RTD Instrumentation for Temperature Sensing

1 2008-2013 Microchip Technology 1AN1154 INTRODUCTIONP recision RTD (Resistive Temperature Detector) Instrumentation is key for high-performance thermalmanagement applications. This application note showshow to use a high resolution Delta-Sigma Analog-to-Digital Converter, and two resistors to measure RTDresistance ratiometrically. A C accuracy and C measurement resolution can be achievedacross the RTD Temperature range of -200 C to+800 C with a single point calibration. A high resolution Delta-Sigma ADC can serve well forhigh-performance thermal management applicationssuch as industrial or medical Instrumentation .

2 Tradi-tionally, RTDs are biased with a constant currentsource. The voltage drop across the RTD is condi-tioned using an Instrumentation Amplifier whichrequires multiple resistors, capacitors and few opera-tion amplifiers and/or a stand-alone instrumentationamplifier. This analog Instrumentation techniquerequires a low noise and stable system to calibrate andaccurately measure Temperature . It also requires anoperator for optimization on the production floor. With the Delta-Sigma ADC solution, the RTD is directlyconnected to the ADC (Microchip s MCP3551 family of22-bit Delta-Sigma ADCs), and a single low-toleranceresistor is used to bias the RTD from the ADCreference voltage (Figure 1) and accurately measuretemperature ratiometrically.

3 A low dropout linearregulator (LDO) is used to provide a reference voltage(refer to Microchip s RTD Reference Design Board [3]).SOLUTIONThis solution uses a common reference voltage to biasthe RTD and the ADC which provides a ratio-metricrelation between the ADC resolution and the RTDtemperature resolution. Only one biasing resistor, RA,is needed to set the measurement resolution ratio(Equation 1).EQUATION 1:RTD RESISTANCEFor instance, a 2V ADC reference voltage (VREF)results in a 1 V/LSb (Least Significant bit) RA = RB = k provides V/ Ctemperature coefficient (PT100 RTD with / Ctemperature coefficient).

4 This provides C/LSbtemperature measurement resolution for the entirerange of 20 to 320 or -200 C to +800 C. A single-point calibration with a 100 resistor provides C accuracy, as shown in Figure approach provides a plug-and-play solution withminimum adjustment. However, the system accuracydepends on several factors such as the RTD type,biasing circuit tolerance and stability, error due topower dissipation or self-heat, and RTD nonlinearcharacteristics. FIGURE 1:RTD Instrumentation Circuit Block Diagram and Output Performance [3].Author:Ezana Haile Microchip Technology :Code = ADC output codeRA= Biasing resistorn = ADC number of bits(22 bits with sign, MCP3551)RRTDRACode2n1 Code ---------------------------------- = ( C)Measured Accuracy ( C)RTDRA 1%LDOVDDSPI3 PIC RB 5%VREFMCP3551+-VREFVLDOC*C*1 f* See LDO Data SheetMCUVDDP recision RTD Instrumentation for Temperature SensingAN1154DS00001154B-page 2 2008-2013 Microchip Technology MeasurementThe key feature of a ratiometric measurementtechnique is that the Temperature accuracy does notdepend on an accurate reference voltage.

5 The ADCreference voltage varies with respect to change in RTDresistance due to the voltage divider relation(Equation 2). This measurement maintains constantresolution. It eliminates the need for a constant biasingcurrent source or a voltage source, which can be costly,while providing a highly accurate temperaturemeasurement solution. Figure 2 shows a circuit blockdiagram with the ADC 2:REFERENCE VOLTAGE FIGURE 2:RTD Biasing and RB must be sufficiently large to minimize errordue to self-heat while providing adequate measure-ment 3 and Equation 4 show that due to the ratio-metric relation, VREF and RB cancel.

6 They do notinfluence the code to RTD-resistance conversion. Thisequation can be easily implemented using a 16-bitmicrocontroller such as the PIC18F 3:VOLTAGE ACROSS RTDS olving for RRTD from Equation 3 gives:EQUATION 4:RTD RESISTANCE AND ADC CODE RELATIONSM easurement Resolution and ADC CharacteristicsEQUATION 5:ADC RESOLUTIONThe key element to this solution is the direct proportion-ality of ADCLSb_quanta and RRTD. The temperaturemeasurement resolution can be determined as shownin Equation 6: Temperature MEASUREMENT RESOLUTIONWhen RA = RB = 6800 , the bias current is ~290 provides < C/LSb Temperature resolution.

7 Asthe RTD resistance varies due to Temperature , the IBIAS(biasing current) varies and Temperature resolutionremains below C/LSb, as shown in Figure 3:TRES vs. RTD +RARBRRTD++----------------------------- ----------=RTDRA 1%SPI3 VREFMCP3551+-VREF1 fVDD1 fVDDW here:VRTD (V) = RTD voltage VREF (V) = Reference VoltageCode = ADC output coden = ADC number of bits(22 bits with sign, MCP3551)VRTDVREFRRTDRARRTD+------------- ------------- VREFCode2n1 ------------- ==RRTDRACode2n1 Code ---------------------------------- =ADCRESOLUTIONVREF2n1 ----------------=Where:VREF(V) = Reference Voltagen = ADC number of bits(22 bits with sign, MCP3551)TRESADCRESOLUTION VRTD------------------------------------ --------=Where.

8 TRES ( C/LSb) = Temperature Measurement ( C)TRES ( C/LSb) 2008-2013 Microchip Technology 3AN1154 The MCP3551 22-bit differential ADC characteristics isoptimum for this type of application. There are fewspecifications that must be carefully considered, suchas conversion accuracy and noise performance. Themaximum full-scale error of the MCP3551 is 10 ppmand the error drift is ppm/C. The maximumintegral nonlinearity is 6 ppm. These specifications areso minute when considering the overall effect totemperature measurement accuracy. If IBIAS is set to~300 A, then the input voltage range to the ADC is~100 mV (VRTD) over the entire RTD temperaturerange.

9 Therefore, the error is much less than the full-scale error specified in the ADC data , the input offset noise is V (typical) and6 V (typical) for MCP3551 and MCP3553 ADCs,respectively. This specification adds offset error thatneeds be considered when converting offset error is specified as 12 V (maximum) at+25 C. This means there is up to 12 LSb flicker or thetemperature measurement precision is Cmaximum (Equation 6). This can be improved by takingthe average of multiple samples to precisely Tolerance and Measurement AccuracyThe variation in RA characteristics introduces tempera-ture accuracy error.

10 When evaluating Equation 4, a 1%tolerance in RA produces greater than C errorwhen using PT100 RTD with C/ temperaturecoefficient (for temperatures greater than 0 C). Forlower tolerance resistors, RA must be calibrated forprecision Temperature order to precisely calibrate RA, a calibration resistorcan be used in place of the RTD, such as 100 resistor and Equation 4 can be rearranged todetermine Temperature CalculationRTDs are significantly nonlinear. Depending on theRTD type and specification, the resistor to temperatureconversion equations have been defined andstandardized.