ADE7761AARS datasheet(21/24 Pages) AD

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ADE7761A

Rev. 0 | Page 21 of 24

APPLICATIONS

INTERFACING TO A MICROCONTROLLER FOR

ENERGY MEASUREMENT

The easiest way to interface the ADE7761A 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 35 illustrates one scheme that could be used to

digitize the output frequency and carry out the necessary

averaging mentioned in the Frequency Output CF section.

TIME

±10%

AVERAGE

FREQUENCY

RIPPLE

MCU

UP/DOWN

COUNTER

LOGIC

REVP1

ADE7761A

FAULT2

1REVP MUST BE USED IF THE METER IS BIDIRECTIONAL OR

DIRECTION OF ENERGY FLOW IS NEEDED.

2FAULT MUST BE USED TO RECORD ENERGY IN FAULT CONDITION.

Figure 35. Interfacing the ADE7761A to an MCU

As shown in Figure 35 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

Timer

Counter

Power

Active

Average

Frequency

Average

The energy consumed during an integration period is

Counter

Time

Counter

Time

Power

Average

Energy

For the purpose of calibration, this integration time could be

10 sec to 20 sec to accumulate enough pulses to ensure correct

averaging of the frequency. In normal operation, the integration

time could be reduced to 1 sec or 2 sec depending on, for

example, 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.

SELECTING A FREQUENCY FOR AN ENERGY

METER APPLICATION

As shown in Table 6, 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 was optimized for a meter constant of

100 impulses/kWh with a maximum current of between 10 A

and 120 A. Table 9 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.

Table 9. F1 and F2 Frequency at 100 Impulses/kWh

IMAX (A)

F1 and F2 (Hz)

12.5

0.083

0.166

0.266

0.4

0.533

120

0.8

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 10

shows the output frequency on F1 and F2 when both analog

inputs are half-scale. The frequencies listed in Table 10 align

well with those listed in Table 9 for maximum load.

Table 10. F1 and F2 Frequency with Half-Scale AC Inputs

F1–4 (Hz)

Frequency on F1 and F2, Ch 1 and Ch 2,

Half-Scale AC Inputs (Hz)

1.72

0.085

3.44

0.17

6.86

0.34

13.5

0.68

When selecting a suitable F1–4 frequency for a meter design, the

frequency output at IMAX (maximum load) with a meter constant

of 100 impulses/kWh should be compared with Column 4 of

Table 10. The frequency that is closest in Table 10 determines

the best choice of frequency (F1-4). For example, if a meter with

a maximum current of 40 A is being designed, the output

frequency on F1 and F2 with a meter constant of 100 impulses

per kWh is 0.266 Hz at 40 A and 240 V (see Table 9).

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