ade7761a Analog Devices, Inc., ade7761a Datasheet - Page 21

• Manufacturer Part Number: ade7761a
• Description: Energy Metering Ic With On-chip Fault And Missing Neutral Detection
• Manufacturer: Analog Devices, Inc.

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.
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
The energy consumed during an integration period is
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.
Average
Energy
FREQUENCY
2
1
FAULT MUST BE USED TO RECORD ENERGY IN FAULT CONDITION.
DIRECTION OF ENERGY FLOW IS NEEDED.
REVP MUST BE USED IF THE METER IS BIDIRECTIONAL OR
AVERAGE
ADE7761A
=
Frequency
FAULT
Average
CF
REVP
Figure 35. Interfacing the ADE7761A to an MCU
CF
2
1
Power
=
Average
×
FREQUENCY
Time
RIPPLE
TIME
Active
=
Counter
Time
Power
UP/DOWN
COUNTER
LOGIC
=
×
MCU
Time
Counter
Timer
±10%
=
Counter
Rev. 0 | Page 21 of 24
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 (I
cases, the meter constant is 100 impulses/kWh.
Table 9. F1 and F2 Frequency at 100 Impulses/kWh
I
12.5
25
40
60
80
120
The F
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
S0
0
0
1
1
When selecting a suitable F
frequency output at I
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 (F
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).
MAX
(A)
S1
0
1
0
1
1–4
frequencies allow complete coverage of this range of
F
1.72
3.44
6.86
13.5
1–4
(Hz)
MAX
MAX
Frequency on F1 and F2, Ch 1 and Ch 2,
Half-Scale AC Inputs (Hz)
0.085
0.17
0.34
0.68
F1 and F2 (Hz)
0.083
0.166
0.266
0.4
0.533
0.8
) with a line voltage of 240 V. In all
(maximum load) with a meter constant
1–4
frequency for a meter design, the
1-4
). For example, if a meter with
ADE7761A