AN3322 datasheet(29/38 Pages) STMICROELECTRONICS

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AN3322

Computational issues and compensation algorithms

2.6

Filtering of input signal for proper zero crossing detection

Proper detection of the zero crossing is the key parameter in order to measure and compute

all the values correctly. Possible problems are caused when the voltage channel contains

more harmonics and such a signal with these harmonics crosses the zero voltage line more

times. Figure 16 shows the voltage signal that is crossing the zero line twice. The same

situation can be caused not only by higher harmonics but also by noise present in all the

circuitry.

Figure 16. Voltage signal crossing zero voltage line twice

In order to avoid an incorrect zero crossing detection, a filter was introduced. The filter is the

simple moving average. The length of the filter is 18. Since the number of the samples per

period of mains is 44, the filter removes all the harmonics starting at 50 Hz with growing

attenuation. Figure 17 shows the signal after the filtering that allows for failure-less zero

crossing detection.

Figure 17. Voltage signal used for correct zero crossing after proper filtering

AN3322 Datasheet(PDF) 29 Page - STMicroelectronics