ADE7759ARSRL datasheet(24/32 Pages) AD

24 / 32 page

REV. 0

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 accumu-

lation of the ADE7759 can be synchronized to the Channel 2

zero crossing so that active energy can be accumulated over an

integral 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 calcu-

lated more accurately and in a shorter time because integration

period can be shortened. By using the Line Cycle Energy Accu-

mulation mode, the energy calibration can be greatly simplified

and the time required to calibrate the meter can be significantly

reduced. The ADE7759 is placed in Line Cycle Energy Accu-

mulation mode by setting Bit 7 (CYCMODE) in the mode register.

In Line Cycle Energy Accumulation mode the ADE7759

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 accumulation.

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.

E t

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:

E t

VIdt

()

= ∫

+ 0

(14)

E t

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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