ADE7758_15 datasheet(34/72 Pages) AD

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ADE7758

Data Sheet

Rev. E | Page 34 of 72

(see the Active Power Calculation section). Equation 14 gives an

expression for the instantaneous power signal. This is filtered by

LPF2, which has a magnitude response given by Equation 23.

()

(23)

–

E(t)

Vlt

×cos(4π ×f1 ×t)

4π ×f1 1 +

2f1

The active power signal (output of the LPF2) can be rewritten as

()

( t

IRMS

VRMS

IRMS

VRMS

cos

⎥

⎦

⎤

⎢

⎣

⎡

−

) (24)

Figure 70. Output Frequency Ripple

where f1 is the line frequency, for example, 60 Hz.

Line Cycle Active Energy Accumulation Mode

From Equation 24, E(t) equals

The ADE7758 is designed with a special energy accumulation

mode that simplifies the calibration process. By using the on-

chip, zero-crossing detection, the ADE7758 updates the watt-hr

accumulation registers after an integer number of zero crossings

(see Figure 71). The line-active energy accumulation mode for

watt-hr accumulation is activated by setting the LWATT bit

(Bit 0) of the LCYCMODE register. The total energy accumu-

lated over an integer number of half-line cycles is written to the

watt-hr accumulation registers after the LINECYC number of zero

crossings is detected. When using the line cycle accumulation

mode, the RSTREAD bit (Bit 6) of the LCYCMODE register

should be set to Logic 0.

()

)

cos(

–

IRMS

VRMS

IRMS

VRMS

⎥

⎦

⎤

⎢

⎣

⎡

(25)

From Equation 25, it can be seen that there is a small ripple in

the energy calculation due to the sin(2ωt) component (see

Figure 70). The ripple gets larger with larger loads. Choosing a

lower output frequency for APCF during calibration by using a

large APCFDEN value and keeping APCFNUM relatively small

can significantly reduce the ripple. Averaging the output

frequency over a longer period achieves the same results.

ZXSEL01

ZERO-CROSSING

DETECTION

(PHASE A)

ZXSEL11

ZERO-CROSSING

DETECTION

(PHASE B)

ZXSEL21

ZERO-CROSSING

DETECTION

(PHASE C)

1ZXSEL[0:2] AREBITS3 TO 5 IN THELCYCMODEREGISTER

CALIBRATION

CONTROL

LINECYC[15:0]

WATTOS[11:0]

WG[11:0]

WDIV[7:0]

WATTHR[15:0]

ACCUMULATE ACTIVE POWER FOR

LINECYC NUMBER OF ZERO-CROSSINGS;

WATT-HR ACCUMULATION REGISTERS

ARE UPDATED ONCE EVERY LINECYC

NUMBER OF ZERO-CROSSINGS

ACTIVE POWER

Figure 71. ADE7758 Line Cycle Active Energy Accumulation Mode

ADE7758 Datasheet(PDF) 34 Page - Analog Devices

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