REV. A
ADE7757
–11–
The HPF in Channel V1 has an associated phase response that
is compensated for on-chip. Figures 11 and 12 show the phase
error between channels with the compensation network acti-
vated. The ADE7757 is phase compensated up to 1 kHz as
shown. This will ensure correct active harmonic power calcula-
tion even at low power factors.
FREQUENCY – Hz
0.30
PHASE – Degrees
0.25
0.20
0.15
0.10
0.05
0
–0.05
–0.10
0 100 200 300 400 500 600 700 800 900 1000
Figure 11. Phase Error between Channels (0 Hz to 1 kHz)
FREQUENCY – Hz
0.30
PHASE – Degrees
0.25
0.20
0.15
0.10
0.05
0
–0.05
–0.10
40 45 50 55 60 65 70
Figure 12. 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,
since this LPF is not an ideal “brick wall” filter implementation,
the output signal also contains attenuated components at the
line frequency and its harmonics, i.e., cos(ht) where h = 1, 2,
3, . . . and so on.
The magnitude response of the filter is given by
|H f
()
=
+
|
.
1
1
445
2
2
f
(5)
For a line frequency of 50 Hz, this would give an attenuation
of the 2 (100 Hz) component of approximately 22 dB. The
dominating harmonic will be at twice the line frequency (2)
due to the instantaneous power calculation.
Figure 13 shows the instantaneous real power signal at the output
of the LPF that still contains a significant amount of instanta-
neous power information, i.e., cos(2t). This signal is then passed
to the digital-to-frequency converter where it is integrated
(accumulated) over time in order to produce an output frequency.
The accumulation of the signal will suppress or average 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 ADE7757 is proportional to the average real
power. Figure 13 shows the digital-to-frequency conversion for
steady load conditions, i.e., constant voltage and current.
LPF
F1
F2
DIGITAL-TO-
FREQUENCY
CF
DIGITAL-TO-
FREQUENCY
MULTIPLIER
F1
TIME
CF
TIME
FREQUENCY FREQUENCY
V
I
0
FREQUENCY (RAD/s)
2
COS (2)
ATTENUATED BY LPF
V I
2
LPF TO EXTRACT
REAL POWER
(DC TERM)
INSTANTANEOUS REAL POWER SIGNAL
(FREQUENCY DOMAIN)
Figure 13. Real Power-to-Frequency Conversion
As can be seen in the diagram, the frequency output CF is seen
to vary over time, even under steady load conditions. This fre-
quency variation is primarily due to the cos(2t) 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 accumu-
lating the instantaneous real power signal over a much shorter
time while converting it to a frequency. This shorter accumula-
tion period means less averaging of the cos(2t) component.
Consequently, some of this instantaneous power signal passes
through the digital-to-frequency conversion. This will not be a
problem in the application. Where CF is used for calibration
purposes, the frequency should be averaged by the frequency
counter, which will remove any ripple. If CF is being used to
measure energy, for example in a microprocessor based applica-
tion, the CF output should also be averaged to calculate power.
Because the outputs F1 and F2 operate at a much lower fre-
quency, a lot 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.
