ADE7761
APPLICATIONS
For the purpose of calibration, this integration time could be
INTERFACING TO A MICROCONTROLLER FOR
ENERGY MEASUREMENT
10 s to 20 s in order to accumulate enough pulses to ensure
correct averaging of the frequency. In normal operation, the
integration time could be reduced to 1 s or 2 s 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 a small amount of ripple, even under
steady load conditions. However, over a minute or more, the
measured energy has no ripple.
The easiest way to interface the ADE7761 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 7). With full-scale ac signals
on the analog inputs, the output frequency on CF is approxi-
mately 5.5 kHz. Figure 33 illustrates one scheme that could be
used to digitize the output frequency and carry out the
necessary averaging mentioned in the previous section.
SELECTING A FREQUENCY FOR AN ENERGY
METER APPLICATION
CF
FREQUENCY
RIPPLE
As shown in Table 5, the user can select one of four frequencies.
This frequency selection determines the maximum frequency
on F1 and F2. These outputs are intended to be used to drive the
energy register (electromechanical or other). Because only four
different output frequencies can be selected, the available
frequency selection has been optimized for a meter constant of
100 impulses/kWh with a maximum current of between 10 A
and 120 A. Table 8 shows the output frequency for several
maximum currents (IMAX) with a line voltage of 240 V. In all
cases, the meter constant is 100 impulses/kWh.
AVERAGE
FREQUENCY
±10%
TIME
MCU
ADE7761
COUNTER
CF
Table 8. F1 and F2 Frequency at 100 Impulses/kWh
UP/DOWN
REVP*
IMAX (A)
12.5
25
F1 and F2 (Hz)
0.083
FAULT**
LOGIC
0.166
40
0.266
*REVP MUST BE USED IF THE METER IS BIDIRECTIONAL OR
DIRECTION OF ENERGY FLOW IS NEEDED.
**FAULT MUST BE USED TO RECORD ENERGY IN FAULT CONDITION.
60
0.4
80
0.533
120
0.8
Figure 33. Interfacing the ADE7761 to an MCU
As shown, the frequency output CF is connected to an MCU
counter or port, which counts the number of pulses in a given
integration time, determined by an MCU internal timer. The
average power, proportional to the average frequency, is given
by
The F1–4 frequencies allow complete coverage of this range of
output frequencies on F1 and F2. When designing an energy
meter, the nominal design voltage on Channel 2 (voltage)
should be set to half-scale to allow for calibration of the meter
constant. The current channel should also be no more than half-
scale when the meter sees maximum load, which accommodates
overcurrent signals and signals with high crest factors. Table 9
shows the output frequency on F1 and F2 when both analog
inputs are half-scale. The frequencies listed in Table 9 align well
with those listed in Table 8 for maximum load.
Counter
AverageFrequency = Average ActivePower =
Timer
The energy consumed during an integration period is given by
Counter
Energy = Average Power ×Time =
×Time = Counter
Time
Rev. A | Page 23 of 28
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