Datasheet

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.
()
2
2
8
1
1
H
f
f
+
=
(23)
–
E(t)
t
Vlt
VI
×cos(4π ×f
1
×t)
4π ×f
1
1 +
2
2f
1
8
04443-069
The active power signal (output of the LPF2) can be rewritten as
()
()
(
tf
f
IRMSVRMS
IRMSVRMStp
1
2
2
1
4cos
8
2
1
π×
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎣
⎡
+
×
−×=
)
(24)
Figure 70. Output Frequency Ripple
where f
1
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.
()
)4cos(
8
2
14
–
1
2
2
1
1
tf
f
f
IRMSVRMS
tIRMSVRMS
π
π
×
⎥
⎥
⎥
⎥
⎥
⎦
⎤
⎢
⎢
⎢
⎢
⎢
⎣
⎡
+
×
××
(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.
ZXSEL0
1
ZERO-CROSSING
DETECTION
(PHASE A)
ZXSEL1
1
ZERO-CROSSING
DETECTION
(PHASE B)
ZXSEL2
1
ZERO-CROSSING
DETECTION
(PHASE C)
1
ZXSEL[0:2]AREBITS3TO5 INTHELCYCMODEREGISTER
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
15 0
40 0
04443-070
Figure 71. ADE7758 Line Cycle Active Energy Accumulation Mode