ADE7758 Analog Devices, ADE7758 Datasheet - Page 34

• Manufacturer Part Number: ADE7758
• Description: Poly Phase Multifunction Energy Metering IC with Per Phase Information
• Manufacturer: Analog Devices

ADE7758
The maximum output frequency (APCFNUM = 0x00 and
APCFDEN = 0x00) with full-scale ac signals on one phase is
approximately 16 kHz.
The ADE7758 incorporates two registers to set the frequency
of APCF (APCFNUM[11:0] and APCFDEN[11:0]). These are
unsigned 12-bit registers that can be used to adjust the frequency
of APCF by 1/2
output frequency is 1.562 kHz, while the contents of CFDIV are
0 (0x000), then the output frequency can be set to 6.103 Hz by
writing 0xFF to the CFDEN register.
If 0 is written to any of the frequency division registers, the
divider would use 1 in the frequency division. In addition, the
ratio APCFNUM/ APCFDEN should be set not greater than
one to ensure proper operation. In other words, the APCF
output frequency cannot be higher than the frequency on the
DFC output.
The output frequency has a slight ripple at a frequency equal to
twice the line frequency. This is due to imperfect filtering of the
instantaneous power signal to generate the active power signal
(see the Active Power Calculation section). Equation 5 gives an
expression for the instantaneous power signal. This is filtered by
LPF2, which has a magnitude response given by Equation 11.
The active power signal (output of the LPF2) can be rewritten as
where f
H
p
( )
( )
1
t
f
is the line frequency, for example, 60 Hz.
=
=
VRMS
1
12
+
to 1 with a step of 1/2
*ZXSEL[0:2] ARE BITS 3 TO 5 IN THE LCYCMODE REGISTER
×
1
ZERO-CROSSING
ZERO-CROSSING
ZERO-CROSSING
IRMS
f
2
DETECTION
DETECTION
DETECTION
(PHASE A)
(PHASE B)
(PHASE C)
8
ACTIVE POWER
2
⎡
⎢
⎢
⎢
⎢
⎢
⎣
VRMS
1
ZXSEL0*
ZXSEL1*
ZXSEL2*
+
( )
2
×
WATTOS[11:0]
f
1
IRMS
+
2
12
+
8
. For example, if the
2
Figure 70. ADE7758 Line Cycle Active Energy Accumulation Mode
⎤
⎥
⎥
⎥
⎥
⎥
⎦
×
cos
WG[11:0]
(
4
π
t f
1
)
WDIV[7:0]
CALIBRATION
LINECYC[15:0]
CONTROL
(11)
(12)
%
Rev. A | Page 34 of 68
+
+
40
15
From Equation 12,
From Equation 13, it can be seen that there is a small ripple in
the energy calculation due to the sin(2ωt) component. Figure 69
shows this. 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. Also, averaging the output
frequency over a longer period of time achieves the same results.
Line Cycle Active Energy Accumulation Mode
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
(Figure 70). 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 over an integer
number of half-line cycles is written to the watt-hr accumulation
registers after the LINECYC number of zero crossings have
been detected. When using the line cycle accumulation mode,
the RSTREAD bit (Bit 6) of the LCYCMODE register should be
set to Logic 0.
WATTHR[15:0]
E
( )
t
=
VRMS
E(t)
0
ACCUMULATE ACTIVE POWER FOR
LINECYC NUMBER OF ZERO-CROSSINGS;
WATT-HR ACCUMULATION REGISTERS
ARE UPDATED ONCE EVERY LINECYC
NUMBER OF ZERO-CROSSINGS
Figure 69. Output Frequency Ripple
×
IRMS
–
×
4 π× f
t
Vlt
0
t
1
–
1 +
VI
⎡
⎢
⎢
⎢
⎢
⎢
⎣
4
π
2f
VRMS
8
t f
1
1
2
1
× sin(4 π × f
+
×
( )
2
IRMS
f
1
2
1
8
× t)
2
⎤
⎥
⎥
⎥
⎥
⎥
⎦
×
cos
(
4
π
(13)
t f
1
)