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ADE7755
DIGITAL-TO-
FREQUENCY
F1
CF
FREQUENCY
RIPPLE
V
MULTIPLIER
LPF
F1
F2
DIGITAL-TO-
FREQUENCY
f OUT
TIME
AVERAGE
FREQUENCY
±10%
I
CF
V×I
2
LPF TO EXTRACT
REAL POWER
(DC TERM)
cos(2 ω t)
ATTENUATED BY LPF
TIME
ADE7755
CF
TIME
MCU
COUNTER
REVP 1
UP/DOWN
0
ω
2 ω
TIMER
FREQUENCY (RAD/s)
INSTANTANEOUS ACTIVE POWER SIGNAL
(FREQUENCY DOMAIN)
Figure 32. Active Power-to-Frequency Conversion
As can be seen in Figure 32, the frequency output CF varies over
time, even under steady load conditions. This frequency variation
is primarily due to the cos(2 ωt) component in the instantaneous
active power signal. The output frequency on CF can be up to
2048 times higher than the frequency on F1 and F2. This higher
1 REVP MUST BE USED IF THE METER IS BIDIRECTIONAL OR
DIRECTION OF ENERGY FLOW IS NEEDED
Figure 33. Interfacing the ADE7755 to an MCU
As shown in Figure 33, the frequency output CF is connected to
an MCU counter or port, which counts the number of pulses in
a given integration time that is determined by an MCU internal
timer. The average power proportional to the average frequency
is given by
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
Average Frequency = Average Active Power =
Counter
Timer
Energy = Average Power × Time =
× Time = Counter
averaging of the cos(2 ωt) component. Consequently, some of
this instantaneous power signal passes through the digital-to-
frequency conversion, which is not a problem in the
application. When CF is used for calibration purposes, the
frequency should be averaged by the frequency counter. This
averaging operation removes any ripple. If CF is measuring
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,
more 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.
INTERFACING THE ADE7755 TO A
MICROCONTROLLER FOR ENERGY MEASUREMENT
The easiest way to interface the ADE7755 to a microcontroller
is to use the CF high frequency output with the output frequency
scaling set to 2048 × F1, F2. This is done by setting SCF = 0 and
S0 = S1 = 1 (see Table 8). With full-scale ac signals on the analog
inputs, the output frequency on CF is approximately 5.5 kHz.
Figure 33 illustrates one scheme that can be used to digitize the
output frequency and carry out the necessary averaging described
in the Digital-to-Frequency Conversion section.
The energy consumed during an integration period is given by
Counter
Time
For the purpose of calibration, this integration time can be
10 seconds to 20 seconds to accumulate enough pulses to ensure
correct averaging of the frequency. In normal operation, the
integration time can be reduced to 1 second or 2 seconds
depending, for example, on the required update rate of a display.
With shorter integration times on the MCU, the amount of
energy in each update may still have some small amount of
ripple, even under steady load conditions. However, over a
minute or more, the measured energy has no ripple.
POWER MEASUREMENT CONSIDERATIONS
Calculating and displaying power information always has some
associated ripple that depends on the integration period used in
the MCU to determine average power and also the load. For
example, at light loads, the output frequency can be 10 Hz. With
an integration period of 2 seconds, only about 20 pulses are
counted. The possibility of missing one pulse always exists because
the ADE7755 output frequency is running asynchronously to the
MCU timer. This possibility results in a 1-in-20 (or 5%) error in
the power measurement.
Rev. A | Page 16 of 20
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