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ADE7754
CFNUM[11:0]
The active power signal (output of the LPF2) can be rewritten as
11
0
??
? ?
1 + ?
? 2 f l ?
? 8 ?
? × cos ( 4 π f l t )
? ?
ACTIVE POWER
PHASE A
ACTIVE POWER +
PHASE B
ACTIVE POWER
PHASE C
+
53 0
TOTAL ACTIVE
POWER
DFC
CF
?
?
p ( t ) = VI ? ?
?
?
VI
?
2
?
?
??
?
?
(12)
11
CFDEN[11:0]
0
where f l is the line frequency (e.g., 60 Hz).
From Equation 8
?? VI
? 2 f l ?
? 4 π f l 1 + ? ?
? ?
? × sin ( 4 π f l t )
? ?
Figure 29. ADE7754 Energy to Frequency Conversion
A digital to frequency converter (DFC) is used to generate the
CF pulsed output. The DFC generates a pulse each time one
LSB in the active energy register is accumulated. An output
pulse is generated when CFDEN/CFNUM pulses are generated
at the DFC output. Under steady load conditions, the output
frequency is proportional to the active power. The maximum
?
?
E ( t ) = VIt ? ?
?
? 8 ?
2
?
?
??
?
?
(13)
output frequency (CFNUM = 00h and CFDEN = 00h) with
full scale ac signals on the three phases (i.e., current channel
and voltage channel is approximately 96 kHz).
The ADE7754 incorporates two registers to set the frequency of
CF (CFNUM[11:0] and CFDEN[11:0]). These are unsigned
12-bit registers that can be used to adjust the frequency of CF
to a wide range of values. These frequency scaling registers are
12-bit registers that can scale the output frequency by 1/2 12 to 1
Equation 13 shows that there is a small ripple in the energy
calculation due to a sin(2 t) component. This is graphically
displayed in Figure 30. The ripple becomes larger as a percentage
of the frequency at larger loads and higher output frequencies.
Choosing a lower output frequency at CF for calibration can
significantly reduce the ripple. Also, averaging the output fre-
quency by using a longer gate time for the counter achieves the
same results.
with a step of 1/2 12 .
If the value 0 is written to any of these registers, the value 1
would be applied to the register. The ratio CFNUM/CFDEN
should be smaller than 1 to ensure proper operation. If the ratio
of the registers CFNUM/CFDEN is greater than 1, the CF
frequency can no longer be guaranteed to be a consistent value.
For example, if the output frequency is 18.744 kHz and the
contents of CFDEN are zero (000h), then the output frequency
E(t)
VIt
( (
can be set to 6.103 Hz by writing BFFh to the CFDEN register.
The output frequency will have a slight ripple at a frequency
equal to twice the line frequency because of imperfect filtering
–
VI
4 f I 1 +
2 f I
8
2
sin ( 4
f I t )
of the instantaneous power signal used to generate the active
power signal. See the Active Power Calculation section. Equa-
tion 5 gives an expression for the instantaneous power signal.
This is filtered by LPF2, which has a magnitude response given
by Equation 11.
t
Figure 30. Output Frequency Ripple
No Load Threshold
| H ( f )| =
1
1 +
f 2
8 2
(11)
The ADE7754 includes a selectable “no load threshold” or
“startup current” feature that eliminates any creep effects in the
active energy measurement of the meter. When enabled, this
function is independently applied on each phase’s active power
calculation. This mode is selected by default and can be disabled
REV. 0
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