AN3322 datasheet(30/38 Pages) STMICROELECTRONICS

Part #	AN3322

Description	Watt-hour meter based on the STM32F101 microcontroller

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Computational issues and compensation algorithms

AN3322

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DocID18304 Rev 1

Since the voltage signal with very low amplitude can lead to incorrect zero crossing

detection, the zero crossing detection is not performed on the voltage signal with an

amplitude lower that 50 V. The measured frequency and locking to the zero crossing is not

updated in this case.

2.7

Phase shift correction

This section describes how to deal with phase shift in the analog circuitry of the STM32

power meter. The evaluation of the error caused by phase shift and the levels of its

correction is computed in Section 2.7.1, Section 2.7.2, and Section 2.7.3.

2.7.1

Phase shift measured for the STM32 meter

The phase shift that needs to be compensated was measured directly in the STM32 meter.

These values help to set the limits and resolution of the phase shift compensation method.

Figure 18. Zero crossing for green - U, blue - ILG, red- IHG

Figure 18 shows that there is a delay between voltage U and currents ILG and IHG. This

delay between U and ILG or between U and IHG is in the same range for both ranges and it

is 5 - 8 µs. This delay is mainly caused by the capacitance and inductance in the signal path

of the input voltage divider (see Section 2.10).

2.7.2

Phase shift correction - error evaluation

The phase shift between voltage and current is also caused by non-simultaneous sampling

of these values, see Section 1.9.

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