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ADE7768
0.30
0.25
DIGITAL-TO-
FREQUENCY
F1
0.20
V
F1
F2
0.15
MULTIPLIER
LPF
≥ 0
DIGITAL-TO-
FREQUENCY
CF
TIME
0.10
0.05
I
LPF TO EXTRACT
REAL POWER
(DC TERM)
CF
0
– 0.05
V × I
2
COS (2 ω )
ATTENUATED BY LPF
TIME
– 0.10
40
45
50
55
60
65
70
FREQUENCY (Hz)
0
ω
2 ω
FREQUENCY (RAD/s)
Figure 25. Phase Error Between Channels (40 Hz to 70 Hz)
Digital-to-Frequency Conversion
As previously described, the digital output of the low-pass filter
after multiplication contains the positive-only real power
information. However, because this LPF is not an ideal brick
wall filter implementation, the output signal also contains
attenuated components at the line frequency and its harmonics,
that is, cos(hωt) where h = 1, 2, 3, ... and so on.
INSTANTANEOUS REAL POWER SIGNAL
(FREQUENCY DOMAIN)
Figure 26. Positive-Only, Real Power-to-Frequency Conversion
In Figure 26, the frequency output CF varies over time, even
under steady load conditions. This frequency variation is
primarily due to the cos(2ωt) component in the instantaneous
positive-only real power signal. The output frequency on CF
can be up to 2048 times higher than the frequency on F1 and
F2. This higher output frequency is generated by accumulating
H ( f ) =
1 +
The magnitude response of the filter is given by
1
f 2
4 . 45 2
(7)
the instantaneous positive-only real power signal over a much
shorter time while converting it to a frequency. This shorter
accumulation period means less averaging of the cos(2ωt)
component. Consequently, 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
For a line frequency of 50 Hz, this gives an attenuation of
the 2ω (100 Hz) component of approximately 22 dB. The
dominating harmonic is twice the line frequency (2ω) due to
the instantaneous power calculation.
Figure 26 shows the instantaneous positive-only real power
signal at the output of the LPF that still contains a significant
amount of instantaneous power information, that is, cos(2ωt).
This signal is then passed to the digital-to-frequency converter
where it is compared to 0 and only positive real power is inte-
grated (accumulated) over time to produce an output frequency.
The accumulation of the signal suppresses or averages out any
non-dc components in the instantaneous positive-only real
power signal. The average value of a sinusoidal signal is 0. Thus,
the frequency generated by the ADE7768 is proportional to the
average positive-only real power. Figure 26 shows the digital-to-
frequency conversion for steady load conditions, that is, the
constant voltage and current.
calibration purposes, the frequency should be averaged by the
frequency counter, which removes any ripple. If CF is used to
measure energy, such as in a microprocessor-based application,
the CF output should also be averaged to calculate power.
Because the F1 and F2 outputs operate at a much lower
frequency, much more averaging of the instantaneous positive-
only real power signal is carried out. The result is a greatly
attenuated sinusoidal content and a virtually ripple-free
frequency output.
Connecting to a Microcontroller for Energy
Measurement
The easiest way to interface the ADE7768 to a microcontroller
is to use the CF high frequency output with the output
frequency scaling set to 2048 × F1, F2. This is done by setting
SCF = 0 and S0 = S1 = 1 (see Table 7). With full-scale ac
signals on the analog inputs, the output frequency on CF is
approximately 2.867 kHz. Figure 27 illustrates one scheme that
could be used to digitize the output frequency and carry out the
necessary averaging mentioned in the previous section.
Rev. A | Page 13 of 20
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