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ADE7755AN-REF View Datasheet(PDF) - Analog Devices

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Description
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ADE7755AN-REF
ADI
Analog Devices ADI
ADE7755AN-REF Datasheet PDF : 16 Pages
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ADE7755
THEORY OF OPERATION
The two ADCs digitize the voltage signals from the current and
voltage transducers. These ADCs are 16-bit second order
sigma-delta with an oversampling rate of 900 kHz. This analog
input structure greatly simplifies transducer interfacing by
providing a wide dynamic range for direct connection to the
transducer and also by simplifying the antialiasing filter design.
A programmable gain stage in the current channel further facili-
tates easy transducer interfacing. A high-pass filter in the current
channel removes any dc component from the current signal.
This eliminates any inaccuracies in the real power calculation
due to offsets in the voltage or current signals (see HPF and
Offset Effects section).
The real power calculation is derived from the instantaneous
power signal. The instantaneous power signal is generated by a
direct multiplication of the current and voltage signals. In order
to extract the real power component (i.e., the dc component),
the instantaneous power signal is low-pass filtered. Figure 2
illustrates the instantaneous real power signal and shows how the
real power information can be extracted by low-pass filtering the
instantaneous power signal. This scheme correctly calculates real
power for nonsinusoidal current and voltage waveforms at all
power factors. All signal processing is carried out in the digital
domain for superior stability over temperature and time.
the voltage by 60. If we assume the voltage and current wave-
forms are sinusoidal, the real power component of the instanta-
neous power signal (i.e., the dc term) is given by:
( ) ÊV ¥ I ˆ
ËÁ 2 ¯˜
¥ cos
60o
This is the correct real power calculation.
INSTANTANEOUS
POWER SIGNAL
INSTANTANEOUS
REAL POWER SIGNAL
V؋I
2
0V
CURRENT
VOLTAGE
INSTANTANEOUS INSTANTANEOUS
POWER SIGNAL REAL POWER SIGNAL
V؋I ؋cos(60؇)
2
0V
CH1
CH2
PGA
ADC
HPF
MULTIPLIER
ADC
DIGITAL-TO-
FREQUENCY
LPF
F1
F2
DIGITAL-TO-
FREQUENCY
CF
INSTANTANEOUS
POWER SIGNAL – p(t)
INSTANTANEOUS REAL
POWER SIGNAL
V؋I
p(t) = i(t)؋v(t)
WHERE:
V؋I
2
v(t) = V؋cos(t)
V؋I
i(t) = I؋cos(t)
2
p(t)
=
V؋I
2
{1+ cos
(2t)}
TIME
Figure 2. Signal Processing Block Diagram
The low-frequency output of the ADE7755 is generated by
accumulating this real power information. This low frequency
inherently means a long accumulation time between output
pulses. The output frequency is therefore proportional to the
average real power. This average real power information can, in
turn, be accumulated (e.g., by a counter) to generate real energy
information. Because of its high output frequency and shorter
integration time, the CF output is proportional to the instanta-
neous real power. This is useful for system calibration purposes
that would take place under steady load conditions.
Power Factor Considerations
The method used to extract the real power information from the
instantaneous power signal (i.e., by low-pass filtering) is still
valid even when the voltage and current signals are not in phase.
Figure 3 displays the unity power factor condition and a DPF
(Displacement Power Factor) = 0.5, i.e., current signal lagging
VOLTAGE
CURRENT
60؇
Figure 3. DC Component of Instantaneous Power Signal
Conveys Real Power Information PF < 1
Nonsinusoidal Voltage and Current
The real power calculation method also holds true for nonsinu-
soidal current and voltage waveforms. All voltage and current
waveforms in practical applications will have some harmonic
content. Using the Fourier Transform, instantaneous voltage
and current waveforms can be expressed in terms of their
harmonic content.
v(t) = VO + 2 ¥ ÂVh ¥ sin(hwt + ah)
(1)
hπ0
where:
v(t) is the instantaneous voltage
VO is the average value
Vh is the rms value of voltage harmonic h
and
h is the phase angle of the voltage harmonic
i(t) = IO + 2 ¥ Â Ih ¥ sin(hwt + bh)
(2)
hπ0
where:
i(t) is the instantaneous current
IO is the dc component
Ih is the rms value of current harmonic h
and
h is the phase angle of the current harmonic
–10–
REV. 0
 

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