Temperature sensor overview

AN3397

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Doc ID 018809 Rev 1

1.3

Temperature sensor accuracy

Without calibration the TS is likely to give error readings in the order of ±30 °C. If the sensor

is calibrated, significantly more accurate temperature measurements can be obtained. For

example, if the TS characteristic slope and offset are calibrated, a five-fold improvement in

accuracy over the full temperature range can be obtained. Even greater accuracy is possible

if the temperature range is limited (see Figure 1).

STM8L15xxx devices are provided with a calibration data point, V90, which is factory

measured at hot temperatures (90 °C ± 5 °C) during the manufacturing process and stored

in the protected memory area of the device at address 0x4911 (see device datasheet for

details). This single calibration point may be sufficient depending on the precision required

by the application, but it is highly recommended to perform a second calibration point (user

calibration point) at ambient temperature.

Devices with a two-point calibration are generally suitable for most embedded applications

because the deviation versus the ideal linear curve is restrained and the individual

characteristics of the temperature sensor are respected.

1.4

Temperature sensor engineering tolerance

The engineering tolerance of the temperature estimation is illustrated in Figure 1. It is limited

by two boundary lines: the minimum biased values (green line) and the maximum biased

values (blue line). This tolerance is directly linked to both the temperature margin of the

factory calibrated data at hot temperatures (V90 = 90 °C ± 5 °C) and the Avg_slope

uncertainty (1.62 mV/°C ± 0.03 mV/°C) specified in the STM8L15xxx datasheets.

Figure 1 shows that the user calibration point significantly improves the accuracy of the

temperature measurement since reducing the whole temperature tolerance area to a lower

range means that more reasonable results are obtained. The recommended position of the

calibration points should be as close as possible to the minimum and maximum values of

the measurement range to cover the full device temperature range. However, the user

calibration point can be performed at ambient temperature (20 °C to 25 °C) as it is the

easiest way to proceed.