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ADE7752/ADE7752A
DIGITAL-TO-FREQUENCY CONVERSION
After multiplication, the digital output of the low-pass filter
contains the real power information of each phase. Because this
LPF is not an ideal brick wall filter implementation, however,
the output signal also contains attenuated components at the
line frequency and its harmonics (cos(h ω t), where h = 1, 2, 3,
and so on).
The magnitude response of the filter is given by
power signal. The average value of a sinusoidal signal is zero.
Thus, the frequency generated by the ADE7752 is proportional
to the average real power. Figure 27 shows the digital-to-
frequency conversion for steady load conditions, constant
voltage, and current.
As can be seen in Figure 27 , the frequency output CF varies
over time, even under steady load conditions. This frequency
1 + ? ?
| H ( f ) | =
1
? f ?
? 8 ?
2
(8)
variation is primarily due to the cos(2 ω t) components in the
instantaneous real power signal. The output frequency on CF
can be up to 160 times higher than the frequency on F1 and F2.
The higher output frequency is generated by accumulating the
instantaneous real power signal over a much shorter time, while
where the ?3 dB cutoff frequency of the low-pass filter is 8 Hz.
For a line frequency of 50 Hz, this would give an attenuation of
the 2 ω (100 Hz) component of approximately –22 dB. The
dominating harmonic is twice the line frequency, cos(2 ω t), due
to the instantaneous power signal. Figure 27 shows the
instantaneous real power signal at the output of the CF, which
still contains a significant amount of instantaneous power
information, cos (2 ω t).
This signal is then passed to the digital-to-frequency converter
where it is integrated (accumulated) over time to produce an
output frequency. This accumulation of the signal suppresses or
averages out any non-dc component in the instantaneous real
converting it to a frequency. This shorter accumulation period
means less averaging of the cos(2 ω t) component. As a conse-
quence, some of this instantaneous power signal passes through
the digital-to-frequency conversion. This is not a problem in
the application. Where CF is used for calibration purposes, the
frequency should be averaged by the frequency counter. This
removes any ripple. If CF is being used to measure energy, such
as in a microprocessor-based application, the CF output should
also be averaged to calculate power. Because the outputs F1 and
F2 operate at a much lower frequency, much more averaging of
the instantaneous real power signal is carried out. The result is a
greatly attenuated sinusoidal content and a virtually ripple-free
frequency output.
VA
LPF
ABS
MULTIPLIER
|X|
F1
IA
DIGITAL-TO-
FREQUENCY
VB
MULTIPLIER
LPF
|X|
Σ
Σ
DIGITAL-TO-
F1
F2
CF
TIME
FREQUENCY
IB
VC
MULTIPLIER
IC
LPF
|X|
Σ
V × I
2
CF
TIME
LPF TO EXTRACT
REAL POWER
(DC TERM)
cos(2 ω t)
ATTENUATED BY LPF
0
ω
2 ω
FREQUENCY – RAD/S
INSTANTANEOUS REAL POWER SIGNAL
(FREQUENCY DOMAIN)
Figure 27. Real Power-to-Frequency Conversion
Rev. C | Page 18 of 24
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