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Measuring Temperature Accurately with Semiconductor Sensors

Introduction

Temperature is one of the most common environmental metrics that is measured in electronic systems, and a variety of sensor types are available to meet the designer’s needs. However careful attention must be paid when selecting a temperature sensor to ensure the overall accuracy meets the design specification. This article summarizes the main considerations for accuracy in a semiconductor temperature sensor.

Temperature Sensor Types

Applications that require temperature sensing typically use one of the following sensor types:

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For basic temperature sensing where accuracy of ±2°C to ±3°C is adequate, thermistors tend to be a popular choice due to their low cost and minimal bill of materials (BOM). However for MCU-based applications if an ADC input is not available, then semiconductor sensors are the preferred option. For designs that require higher accuracy better than ±2°C, lower power consumption and/or high-resolution measurements, semiconductor sensors provide the best performance for the price as long as the sensing range is within –40 to +125 °C. Resistance temperature detectors (RTDs) traditionally offer the highest accuracy as good as ±0.2°C, albeit at high cost and with additional external BOM cost necessary to maintain accurate analog to digital conversions. New semiconductor temperature sensors like the Si705x from Silicon Labs now offer accuracy levels comparable to RTDs without the need for external BOM.

Thermopiles offer the advantage of non-contact measurement and can therefore also measure a wide range of temperatures. Traditional thermopile designs use TO-5 metal can packaging which either require external amplification or contain an embedded ASIC. This, coupled with the need for proper opto-mechanical design, necessitates higher cost and limits thermopiles to applications where the non-contact measurement provides significant advantages. Examples of this are in-ear thermometers and industrial process controls at high temperatures.

Factors that Influence Accuracy for Semiconductor Sensors

Semiconductor temperature sensors commonly use a bandgap element which measures variations in the forward voltage of a diode to determine temperature. To achieve reasonable accuracy, these are calibrated at a single temperature point, typically 25 °C. Therefore, highest accuracy is achieved at the calibration point and accuracy then deteriorates for higher or lower temperatures. For higher accuracy across a wide temperature range, additional calibration points or advanced signal processing techniques can be employed.

Manufacturers of semiconductor temperature sensors will specify typical and maximum temperature accuracy within certain temperature ranges. While typical values can give some idea of the accuracy for a few devices under ideal conditions, customers should rely on the maximum values for a true indication of accuracy across multiple devices and under a variety of conditions.

Power supply voltage can also affect temperature accuracy in a semiconductor sensor. Sensor devices with a lower level of internal voltage regulation will exhibit greater reductions in accuracy when the power supply deviates from nominal voltages. Most manufacturers will include this in their datasheet specifications, with maximum values in the range of ±0.2°C/V to ±0.3°C/V.

In higher accuracy devices with <±0.5°C error, secondary effects will begin to emerge that can also play a role in overall accuracy. These are often specified separately from the overall accuracy specification in manufacturer’s datasheets and should therefore be added. Some of these include:

  • Repeatability/noise: This is the error introduced by the analog front end and ADC, and can range from ±0.1°C in lower performance products to ±0.01°C in higher performance products.
  • Drift/stability: Additional error introduced over time as the device ages.

 

Design Considerations for High Accuracy

Proper placement of temperature sensors on a PCB is critical to maintaining accuracy. For measuring ambient air temperatures, sensors should be placed as far away from other heat sources as possible. These include MCUs, voltage regulators and other components. If the sensor is placed on the same PCB as other heat-generating components, cutouts in the PCB can help provide thermal isolation. Adequate ventilation must also be provided to allow the sensor to contact the ambient air.

If the temperature of PCB components is to be measured, temperature sensors should be placed as close as possible to the critical components. GND and VDD planes can be used to provide a thermal conduction path from the components to the sensors.

The response time of a temperature sensor is closely related to the thermal mass of the PCB and enclosure that it is mounted in. For example, a temperature sensor attached to the center of a large PCB will show a very slow response to changes in air temperature. For fastest response time, mount temperature sensors to small PCBs, or provide cutouts to insulate the sensors from the rest of the system’s thermal mass.

For more information about PCB placement, refer to Silicon Labs Application Note AN607.

Silicon Labs’ Si705x vs. Competing Temperature Sensors

Silicon Labs’ Si705x Digital Temperature Sensors have been designed to maintain their accuracy across the entire operating voltage and temperature ranges. With an operating voltage range of 1.9V to 3.6V, the Si705x devices can be directly connected to a battery in remote sensing applications. As seen in the plot below, the Si705x devices provide noticeable improvements in accuracy at high and low temperatures, minimal loss of accuracy from noise and no sensitivity to power supply voltage.

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For highest accuracy, devices like the Si7053 also provide acceptable alternatives to resistance temperature detectors, without the added cost and BOM complexity.

Example Temperature Accuracy Considerations in Real-World Applications:

  • Cold Chain Pharmaceutical / Food Storage: World Health Organization Specification E06/TR07.1 requires accuracy of ±0.5°C within -5°C to +25°C, and ±1°C within -20°C to -5°C and +25°C to +55°C.
  • HVAC & Refrigeration: In addition to absolute accuracy, long-term stability is important for these applications, since these devices are expected to last for many years. For example, a device specified with ±0.05°C/year will have 0.5°C worse accuracy after 10 years.
  • Cold Junction Compensation: Thermocouples are a common device used to measure very wide temperature ranges. However these require an accurate room temperature reference measurement to provide an accurate temperature measurement.
  • Remote Sensors: Considerations for the power supply voltage must be made when designing remote sensor nodes and portable data loggers. Without proper voltage regulation, variations in battery voltage can cause a reduction in temperature accuracy.

 

 

 

 

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