ADE7757ARN Analog Devices Inc, ADE7757ARN Datasheet - Page 8

• Manufacturer Part Number: ADE7757ARN
• Description: IC ENERGY METERING 16-SOIC
• Manufacturer: Analog Devices Inc

Specifications of ADE7757ARN

• Rohs Status: RoHS non-compliant
• 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

ADE7757
THEORY OF OPERATION
The two ADCs digitize the voltage signals from the current
and voltage sensors. These ADCs are 16-bit - 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 chan-
nel 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 operate only with ac input (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 3
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 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.
CH1
CH2
Figure 3. Signal Processing Block Diagram
TIME
POWER SIGNAL – p(t)
INSTANTANEOUS
ADC
ADC
MULTIPLIER
HPF
LPF
INSTANTANEOUS REAL
POWER SIGNAL
TIME
DIGITAL-TO-
FREQUENCY
DIGITAL-TO-
FREQUENCY
F1
F2
CF
–8–
The low frequency outputs (F1, F2) of the ADE7757 are gener-
ated 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 proportional 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 4 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
This is the correct real power calculation.
V
2
I
COS (60 )
Figure 4. DC Component of Instantaneous Power
Signal Conveys Real Power Information, PF < 1
V
2
POWER
POWER
0V
0V
I
VOLTAGE
CURRENT
VOLTAGE
INSTANTANEOUS
POWER SIGNAL
INSTANTANEOUS
POWER SIGNAL
V
2
1
60
INSTANTANEOUS REAL
cos
POWER SIGNAL
INSTANTANEOUS REAL
60
CURRENT
POWER SIGNAL
REV. A
TIME
TIME