ADE7759_15 datasheet(24/36 Pages) AD

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REV. A

ADE7759

–24–

4 ƒl(1 + 2ƒl /8.9Hz)

–

sin(4 ƒlt)

E(t)

Vlt

Figure 39. Output Frequency Ripple

LINE CYCLE ENERGY ACCUMULATION MODE

In line cycle energy accumulation mode, the energy accumula-

tion of the ADE7759 can be synchronized to the Channel 2 zero

crossing so that active energy can be accumulated over an inte-

gral number of half line cycles. The advantage of summing the

active energy over an integer number of half-line cycles is that

the sinusoidal component in the active energy is reduced to zero.

This eliminates any ripple in the energy calculation. Energy is

calculated more accurately and in a shorter time because the

integration period can be shortened. By using the line cycle

energy accumulation mode, the energy calibration can be greatly

simplified and the time required to calibrate the meter can be sig-

nificantly reduced. The ADE7759 is placed in line cycle energy

accumulation mode by setting Bit 7 (CYCMODE) in the mode

accumulates the active power signal in the LENERGY register

(Address 14h) for an integral number of half cycles, as shown in

Figure 40. The number of half-line cycles is specified in the

LINECYC register (Address 14h). The ADE7759 can accumu-

late active power for up to 16,383 half cycles. Because the active

power is integrated on an integral number of half-line cycles, at

the end of a line cycle energy accumulation cycle, the CYCEND

flag in the interrupt status register is set (Bit 2). If the CYCEND

enable bit in the interrupt enable register is enabled, the

IRQ

output will also go active low. Thus the

IRQ line can also be

used to signal the completion of the line cycle energy accumula-

tion. Another calibration cycle will start as long as the CYCMODE

bit in the mode register is set. Note that the result of the first

calibration is invalid and should be ignored. The result of all

subsequent line cycle accumulation is correct.

From Equations 5 and 11:

VIdt

wt dt

()

–

()

= ∫

()

















∫

41 2

8 9

cos

(13)

where n is an integer and T is the line cycle period.

Since the sinusoidal component is integrated over an integer

number of line cycles, its value is always zero. Therefore:

VIdt

()

= ∫

(14)

VInT

()

(15)

Note that in this mode, the 14-bit LINECYC register can hold a

maximum value of 16,383. In other words, the line cycle

energy accumulation mode can be used to accumulate active

energy for a maximum duration over 16,383 half-line cycles. At

60 Hz line frequency, it translates to a total duration of 16,383/

120 Hz = 136.5 seconds. The 40-bit signed LENERGY register

can overflow if large signals are present at the inputs. The

LENERGY register can only hold up to 11.53 seconds of active

energy when both its input channels are at ac full-scale—see

Integration Time Under Steady Load section. Large LINECYC

content is meant to be used only when the input signal is low

and extensive averaging is required to reduce the noise.

CALIBRATING THE ENERGY METER

Calculating the Average Active Power

When calibrating the ADE7759, the first step is to calibrate the

frequency on CF to some required meter constant, e.g.,

3200 imp/kWh.

To determine the output frequency on CF, the average value of

the active power signal (output of LPF2) must first be deter-

mined. One convenient way to do this is to use the line cycle

energy accumulation mode. When the CYCMODE (Bit 7) bit

in the mode register is set to a Logic 1, energy is accumulated

over an integer number of half-line cycles as described in the

last section. Since the line frequency is fixed at, say, 60 Hz, and

the number of half cycles of integration is specified, the total

integration time is given as:

260

number of half cycles

SIGN 26

2–1 2–2 2–3 2–4 2–5 2–6 2–7 2–8

APOS [15:0]

LPF2

WAVEFORM [23:0]

LENERGY [39:0]

LINECYC [13:0]

ACTIVE POWER

SIGNAL – P

CCCDh

00h

FROM

MULTIPLIER

LPF1

CHANNEL 2

ADC

ZERO CROSS

DETECT

CALIBRATION

CONTROL

Figure 40. Energy Calculation in Line Cycle Energy Accumulation Mode

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