Methods for precision improvement

AN2719

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15th order. The amplitude of 15th harmonics in the main signal is very small (the next order

harmonics can be neglected). The calculated effective value of the mains signal can be

obtained with high precision because the effective values of harmonics are added to the

total AC harmonic value as:

So if the 15th harmonics amplitude is only 1% (0.01) from 1st harmonics (50 Hz) then its

contribution to the total effective value will be only 0.01% (because of above equation -

The principle of this method is therefore to sample the AC signal with a known frequency

and then perform FFT postprocessing on the data of each measured period. Because the

number of sampling points per measured signal period is small (32 points for example) then

the performance needed for FFT processing is not so high (only 32-point FFT for example).

If there is no requirement for real-time processing, for example, with a stable input signal

shape, and measuring only one period per second, as in the case of the mains signal, then

FFT can be calculated even by an 8-bit microcontroller.

Advantages: this method is good for AC measurement of a stable input signal. The

disadvantage is the requirement for precise signal sampling. The frequency of the measured

multiplier of the measured frequency. The input signal frequency can be measured by

another method. The ADC sampling frequency can be tuned by programming the prescaler

and MCU master clock or interpolation can used to insert sample points at the required

frequency if sampling is performed with an inaccurate clock.

3.3.4

ADC calibration

This method requires knowledge of internal ADC structure and how the ADC converter is

implemented inside the microcontroller. This knowledge is necessary in order to design a

physical/mathematical model of the ADC implementation. In case of an unknown ADC

implementation we can use standard models - linear or polynomic.

A proper physical model (which is usually a schematic diagram) can be used as the base for

describing it mathematically. From the mathematical model each element in the model can

be obtained by set of equations (for example, resistor values which represent bit weights).

To solve these equations is necessary to perform a set of practical measurements and

obtain a set of solvable equations (for example, measurement: input signal versus the

proper ADC digital output words).

From the measured values and mathematical computation of the model, all known values of

model elements (resistors, voltages, capacitors,...) can be put into the schematic diagram.

So instead of the ADC converter schematic with the designed values you obtain an ADC

converter schematic with the real values for a given microcontroller.

Computed model parameters can be stored in the microcontroller memory after calibration

and used in postprocessing to correct ADC values.

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