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AN2834

How to get the best ADC accuracy

3.3.3

FFT for AC measurement

In some specific cases the application needs to know the amplitude of an AC signal with a

given frequency. In this case the effective value of an AC signal can also be obtained by

using a relatively slow sampling speed (in comparison to the measured signal frequency).

For example, when measuring an AC mains signal (which is near-to-sinusoidal and has

relatively low harmonics content), it is sufficient to choose a sampling frequency 32 times

greater than the mains frequency (50 Hz). In this case you can obtain harmonics of up to the

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 is 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 the 1st harmonics (50 Hz) then its

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

square addition: 0.012 = 0.0001).

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

and then perform post-processing onthe FFT for 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).

Advantages: this method is good for AC measurement of signals with lower distortion.



The disadvantage is the requirement for precise signal sampling:



The frequency of the measured signal must be known and the ADC sampling

frequency must be set exactly as a 2n multiplier of the measured frequency.



The input signal frequency is measured by another method.



The ADC sampling frequency is tuned by programming the prescaler and MCU master

clock selection (if sampling is performed with an inaccurate clock an interpolation can

be used to obtain samples at the required points).

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