ADE7752BARW AD [Analog Devices], ADE7752BARW Datasheet - Page 13

• Manufacturer Part Number: ADE7752BARW
• Description: Polyphase Energy Metering IC with Pulsed Output
• Manufacturer: AD [Analog Devices]

Preliminary Technical Data
THEORY OF OPERATION
The six signals from the current and voltage transducers are
digitized with ADCs. These ADCs are 16-bit second-order ∑-Δ
with an oversampling rate of 833 kHz. This analog input
structure greatly simplifies transducer interface by providing a
wide dynamic range and bipolar input for direct connection to
the transducer. High-pass filters in the current channels remove
the dc component from the current signals. This 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 of each
phase. In order to extract the active power component, the dc
component, the instantaneous power signal is low-pass filtered
on each phase. Figure 15 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 method is used to extract the active power
information on each phase of the polyphase system. The total
active power information is then obtained by adding the
individual phase active power. 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.
VBP
VCP
VAP
V × I
V × I
IAN
IBN
ICN
IAP
IBP
ICP
VN
2
TIME
ADC
ADC
ADC
ADC
ADC
ADC
p(t) = i(t) × v(t)
WHERE:
POWER SIGNAL - p(t)
v(t) = V × cos (ωt)
i(t) = I × cos (ωt)
p(t) = V × I
INSTANTANEOUS
MUL TIPLIER
MUL TIPLIER
MUL TIPLIER
2
HPF
HPF
HPF
{1+ cos (2ωt)}
Figure 15. Signal Processing Block Diagram
LPF
LPF
LPF
Rev. PrA | Page 13 of 27
V × I
2
ACTIVE POWER SIGNAL
ABS
|X|
|X|
|X|
INSTANTANEOUS
The low frequency output of the ADE7752B is generated by
accumulating the total 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 therefore shorter integration time, the CF output
is proportional to the instantaneous active power. This pulse is
useful for system calibration purposes that would take place
under steady load conditions.
POWER FACTOR CONSIDERATIONS
Low-pass filtering, the method used to extract the active power
information from the individual instantaneous power signal, is
still valid when the voltage and current signals of each phase are
not in phase. Figure 16 displays the unity power factor
condition and a displacement power factor (DPF) = 0.5, that is,
current signal lagging the voltage by 60°, for one phase of the
polyphase. Assuming that the voltage and current waveforms
are sinusoidal, the active power component of the instantaneous
power signal (the dc term) is given by
Σ
⎛ ×
⎜
⎝
V
2
1
VA × IA + VB × IB +
⎞
× ⎟
⎠
cos
Σ
Σ
VC × IC
INSTANTANEOUS
POWER SIGNAL
FREQUENCY
FREQUENCY
2
DIGITAL-TO-
DIGITAL-TO-
(
60
TOTAL
°
)
CF
F1
F2
ADE7752B
(1)