ADE7978ACPZ datasheet(71/125 Pages) AD

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Data Sheet

ADE7978/ADE7933/ADE7932/ADE7923

Rev. D | Page 71 of 125

TOTAL HARMONIC DISTORTION CALCULATION

The ADE7978 computes the total harmonic distortion (THD)

on all phase currents and voltages. The THD expressions are

shown in the following equations:

THD

−

(49)

THD

−

where:

I and V are the rms values of the phase currents and voltages

stored in the AIRMS, AVRMS, BIRMS, BVRMS, CIRMS, and

CVRMS registers.

I1 and V1 are the fundamental rms values stored in the AFIRMS,

AFVRMS, BFIRMS, BFVRMS, CFIRMS, and CFVRMS

registers.

The THD calculations are stored in the AVTHD, AITHD,

BVTHD, BITHD, CVTHD, and CITHD registers (Address 0xE521

to Address 0xE526). These registers are 24-bit registers in 3.21

signed format. This means the ratios are limited to +3.9999 and

all greater results are clamped to it.

Like the xIRMS and xFIRMS registers, the 24-bit signed xITHD

and xVTHD registers are transmitted as 32-bit registers with

the eight MSBs padded with 0s (see Figure 75).

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