EVAL-ADE7880EBZ datasheet(66/104 Pages) AD

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ADE7880

Data Sheet

Rev. A | Page 66 of 104

By default, the TERMSELx bits are all 1 and the CF1SEL bits are

000, the CF2SEL bits are 100, and the CF3SEL bits are 010. This

means that by default, the CF1 digital-to-frequency converter

produces signals proportional to the sum of all 3-phase total

active powers, CF2 produces signals proportional to fundamental

reactive powers, and CF3 produces signals proportional to

apparent powers.

Similar to the energy accumulation process, the energy-to-

frequency conversion is accomplished in two stages. The first

stage is the same stage illustrated in the energy accumulation

sections of active, reactive and apparent powers (see Active

Energy Calculation, Fundamental Reactive Energy Calculation,

Apparent Energy Calculation sections). The second stage

consists of the frequency divider by the CFxDEN 16-bit

unsigned registers. The values of CFxDEN depend on the meter

constant (MC), measured in impulses/kWh and how much

energy is assigned to one LSB of various energy registers:

xWATTHR, xFVARHR, and so forth. Suppose a derivative of

Wh [10n Wh] where n is a positive or negative integer, is desired

as one LSB of xWATTHR register. Then, CFxDEN is as follows:

CFxDEN

]

imp/kwh

[

(49)

The derivative of wh must be chosen in such a way to obtain a

CFxDEN register content greater than 1. If CFxDEN = 1, then

the CFx pin stays active low for only 1 μs. Thus, CFxDEN

accommodate fractional results; the result of the division must be

rounded to the nearest integer. If CFxDEN is set equal to 0, then

the ADE7880 considers it to be equal to 1.

The CFx pulse output stays low for 80 ms if the pulse period is

larger than 160 ms (6.25 Hz). If the pulse period is smaller than

160 ms and CFxDEN is an even number, the duty cycle of the

pulse output is exactly 50%. If the pulse period is smaller than

160 ms and CFxDEN is an odd number, the duty cycle of the

pulse output is

(1+1/CFxDEN) × 50%

INTERNAL

ACCUMULATOR

THRESHOLD

WTHR

27 26

CFxDEN

FREQ DIVIDER

REVPSUMx BIT OF

STATUS0[31:0]

CFx PULSE

OUTPUT

WATT

FWATT

FVAR

CFxSEL BITS IN

CFMODE

TERMSELx BITS IN

COMPMODE

INSTANTANEOUS

PHASE A

ACTIVE POWER

INSTANTANEOUS

PHASE B

ACTIVE POWER

INSTANTANEOUS

PHASE C

ACTIVE POWER

DIGITAL SIGNAL

PROCESSOR

Figure 87. Energy-to-Frequency Conversion

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