One-point Calibration and Compensation

If one-point calibration is the only option, compensation should be avoided when the temperatures of interest are far from the calibration point (>20°C). Most devices benefit from the use of offset compensation, and for some devices it may even suffice. However, for other devices one-point calibration can result in larger errors. If one-point calibration is used, the operation window of the temperature sensor with ±1°C accuracy is no more than 20-30°C.

Figure 1. ADC Output Error in ADC Noise Reduction Mode, 1MHz, 3.3V, Offset Compensated

The Figure 1 above shows the results from measurements carried out on three different devices in ADC Noise Reduction Mode. In this figure simple offset calibration and compensation has been applied. The ADC reading of the temperature sensor at 25°C is used as the calibration point. The chart shows high, low and average data. The dot gives the average value, while the top of the individual lines give the largest value and the bottom give the lowest. As the figure shows, on these devices this kind of compensation gave good results over a wide temperature range. If ±1°C was required, the compensation gave satisfactory results from 20°C to 55°C.

Applying only gain compensation on the same readings gives the following results:

Figure 2. ADC Output Error in ADC Noise Reduction Mode, 1MHz, 3.3V, Gain Compensated

If one-point calibration/compensation is used or if no compensation is applied, entering ADC Noise Reduction Mode before the conversion starts is necessary to obtain good results. ADC Noise Reduction Mode works well, but at low CPU frequencies the advantage over active mode is quite small. At 8MHz however, the ADC Noise Reduction Mode is essential.