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ADE7769
0.30
0.25
DIGITAL-TO-
FREQUENCY
F1
0.20
V
F1
F2
TIME
0.15
0.10
MULTIPLIER
I
LPF
DIGITAL-TO-
FREQUENCY
CF
CF
0.05
LPF TO EXTRACT
REAL POWER
(DC TERM)
0
V × I
TIME
– 0.05
– 0.10
2
COS (2 ω )
ATTENUATED BY LPF
40
45
50
55
60
65
70
FREQUENCY (Hz)
0
ω
2 ω
Figure 24. 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 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(h ω t) where h = 1, 2, 3, … and so on.
The magnitude response of the filter is given by
FREQUENCY (RAD/s)
INSTANTANEOUS REAL POWER SIGNAL
(FREQUENCY DOMAIN)
Figure 25. Real Power-to-Frequency Conversion
In Figure 25, the frequency output, CF, varies over time, even
under steady load conditions. This frequency variation is
primarily due to the cos(2 ω t) component in the instantaneous
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 the instan-
taneous real power signal over a much shorter time while
H ( f ) =
1 +
1
f 2
4 . 45 2
(7)
converting it to a frequency. This shorter accumulation period
means less averaging of the cos(2 ω t) component. Consequently,
some of this instantaneous power signal passes through the
digital-to-frequency conversion. This is not a problem in the
For a line frequency of 50 Hz, this gives an attenuation of
the 2 ω (100 Hz) component of approximately 22 dB. The
dominating harmonic is twice the line frequency (2 ω ) due to
the instantaneous power calculation.
Figure 25 shows the instantaneous real power signal at the
output of the LPF that still contains a significant amount of
instantaneous power information, that is, 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. The accumulation of the signal suppresses or
averages out any non-dc components in the instantaneous real
power signal. The average value of a sinusoidal signal is zero.
Thus, the frequency generated by the ADE7769 is proportional
to the average real power. Figure 25 shows the digital-to-
frequency conversion for steady load conditions, that is,
constant voltage and current.
application. Where CF is used for calibration purposes, the
frequency should be averaged by the frequency counter, which
removes any ripple. If CF is being used to measure energy, for
example 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 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 ADE7769 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 26 shows 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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