ADE7762(Rev0) Ver la hoja de datos (PDF) - Analog Devices
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ADE7762 Datasheet PDF : 28 Pages
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ADE7762
DIGITAL-TO-FREQUENCY CONVERSION
After multiplication, the digital output of the low-pass filter
contains the active power information of each phase. 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 …
The magnitude response of the filter is given by
|H( f )| = 1
(11)
1
+
⎧
⎨
f
⎫2
⎬
⎩8⎭
where the −3 dB cutoff frequency of the low-pass filter is 8 Hz.
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, that is,
cos(2ωt), due to the instantaneous power signal. Figure 26
shows the instantaneous active power signal at the output of
the CF, which still contains a significant amount of instantane-
ous power information, cos(2ωt).
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 nondc component in the instantaneous active
power signal.
The average value of a sinusoidal signal is zero. Thus, the
frequency generated by the ADE7762 is proportional to the
average active power. Figure 26 shows the digital-to-frequency
conversion for steady load conditions, that is, constant voltage
and current.
The frequency output CF varies over time, even under steady load
conditions (see Figure 26). This frequency variation is primarily
due to the cos(2ωt) components in the instantaneous active power
signal. The output frequency on CF can be up to 160× higher than
the frequency on F1 and F2. The higher output frequency is
generated by accumulating the instantaneous active 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. Therefore, some of this instantaneous power signal
passes through the digital-to-frequency conversion.
Where CF is used for calibration purposes, the frequency counter
should average the frequency to remove the ripple and obtain a
stable frequency. If CF is used to measure energy, for example, 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, significant averaging of the
instantaneous active power signal is carried out. The result is a
greatly attenuated sinusoidal content and a virtually ripple-free
frequency output on F1 and F2, which are used to measure energy
in a stepper motor-based meter.
VA
MULTIPLIER
IA
VB
MULTIPLIER
IB
VC
MULTIPLIER
IC
ABS
LPF
|X|
LPF
|X|
Σ
F1
DIGITAL-TO -
FREQUENCY
Σ
F1
F2
DIGITAL-TO -
CF
FREQUENCY
Σ
CF
TIME
LPF
|X|
V× I
2
LPF TO EXTRACT
REAL POWER
(DC TERM)
TIME
cos(2ωt)
ATTENUATED BY LPF
0
ω
2ω
FREQUENCY – RAD/S
INSTANTANEOUS REAL POWER SIGNAL
(FREQUENCY DOMAIN)
Figure 26. Active Power-to-Frequency Conversion
Rev. 0 | Page 21 of 28
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