RTD thermometer

Description: This thermometer achieves an accuracy of 0.01°C across a temperature range of -50°C to +150°C. A distinctive trimming arrangement eliminates complex interactions during the trimming process, allowing for simultaneous adjustments of zero gain and nonlinearity correction. Additional operational amplifiers facilitate full Kelvin sensing on the sensor without introducing drift and offset terms commonly associated with alternative designs. Operational amplifier A1 is configured as a Howland current pump, providing the sensor with a constant current bias. Resistors R2, R3, R4, and R5 create a bridge configuration that is balanced by A1. When balanced, both inputs of operational amplifier A2 receive the same voltage. With R6 equal to R7, A1 draws equal currents from both sides of the bridge. If the sensor loads the R4/R5 leg, it would disrupt the balance of the bridge; thus, both bridge taps deliver the sensor's open circuit voltage without drawing current.

The described thermometer circuit utilizes a precision measurement methodology to deliver high accuracy over a specified temperature range. The unique trimming arrangement is crucial for maintaining calibration without the complications typically associated with traditional trimming methods. By allowing zero gain and nonlinearity adjustments to occur simultaneously, the design enhances ease of use and reliability.

The operational amplifier A1, configured as a Howland current pump, plays a pivotal role in providing a stable bias current to the temperature sensor. This configuration is beneficial as it minimizes the effects of drift and offset, which are prevalent in other designs, ensuring that the measurement remains accurate over time and varying environmental conditions.

The bridge circuit formed by resistors R2, R3, R4, and R5 is central to the operation of the thermometer. When balanced, the integrity of the measurement is maintained, as both inputs to operational amplifier A2 are equal. The equal values of R6 and R7 ensure that A1 draws equal currents from the bridge, which is essential for accurate readings. The design's consideration of sensor loading is critical; by ensuring that the sensor does not draw current from the bridge taps, the circuit guarantees that the open circuit voltage remains stable and reliable for measurement purposes.

Overall, this thermometer circuit exemplifies a sophisticated approach to temperature measurement, combining advanced operational amplifier configurations with a well-designed resistive bridge to achieve high precision and stability across a wide temperature range.This thermometer is capable of 0.01°C accuracy over - 50°C to +150°C. A unique trim arrangement eliminates cumbersome trim interactions so that zero gain, and nonlinearity correction can be trimmed in one even trip. Extra op amps provide full Kelvin sensing on the sensor without adding drift and offset terms found in other designs.

Al is configured as a Howland current pump, biasing the sensor with a fixed current. Resistors R2, R3, R4 and R5 form a bridge driven into balance by Al. In balance, both inputs of A2 are at the same voltage. Since R6 = R7, Al draws equal currents from both legs of the bridge. Any loading of the R4/R5 leg by the sensor would unbalance the bridge; therefore, both bridge taps are given to the sensor open circuit voltage and no current is drawn.