AN3137 datasheet(25/42 Pages) STMICROELECTRONICS

25 / 42 page

AN3137

Methods for precision improvement

Doc ID 16983 Rev 3

25/42

4.3.3

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:

… U

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 -

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 FFT post-processing 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

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 or interpolation can be used to insert sample points at the required frequency if

sampling is performed with an inaccurate clock.

4.3.4

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.

A proper physical model (which is usually a schematic diagram) is 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/capacitor values which represent bit

weights). To solve these equations, it 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 schematic with the designed values you obtain an ADC schematic

with the real values for a given microcontroller.

AN3137 Fiches technique(PDF) 25 Page - STMicroelectronics

AN3137 Fiches technique(HTML) 25 Page - STMicroelectronics

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

Fiches technique Télécharger

Lien URL