EVAL-ADE7755ZEB Datasheet, PDF (12/20 Pages) Analog Devices – Energy Metering IC with Pulse Output

ADE7755
THEORY OF OPERATION
The two ADCs of the ADE7755 digitize the voltage signals from
the current and voltage transducers. These ADCs are 16-bit,
second-order Σ-Δ 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
facilitates easy transducer interfacing. A high-pass filter in the
current channel removes any dc components from the current
signal. This removal eliminates any inaccuracies in the active
power calculation due to offsets in the voltage or current signals
(see the HPF and Offset Effects section).
The active 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 active power component (that is, the dc component),
the instantaneous power signal is low-pass filtered. Figure 22
illustrates the instantaneous active power signal and shows how
the active power information can be extracted by low-pass filtering
the instantaneous power signal. This scheme correctly calculates
active 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.
CH1
PGA
ADC
HPF
MULTIPLIER
LPF
CH2
ADC
DIGITAL-TO-
FREQUENCY
F1
F2
DIGITAL-TO-
FREQUENCY
CF
V×I
V×I
2
INSTANTANEOUS
POWER SIGNAL {p(t)}
INSTANTANEOUS ACTIVE
POWER SIGNAL
TIME
p(t) = i(t) × v(t)
WHERE:
v(t) = V × cos(ωt)
i(t) = I × cos(ωt)
p(t)
=
V×
2
I
{1+cos
(2ωt)}
V×I
2
Figure 22. Signal Processing Block Diagram
The low frequency output of the ADE7755 is generated by
accumulating this active power information. This low frequency
inherently means a long accumulation time between output
pulses. The output frequency is therefore proportional to the
average active power. This average active power information
can, in turn, be accumulated (for example, by a counter) to
generate active energy information. Because of its high output
frequency and shorter integration time, the calibration frequency
(CF) output is proportional to the instantaneous active power.
This is useful for system calibration purposes that take place
under steady load conditions.
POWER FACTOR CONSIDERATIONS
The method used to extract the active 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 23 displays the unity power factor condition and a
displacement power factor (DPF) = 0.5, that is, current signal
lagging the voltage by 60°. Assuming that the voltage and current
waveforms are sinusoidal, the active power component of the
instantaneous power signal (that is, the dc term) is given by
⎜⎛ ^{V × I} ⎟⎞ × cos(60°)
⎝²âŽ 
This is the correct active power calculation.
INSTANTANEOUS
POWER SIGNAL
INSTANTANEOUS ACTIVE
POWER SIGNAL
V× I
2
0V
CURRENT
VOLTAGE
INSTANTANEOUS ACTIVE
INSTANTANEOUS
POWER SIGNAL
POWER SIGNAL
V×
2
I
cos(60°)
0V
VOLTAGE
CURRENT
60°
Figure 23. DC Component of Instantaneous Power Signal Conveys
Active Power Information PF < 1
Rev. A | Page 12 of 20