AN3322 datasheet(33/38 Pages) STMICROELECTRONICS

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AN3322

Computational issues and compensation algorithms

Figure 21. Error of instant power caused by phase shift for PF = 0.5

2.7.3

Phase shift correction - filter implementation

The phase shift in the time domain is not as simple as in the frequency domain or in

a system where more ADC are employed for current and voltage measurements. In this HW

implementation we need to use an approach that allows us effectively interpolate values of

the voltage so that they are correctly aligned to current. This approach is described in

a separate document supplied on request.

The phase compensation range that may be reached with this approach is:

With the step of 0.73 µs (0.013 deg.)

Maximum positive shift = -6.2 µs (0.11 deg.)

Maximum negative shift = +6.2 µs (0.11 deg.)

With the step of 0.77 µs (0.014 deg.)

Maximum positive shift = -12.4 µs (0.22 deg.)

Maximum negative shift = +12.4 µs(0.22 deg.)

When a higher value of the phase shift needs to be compensated, an extension to the

method used is introduced.

2.8

Calibration procedure

Calibration (computation of the correction factor between measured and expected output) is

a very important process and its correct performance results in the overall desired precision.

Even when the power meter measures the current and voltage with the maximum error

0.5%, we need to perform calibration on an even higher level of accuracy. If this is not done,

calibrate the meter with the error of 0.5%, in the worst case (over the whole measured

range) the maximum error may be an additional 0.5%, which results in an error of 1%

(0.5 + 0.5) in total. This means that the measured value should be used for calibration after

its error is the lowest possible. This happens after averaging the value over a longer period

AN3322 Datasheet(PDF) 33 Page - STMicroelectronics