ADE7752BARW AD [Analog Devices], ADE7752BARW Datasheet - Page 20

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

ADE7752B
DIGITAL-TO-FREQUENCY CONVERSION
After multiplication, the digital output of the low-pass filter
contains the active power information of each phase. However,
since this LPF is not an ideal brick wall filter implementation, the
output signal also contains attenuated components at the line
frequency and its harmonics, that is, cos(hωt), where h = 1, 2, 3 ….
The magnitude response of the filter is given by
where the −3 dB cutoff frequency of the low-pass filter is 8 Hz.
For a line frequency of 50 Hz, this would give an attenuation of
the 2ω(100 Hz) component of approximately −22 dB. The
dominating harmonic is twice the line frequency, that is,
cos(2ωt), due to the instantaneous power signal. Figure 27
shows the instantaneous active power signal at the output of the
CF, which still contains a significant amount of instantaneous
power information, cos(2 ω t).
This signal is then passed to the digital-to-frequency converter
where it is integrated (accumulated) over time to produce an
output frequency. This accumulation of the signal suppresses or
averages out any non-dc component in the instantaneous active
power signal.
|
H
( )
f
|
=
1
+
1
⎧
⎨
⎩
8
MULTIPLIER
MULTIPLIER
MULTIPLIER
f
⎫
⎬
⎭
2
VA
VB
VC
IA
IB
IC
LPF
LPF
LPF
Figure 27. Active Power-to-Frequency Conversion
ABS
|X|
|X|
|X|
LPF TO EXTRACT
REAL POWER
(DC TERM)
Rev. PrA | Page 20 of 27
(10)
Σ
Σ
Σ
V× I
FREQUENCY
FREQUENCY
The average value of a sinusoidal signal is zero. Thus, the
frequency generated by the ADE7752B is proportional to the
average active power. Figure 27 shows the digital-to-frequency
conversion for steady load conditions, that is, constant voltage
and current.
The frequency output CF varies over time, even under steady load
conditions (see Figure 27). This frequency variation is primarily
due to the cos(2ωt) components in the instantaneous active power
signal. The output frequency on CF can be up to 160× higher than
the frequency on F1 and F2. The higher output frequency is
generated by accumulating the instantaneous active power signal
over a much shorter time, while converting it to a frequency. This
shorter accumulation period means less averaging of the cos(2ωt)
component. Therefore, some of this instantaneous power signal
passes through the digital-to-frequency conversion.
Where CF is used for calibration purposes, the frequency counter
should average the frequency to remove the ripple and obtain a
stable frequency. If CF is being used to measure energy, for
example, in a microprocessor-based application, the CF output
should also be averaged to calculate power. Because the outputs F1
and F2 operate at a much lower frequency, significant averaging of
the instantaneous active power signal is carried out. The result is a
greatly attenuated sinusoidal content and a virtually ripple-free
frequency output on F1 and F2, which are used to measure energy
in a stepper-motor based meter.
DIGITAL-TO-
DIGITAL-TO-
2
0
INSTANTANEOUS REAL POWER SIGNAL
CF
(FREQUENCY DOMAIN)
F1
F2
FREQUENCY – RAD/S
ω
CF
F1
ATTENUATED BY LPF
Preliminary Technical Data
TIME
TIME
cos(2ωt)
2ω