (°C R 5% B 0.05 cy V REF 1µf 0 1%R A