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| 型號： | ADE7757 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Energy Metering IC with Integrated Oscillator |
| 中文描述： | 電能計量IC集成振蕩器 |
| 文件頁數(shù)： | 8/14頁 |
| 文件大?。?/td> | 534K |
| 代理商： | ADE7757 |
PRELIMINARY TECHNICAL DATA
REV. PrC.
ADE7757
–8–
THEORY OF OPERATION
The two ADCs digitize the voltage signals from the cur-
rent and voltage sensors. These ADCs are 16-bit sigma-
delta with an oversampling rate of 450 kHz. This analog
input structure greatly simplifies sensor interfacing by
providing a wide dynamic range for direct connection to
the sensor and also simplifies the antialiasing filter design.
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. Because the HPF is always
enabled, the IC will only operate with AC Input
—
see HPF
and Offset Effects.
The real power calculation is derived from the instanta-
neous 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 compo-
nent (i.e., the dc component), the instantaneous power
signal is low-pass filtered. Figure 11 illustrates the instan-
taneous real power signal and shows how the real power
information can be extracted by low-pass filtering the in-
stantaneous power signal. This scheme correctly calculates
real power for sinusoidal 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.
LPF
DIGITAL-TO-
FREQUENCY
F1
F2
CH1
INSTANTANEOUS REAL
POWER SIGNAL
MULTIPLIER
CH2
ADC
INSTANTANEOUS
POWER SIGNAL p (t)
V
I
2
V I
V I
2
p(t) = i(t) v(t)
WHERE:
v(t) = V
i(t) = I
p(t) = V I
ccos( t)
{
1+cos (2 t)}
2
ADC
TIME
HPF
DIGITAL-TO-
FREQUENCY
CF
PGA
Figure 11. Signal Processing Block Diagram
The low frequency outputs (F1, F2) of the ADE7757 is
generated by accumulating this real power information.
This low frequency inherently means a long accumulation
time between output pulses. Consequently, the resulting
output frequency is proportional to the average real power.
This average real power information is then accumulated
(e.g., by a counter) to generate real energy information.
Conversely, due to its high output frequency and hence
shorter integration time, the CF output frequency is pro-
portional to the instantaneous real power. This is useful
for system calibration, which can be done faster 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 12 displays the unity power factor condition
and a DPF (Displacement Power Factor) = 0.5, i.e., cur-
rent signal lagging the voltage by 60
°
. If we assume the
voltage and current waveforms are sinusoidal, the real
power component of the instantaneous power signal (i.e.,
the dc term) is given by:
)
60
(
cos
2
I
V
°
×
×
This is the correct real power calculation.
INSTANTANEOUS REAL
POWER SIGNAL
INSTANTANEOUS
POWER SIGNAL
INSTANTANEOUS
POWER SIGNAL
INSTANTANEOUS REAL
POWER SIGNAL
CURRENT
CURRENT
VOLTAGE
0V
0V
VOLTAGE
POWER
POWER
TIME
TIME
°
60
2
I
V
×
)
60
(
cos
2
I
V
°
×
Figure 12. 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
nonsinusoidal 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.
∑
h
α
+
ω
×
×
+
=
0
h
0
)
t
h
sin
V
2
V
)
(1)
where:
v(t)
is the instantaneous voltage
V
O
is the average value
V
h
is the rms value of voltage harmonic
h
and
h
is the phase angle of the voltage harmonic.
∑
h
β
+
ω
×
×
+
=
0
h
0
)
t
h
sin
I
2
I
)
(2)
where:
i
(
t
)
I
O
I
h
and
h
is the instantaneous current
is the dc component
is the rms value of current harmonic
h
is the phase angle of the current harmonic.
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