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參數資料
| 型號： | AD7751AAN |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 模擬信號調理 |
| 英文描述： | Energy Metering IC With On-Chip Fault Detection |
| 中文描述： | SPECIALTY ANALOG CIRCUIT, PDIP24 |
| 封裝： | PLASTIC, DIP-24 |
| 文件頁數： | 10/16頁 |
| 文件大小： | 249K |
| 代理商： | AD7751AAN |
REV. A
AD7751
–10–
THEORY OF OPERATION
The two ADCs digitize the voltage and current signals from the
current and voltage transducers. These ADCs are 16-bit second
order sigma-delta converters 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 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 compo-
nent) 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.
LPF
DIGITAL-TO-
FREQUENCY
F1
F2
CH1
MULTIPLIER
PGA
CH2
ADC
V
I
2
V
I
V I
2
p(t) = i(t)
WHERE:
v(t) = V
i(t) = I
p(t) =V
v(t)
cos( t)
cos( t)
I
2
{
1+cos(2 t)}
ADC
TIME
HPF
DIGITAL-TO-
FREQUENCY
CF
INSTANTANEOUS REAL-
POWER SIGNAL
INSTANTANEOUS
POWER SIGNAL
–
p(t)
Figure 2. Signal Processing Block Diagram
The low frequency output of the AD7751 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 hence
shorter integration time, the CF output is proportional to the
instantaneous 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
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:
V
I
×
2
×
°
(
)
60
cos
(1)
This is the correct real-power calculation.
INSTANTANEOUS
REAL-POWER SIGNAL
INSTANTANEOUS
POWER SIGNAL
V I
2
cos(60 )
V I
2
INSTANTANEOUS
POWER SIGNAL
INSTANTANEOUS
REAL-POWER SIGNAL
60
CURRENT
VOLTAGE
CURRENT
VOLTAGE
0V
0V
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.
∞
2
0
where:
v t
( )
V
V
h t
O
h
h
sin(
)
=
+
× ∑
×
+
≠
α
(2)
v
(
t
)
V
O
V
h
and
is the instantaneous voltage
is the average value
is the rms value of voltage harmonic
h
h
is the phase angle of the voltage harmonic.
i t
( )
I
I
h t
O
h
h
sin(
)
=
+
× ∑
×
+
≠
∞
2
0
β
(3)
where:
i
(
t
)
I
O
I
h
is the instantaneous current
is the dc component
is the rms value of current harmonic
h
and
h
is the phase angle of the current harmonic.
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相關代理商/技術參數 |
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