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Precise temp measurement with semiconductor sensors

27 Jul 2015  | Bill Simcoe

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Temperature is one of the most common environmental metrics that is measured in electronic systems, and numerous sensor types are available to meet the various application requirements of embedded system designers. However, designers must pay careful attention when selecting a temperature sensor to ensure that the overall sensor accuracy meets design specifications. We will examine some of the primary considerations for ensuring accuracy in a CMOS-based semiconductor temperature sensor.


Temperature sensor types
Applications that require temperature sensing typically use one of the following sensor types as shown in the table.


Tab: Sensor types.


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 analogue to digital converter (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 (up to ±0.2°C), albeit at high cost and with additional external BOM cost necessary to maintain accurate analogue to digital conversions.

New semiconductor temperature sensors such as the Si705x sensor family from Silicon Labs now offer accuracy levels comparable to RTDs but without the need to add external devices, which can increase system-level BOM cost.

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 approach, 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 applications appropriate for thermopiles include in-ear thermometers and industrial process controls operating at high temperatures.


Factors that influence accuracy for semiconductor sensors
Semiconductor temperature sensors commonly use a bandgap element that 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, as shown in figure 1. While typical values can give some idea of the accuracy for a few devices under ideal conditions, developers should rely on the maximum values for a true indication of accuracy across multiple devices and under a variety of conditions.


Figure 1: Example of typical and maximum temperature accuracy for semiconductor sensor.


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.

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