ADE7757 Analog Devices, ADE7757 Datasheet - Page 11

• Manufacturer Part Number: ADE7757
• Description: Energy Metering IC with Integrated Oscillator
• Manufacturer: Analog Devices

As previously described, the digital output of the low-pass filter
after multiplication contains the real power information. How-
ever, since this LPF is not an ideal “brick wall” filter imple-
mentation, the output signal also contains attenuated
components at the line frequency and its harmonics, i.e.,
cos(h t) where h = 1, 2, 3, . . . etc.
The magnitude response of the filter is given by:
For a line frequency of 50 Hz this would give an attenua-
tion of the 2
22 dB. The dominating harmonic will be at twice the line
frequency (2 ) due to the instantaneous power calculation.
Figure 21 shows the instantaneous real power signal at the
output of the LPF which still contains a significant amount
of instantaneous power information, i.e., cos (2 t). This
signal is then passed to the digital-to-frequency converter
where it is integrated (accumulated) over time in order to
produce an output frequency. The accumulation of the
signal will suppress or average out any non-dc components
in the instantaneous real power signal. The average value
of a sinusoidal signal is zero. Hence the frequency gener-
ated by the ADE7757 is proportional to the average real
power. Figure 21 shows the digital-to-frequency conver-
sion for steady load conditions, i.e., constant voltage and
current.
As can be seen in the diagram, the frequency output CF is
seen to vary over time, even under steady load conditions.
This frequency variation is primarily due to the cos (2 t)
component in the instantaneous real power signal. The
output frequency on CF can be up to 2048 times higher
than the frequency on F1 and F2. This higher output fre-
quency is generated by accumulating the instantaneous
real power signal over a much shorter time while convert-
REV. PrC.
MULTIPLIER
V
INSTANTANEOUS REAL POWER SIGNAL
2
I
0
(FREQUENCY DOMAIN)
V
I
LPF TO EXTRACT
REAL POWER
(100 Hz) component of approximately –
(DC TERM)
ATTENUATED
LPF
FREQUENCY (RAD/S)
H
(
cos
f
)
2
2 (
) t
PRELIMINARY TECHNICAL DATA
BY
1
LPF
DIGITAL-TO-
FREQUENCY
DIGITAL-TO-
FREQUENCY
1
8
f
.
9
2
2
F1
F2
CF
F1
CF
TIME
TIME
(5)
–11–
ing it to a frequency. This shorter accumulation period
means less averaging of the cos (2 t) component. Conse-
quently, some of this instantaneous power signal passes
through the digital-to-frequency conversion. This will not
be a problem in the application. Where CF is used for
calibration purposes, the frequency should be averaged by
the frequency counter which will remove any ripple. If CF
is being used to measure energy; for example, in a micro-
processor-based application, the CF output should also be
averaged to calculate power.
Because the outputs F1 and F2 operate at a much lower
frequency, a lot more averaging of the instantaneous real
power signal is carried out. The result is a greatly attenu-
ated sinusoidal content and a virtually ripple-free fre-
quency output.
The easiest way to interface the ADE7757 to a
microcontroller is to use the CF high frequency output
with the output frequency scaling set to 2048 x F1, F2.
This is done by setting SCF = 0 and S0 = S1 = 1, see
Table III. With full-scale ac signals on the analog inputs,
the output frequency on CF will be approximately
2.867 kHz. Figure 22 illustrates one scheme which could
be used to digitize the output frequency and carry out the
necessary averaging mentioned in the previous section.
As shown, the frequency output CF is connected to an
MCU counter or port. This will count the number of
pulses in a given integration time which is determined by
an MCU internal timer. The average power is propor-
tional to the average frequency is given by:
The energy consumed during an integration period is
given by:
Energy
Average
FREQUENCY
AVERAGE
Average
ADE7757
CF
Frequency
CF
Power
FREQUENCY
Time
RIPPLE
Average
TIME
Counter
Time
Power
COUNTER
TIMER
MCU
Time
Counter
Time
10%
Counter