ADE7759 Analog Devices, ADE7759 Datasheet - Page 24

• Manufacturer Part Number: ADE7759
• Description: Active Energy Metering IC with di/dt Sensor Interface
• Manufacturer: Analog Devices

ADE7759
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.
E(t)
–
4 ƒ
l
MULTIPLIER
(1 + 2ƒ
15
CHANNEL 2
Vlt
SIGN 2
VI
CCCDh
FROM
l
/8.9Hz)
ADC
00h
6
2
5
2
LPF2
LPF1
sin(4 ƒ
4
ACTIVE POWER
2
3
SIGNAL – P
l
t)
2
2
+
APOS [15:0]
2
1
+
2
0
23
39
2
–1
ZERO CROSS
2
–2
DETECT
2
–3
From Equations 5 and 11.
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:
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:
E t
WAVEFORM [23:0]
LENERGY [39:0]
2
E t
E t
( )
–4
( ) =
( ) = ∫
= ∫
2
–5
nT
O
VInT
2
VIdt
nT
O
–6
LINECYC [13:0]
VIdt
CALIBRATION
2
CONTROL
–7
–
2 60
2



 
×
–8
+ 0
4
0
1
π
f
Hz
l
(
0
0
1 2
+
×
VI
number of half cycles
f
l
/ .
8 9
+
+
Hz
)



 
nT
O
∫
cos
(
2
wt dt
)
(13)
(14)
(15)