Datasheet

REV. 0–26–
ADE7754
APPARENT ENERGY CALCULATION
The apparent energy is given as the integral of the apparent
power.
Apparent Energy Apparent Power t dt=
∫
()
(21)
The ADE7754 achieves the integration of the apparent power
signal by continuously accumulating the apparent power signal
in an internal nonreadable 49-bit register. The apparent energy
register (VAENERGY[23:0]) represents the upper 24 bits of
this internal register. This discrete time accumulation or summa
-
tion is equivalent to integration in continuous time. Equation 22
expresses the relationship, where n is the discrete time sample
number and T is the sample period.
Apparent Energy Lim Apparent Power nT T
T
n
=×










→
=
∞
∑
0
0
()
(22)
The discrete time sample period (T) for the accumulation regis-
ter in the ADE7754 is 1.2 µs (12/10 MHz).
Figure 37 shows a graphical representation of this discrete
time integration or accumulation. The apparent power signal
is continuously added to the internal register. This addition is
a signed addition even if the apparent energy theoretically
always remains positive.
48
0
+
48
0
VAENERGY[23:0]
VADIV
23
0
%
TOTAL APPARENT POWER IS
ACCUMULATED (INTEGRATED) IN
THE APPARENT ENERGY REGISTER
TOTAL APPARENT POWER
T
+
00000h
D1B71h
TIME (nT)
T
APPARENT POWER
SIGNAL – P
Figure 37. Apparent Energy Calculation
The upper 49-bit value of the internal register is divided by
VADIV. If the value in the VADIV register is 0, then the internal
active energy register is divided by 1. VADIV is an 8-bit unsigned
register. The upper 24-bit values are then written in the 24-bit
apparent energy register (VAENERGY[23:0]). RVAENERG
Y
register (24 bits long) is provided to read the apparent energy. This
register is reset to 0 after a read operation.
Figure 38 shows this apparent energy accumulation for full-scale
(sinusoidal) signals on the analog inputs. The three curves illus-
trate the minimum time it takes the energy register to roll over
when the individual VA gain registers contents all equal 3FFh,
000h, and 800h. The VA gain registers are used to carry out an
apparent power calibration in the ADE7754. The fastest integra-
tion time occurs when the VA gain registers are set to maximum
full scale (i.e., 3FFh).
00,0000h
7F,FFFFh
80,0000h
3F,FFFFh
40,0000h
VAENERGY[23:0]
TIME (sec)
AVAG = BVAG = CVAG = 3FFh
131
262
393
65.5
AVAG = BVAG = CVAG = 000h
AVAG = BVAG = CVAG = 800h
196.5
327.5
Figure 38. Energy Register Roll Over Time for Full-
Scale Power (Minimum and Maximum Power Gain)
Note that the apparent energy register contents roll over to full-
scale negative (80,0000h) and continue increasing in value when
the power or energy flow is positive, as shown in Figure 38. By
using the interrupt enable register, the ADE7754 can be config-
ured to issue an interrupt (IRQ) when the apparent energy
register is half full (positive or negative).
Integration Times under Steady Load
As described in the preceding section, the discrete time sample
period (T) for the accumulation register is 1.2 µs (12/CLKIN).
With full-scale sinusoidal signals on the analog inputs and the
VA gain registers set to 000h, the average word value from each
apparent power stage is D1B71h. See the Apparent Power
Calculation section. The maximum value that can be stored in
the apparent energy register before it overflows is 22
3
– 1 or
FF,FFFFh. As the average word value is added to the internal
register that can store 24
8
– 1 or FFFF,FFFF,FFFFh before it
overflows, the integration time under these conditions with
VADIV = 0 is calculated as follows:
Time
FFFF FFFF FFFFh
DB h
ss s=
×
×==
,,
. min
3171
12 131 2 11µ
When VADIV is set to a value different from 0, the integration
time varies as shown in Equation 23.
Time Time VADIV
WDIV
=×
=0
(23)
LINE APPARENT ENERGY ACCUMULATION
The ADE7754 is designed with a special apparent energy accu-
mulation mode that simplifies the calibration process. By using
the on-chip zero-crossing detection, the ADE7754 accumulates
the apparent power signal in the LVAENERGY register for an
integral number of half cycles, as shown in Figure 39. The line
apparent energy accumulation mode is always active.
Each of three zero-crossing detection phases can contribute to
the accumulation of the half line cycles. Phase A, B, and C zero
crossings are taken into account when counting the number of
half line cycles by setting Bits 4 to 6 of the MMODE register to
Logic 1. Selecting phases for the zero-crossing counting also has
the effect of enabling the zero-crossing detection, zero-crossing
timeout, and period measurement for the corresponding phase
as described in the zero-crossing detection paragraph.