Electronic Design

Precision Thermometer Linearizes, Digitally Calibrates Platinum RTDs

Among temperature transducers, the platinum resistance temperature detector (PRTD) is generally accepted as the “gold standard.” PRTDs are available with interchangeability accuracies as tight as 0.1°C over operational ranges extending from −200°C (colder than liquid nitrogen) to +850°C (hotter than liquid aluminum). As a result, PRTDs are ubiquitous in aviation, environmental, industrial, and scientific instrumentation.

The PRTD temperature response consists of resistance variations on the order of only tenths of ohms/°C, so strict attention must be paid to the effects of transducer lead-wire resistance. Either the signal-conditioning circuitry must be packaged so that it’s close to the sensor (making the lead wires short and their resistance insignificant), or Kelvin-sensing arrangements will be needed to explicitly sense and cancel out wiring resistance. The excitation current must be kept at less than 1 mA or excessive I2R PRTD power dissipation will cause unacceptable self-heating measurement errors.

The combination of low excitation currents and small resistance changes means that the PRTD signal will typically be on the order of only tens of microvolts/°C. As a result, stable highgain dc amplification in the signal chain is required. Also, although the PRTD temperature coefficient is extremely stable and reproducible, it's only “reasonably” invariant with temperature. Thus, the PRTD’s response is significantly nonlinear and deviates from a linear approximation by tens of degrees over large temperature spans.

The accurate measurement of temperature over wide ranges therefore depends on provisions for linearization of the PRTD signal. These design considerations, which are incorporated in the circuit in the figure, result in a precision thermometer with output span of −1 V to +3.5 V, corresponding to a temperature range of −100 to +350°C. The maximum error over this span can be adjusted to ±0.02°C at 0°C and ±0.05°C elsewhere. Current excitation (approximately 250 µA) for the PRTD is sourced by the 2.5-V voltage reference VR1 via R1. A 256-tap digitally controlled potentiometer (DCP1) provides automated adjustment of the thermometer scale factor and span.

Amplifier A1 is used to scale up the ~100-µV/°C raw PRTD temperature signal to 0.01 V/°C. The DCP2 network implements a high-resolution zero adjustment in which each increment in DCP2’s setting will result in a 200-µV shift in A1’s output—a 0.02°C zero adjustment. The symmetry of the R6-R9 network surrounding DCP2 causes zero adjustment to have no effect on A1’s gain and, therefore, no effect on the thermometer’s span/scale factor. Likewise, performing span adjustments via trimming of the VR1 reference prevents interaction between DCP1 and the zero calibration point established by DCP2.

Positive feedback provided by R2 linearizes the thermometer’s response curve by providing the Thevenin equivalent of a negative resistance (−2064 W) in parallel with R1. This ploy introduces a positive slope (roughly +0.016%/°C), which effectively cancels the tendency of the PRTD temperature coefficient to decline with increasing temperature. The result is better than a factor of 100 improvement in linearity over the raw PRTD response.

The illustrated circuit implements a signal-conditioned precision temperature sensor that’s compatible (thanks to DCP1 and DCP2) with full automation of the calibration process, low in total power draw, and low in cost. Substituting digitally controlled potentiometers in place of manually adjusted trimmers can dramatically improve reliability. At the same time, it allows packaging of the signal conditioning circuitry that’s compatible with rugged encapsulation appropriate for mounting close to the PRTD itself. This avoids problems related to lead length and resistance.

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