Overview

There are many kinds of temperature measurement sensors, including thermocouples, PRTD (Platinum Resistance Temperature Detectors), thermistors, thermal diodes and so on.

The accuracy of the system is determined by the accuracy of the temperature sensor and the high performance ADC that digitizes the sensor data. In industrial and medical applications many temperature measurements typically require ±0.1°C or better measurement accuracy, reasonable cost, and lower power consumption.

The temperature measurement range in these fields is generally -200°C to +1750°C. The platinum resistance temperature sensor is used in high-precision temperature measurement because of its excellent accuracy and interchangeability. The platinum resistance sensor uses metal platinum. A temperature sensor made of a physical characteristic in which the resistance value of (Pt) changes with a change in temperature, PRTD is generally used.

Influence of measurement accuracy

The resistance of the PRTD exhibits a nearly linear corresponding temperature measurement over the temperature range.

PRTD temperature detection interface can be used to provide differential signals for ADCs using 2-wire, 3-wire, and 4-wire interfaces. Each type of interface has its advantages and disadvantages. The 4-wire connection method can obtain the most accurate measurement results. The disadvantage is that the reliability is reduced and the system cost is increased. The 3-wire connection is accurate, the cost is more economical, and the current use is relatively common. Wiring is the most economical and usually requires the use of a microprocessor or DSP to compensate for the IR error of the leads.

The error in PRTD measurement mainly includes:

1. Error caused by wire resistance: PRTD acts as a resistor, and the resistance of the wire between it and the control terminal can cause errors.

2. Error caused by self-heating: PRTD itself is a kind of metal resistance. When the excitation current passes through the PRTD, the power consumption caused by the sensor itself increases, causing the resistance of the PRTD to become large.

3, PRTD's linearity error: PRTD has the approximate linear characteristic, in 0°C resistance value is 100 European PRTD, when the temperature reaches 266°C, the resistance value will reach 200 Europe, but the linear error will change with the temperature.

High-precision design

Temperature measurement systems are generally low-speed, sampling no more than 100 times per second, narrow-band ADC can meet the demand, but require ADC with high accuracy, this article on how to ensure the high accuracy of the ADC as follows:

1) Using a three-wire constant-current source to drive Pt1000 platinum resistance, effectively overcoming the influence of wire resistance and self-heating effect on measurement accuracy;

2) Using the single-chip computer to calculate the two measurement voltages under the bipolar driving current can effectively avoid the system error of the wiring barrier voltage and the offset and drift of the amplifier and the A/D converter;

3) The constant current source and the A/D converter share a common reference, effectively eliminating the error caused by the instability of the reference reference.

4) The MLS numerical algorithm is used to cancel the noise in the MCU, further overcoming the influence of noise and random errors on the measurement accuracy and stability, and greatly improving the temperature measurement accuracy and stability.

5) The ADC has a differential analog input and accepts a differential reference voltage, allowing ratio configuration. As can be seen from Figure 1, the constant current source is provided to the ADC reference. The voltage generated by the precision constant current source is provided to the differential reference input of the ADC. The analog input voltage to the ADC is proportional to the reference voltage. Any error in the analog input voltage caused by PRTD current source temperature drift can be compensated by the deviation of its reference voltage.

Figure 1 Three-wire high-precision temperature measurement system

In addition, the temperature measurement system requires high requirements for the ADC and other electronic components of the system. The tiny analog signal generated by the temperature sensor needs to be amplified by a low-noise gain amplifier, and it is required that the noise of the amplifier circuit cannot submerge the weak signal of the sensor. The analog-to-digital conversion part needs a high-precision ADC to digitize the analog signal output from the sensor, and a high-precision ∑-Δ ADC is suitable. In addition to the ADC and the amplified power supply, the excitation current source and the reference voltage source must have low drift and low noise to provide better system accuracy.

Author: Yan of Shanghai Science and Technology Co., Ltd., a research agency

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