AD71056ARZ-RL Analog Devices Inc, AD71056ARZ-RL Datasheet - Page 10

• Manufacturer Part Number: AD71056ARZ-RL
• Description: IC,Power Metering,CMOS,SOP,16PIN,PLASTIC
• Manufacturer: Analog Devices Inc

Specifications of AD71056ARZ-RL

• Input Impedance: 320 KOhm
• Measurement Error: 0.1%
• Voltage - I/o High: 2.4V
• Voltage - I/o Low: 0.8V
• Current - Supply: 5mA
• Voltage - Supply: 4.75 V ~ 5.25 V
• Operating Temperature: -40°C ~ 85°C
• Mounting Type: Surface Mount
• Package / Case: 16-SOIC (0.154", 3.90mm Width)
• Meter Type: Single Phase
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

AD71056
THEORY OF OPERATION
The two ADCs in the AD71056 digitize the voltage signals from
the current and voltage sensors. These ADCs are 16-bit, Σ-Δ
with an oversampling rate of 450 kHz. This analog input struc-
ture 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.
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. To
extract the real power component (that is, the dc component),
the instantaneous power signal is low-pass filtered. Figure 15
illustrates the instantaneous real power signal and shows how
the real power information is extracted by low-pass filtering the
instantaneous 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.
The low frequency outputs (F1, F2) of the AD71056 are
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 (for
example, 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 proportional to the
instantaneous real power. This is useful for system calibration
that can be done faster under steady load conditions.
CH1
CH2
TIME
POWER SIGNAL – p(t)
INSTANTANEOUS
Figure 15. Signal Processing Block Diagram
ADC
ADC
MULTIPLIER
HPF
LPF
INSTANTANEOUS REAL
POWER SIGNAL
TIME
FREQUENCY
DIGITAL-TO-
FREQUENCY
DIGITAL-TO-
F1
F2
CF
Rev. A | Page 10 of 20
POWER FACTOR CONSIDERATIONS
The method used to extract the real power information from
the instantaneous power signal (that is, by low-pass filtering) is
valid even when the voltage and current signals are not in
phase. Figure 16 displays the unity power factor condition and
a displacement power factor (DPF) = 0.5; that is, the current
signal lagging the voltage by 60°. Assuming the voltage and
current waveforms are sinusoidal, the real power component of
the instantaneous power signal (the dc term) is given by
This is the correct real power calculation.
NONSINUSOIDAL VOLTAGE AND CURRENT
The real power calculation method also holds true for non-
sinusoidal current and voltage waveforms. All voltage and
current waveforms in practical applications have some
harmonic content. Using the Fourier transform, instantaneous
voltage and current waveforms can be expressed in terms of
their harmonic content.
where:
v(t) is the instantaneous voltage.
V
V
V × I
α
2
0
h
h
is the average value.
is the rms value of Voltage Harmonic h.
is the phase angle of the voltage harmonic.
COS (60°)
Figure 16. DC Component of Instantaneous Power Signal Conveys
⎛
⎜
⎝
v
V
) (
V × I
t
2
POWER
POWER
2
×
0V
0V
=
I
V
⎞
⎟
⎠
0
×
INSTANTANEOUS
POWER SIGNAL
+
INSTANTANEOUS
VOLTAGE
cos
CURRENT
POWER SIGNAL
VOLTAGE
AND
2
Real Power Information, PF < 1
(
×
60
h
∑
∞
≠
°
0
)
V
h
×
60°
sin
INSTANTANEOUS REAL
(
POWER SIGNAL
INSTANTANEOUS REAL
h
ω
POWER SIGNAL
t
CURRENT
+
α
h
)
TIME
TIME
(1)
(2)