DESIGN NOTES Number 45 in a series from Linear Technology Corporation March, 1991 Signal Conditioning for Platinum Temperature Transducers Jim Williams High accuracy, stability, and wide operating range make platinum RTDs (resistance temperature detectors) popular temperature transducers. Signal conditioning these devices requires care to utilize their desirable characteristics. Figure 1s bridge based circuit is highly accurate and features aground referred RTD. The ground connection is often desirable for noise rejection. The bridges RTD leg is driven by a current source while the opposing bridge branch is voltage biased. The current drive allows the voltage across the RTD to vary directly with its temperature induced resistance shift. The differ- ence between this potential and that of the opposing bridge leg forms the bridges output. A1A and instrumentation amplifier A2 form a voltage controlled current source. A1A, biased by the LT1009 reference, drives currentthrough the 88.7Qresistor and the RTD. A2, sensing differentially across the 88.7 resistor, closes a loop back to A1A. The 2k-0.1pF combination sets amplifier rolloff, and the configuration is stable. Because A1As loop forces a fixed voltage across the 88.7Q resistor, the current through Rp is constant. A1s operating point is primarily fixed by the 2.5V LT1009 voltage reference. The RTDs constant current forces the voltage across it to vary with its resistance, which has a nearly linear positive temperature coefficient. The non-linearity could cause several degrees of error over the circuits 0C- 400C operating range. The bridges output is fed to instrumentation amplifier A3, which provides differen- tial gain while simultaneously supplying non-linearity + 15V > 27k > +15V 40k" Wir + LT1009 AA 25V 1/2 LT1078 , - > 274K" > 2 > 0.1nF [+4 + 2k Aa P LIM01} A=10 AA 1% FILM RESISTOR Rp = ROSEMOUNT 118MFRTD OV-10VouT = 0C-400C + 0.05C = 4 > GAIN 5 aK! e 13k Figure 1. Linearized Platinum RTD Bridge. Feedback to Bridge from A3 Linearizes the Circuit. LD tineSVOUT +15V 111027 . 500k 2 12k 12.5k cay + 15V > >, 2 1k* Keo "TRW-RC MAR-6 RESISTOR0.1% **1% FILM RESISTOR Rpiat. = 1k0 AT 0CROSEMOUNT #118MF + 10uF HH SERIAL OUT TO VREF [ 68HC05 PROCESSOR LTG1290 +V | +15V Figure 2. Digitally Linearized Platinum RTD Signal Conditioner correction. The correction is implemented by feeding a portion of A3s output back to A1s input via the 1 Ok-250k divider. This causes the current supplied to Rp to slightly shift with its operating point, compensating sensor non- linearity to within +0.05C. A1B, providing additional scaled gain, furnishes the circuit output. To calibrate this circuit, substitute a precision decade box (e.g., General Radio 1432k) for Rp. Set the box to the 0C value (100.002) and adjust the zero trim fora 0.00V output. Next, set the decade box for a 140C output (154.260) and adjust the gain trim for a 3.500V output reading. Finally, set the box to 249.0Q (400.00C) and trim the linearity adjustment for a 10.000V output. Repeat this sequence untilall three points are fixed. Total error over the entire range will be within +0.05C. The resistance values given are for a nominal 100.002 (0C) sensor. Sensors deviating from this nominal value can be used by factoring in the deviation from 100,00Q. This deviation, which is manufacturer specified for each individual sensor, is an offset term due to winding tolerances during fabrication of the RTD. The gain slope of the platinum is primarily fixed by the purity of the materiai and has a very small error term. The previous example relies on analog techniques to achieve a precise, tinear output from the platinum RTD bridge. Figure 2 uses digital corrections to obtain similar results. A processor is used to correct residual! RTD non-linearities. The bridges inherent non-linear output is also accommodated by the processor. The LT1027 drives the bridge with 5V. The bridge differential output is extracted by instrumentation am- plifier A1. Ais output, via gain scaling stage A2, is fed to the LTC1290 12-bit A-D. The LTC1290s raw output codes reflect the bridges non-linear output versus tem- perature. The processor corrects the A-D output and presents linearized, calibrated data out. RTD and resis- tor tolerances mandate zero and full scale trims, but no linearity correction is necessary. A2s analog output is available for feedback control applications. The com- plete software code for the 68HC05 processor, devel- oped by Guy M. Hoover, appears in Application Note 43. For literature on our Amplifiers and Data Converters, call (800) 637-5545. For applications help, call (408) |432-1900, Ext. 456 . DN45-2