2.2 Two-point Calibration/Compensation

Figure 2-5. ADC Output Error in ADC Noise Reduction Mode, 1MHz, 3.3V, Two-point Compensated

The figure above shows the same results as Figure 2-3 and Figure 2-4, but with two-point calibration and compensation. The first calibration temperature is 25°C and the second is 85°C. If ±1°C accuracy is required, the temperatures where the results are satisfactory range from 5°C to 95°C with three exceptions. The errors at the temperature extremes are much smaller, and the compensation gives ±3°C accuracy over the whole temperature range.

Figure 2-6. ADC Output Error, ADC Noise Reduction Mode, 1MHz, 3.3V, no Compensation

As seen in the Figure 2-6 above, the curve of the ADC characteristic of these devices is approximately piecewise linear.

Figure 2-7. ADC Output Error in Active Mode, 8MHz, 3.3V, Two-point Compensated

It is possible that making the measurements at a high CPU frequency and in active mode actually makes the results more correct, if subsequent two-point calibration and compensation is applied. The Figure 2-7 above illustrates this. Here, the measurements are carried out in active mode at 8MHz CPU frequency. If ±1°C accuracy is needed, this approach results in satisfactory readings between the calibration points (25°C and 85°C) with no exceptions, in contrast to the measurements in the Figure 2-5 where three values had an error larger than ±1°C. If the second calibration point isn’t accurate, two-point calibration may actually make the results worse. Let’s say the second calibration point is ±5°C accurate. If T2 is 5°C above the actual value at 85°C, two-point calibration/compensation would result in the following results:

Figure 2-8. Two-point Compensated with T2 5°C Above Ideal Value

If T2 is 5°C smaller than the target temperature of 85°C, the results of the compensation would be like this:

Figure 2-9. Two-point Compensated with T2 5°C Below Ideal Value

The two figures above Figure 2-8 and Figure 2-9 here show that if one or both of the calibration temperatures are not accurate, there is no sense in using two-point calibration and compensation. With only one accurate calibration temperature, one-point calibration and compensation gives the best results.