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ADE7755 Datasheet(PDF) 18 Page - Analog Devices

Part No. ADE7755
Description  Energy Metering IC with Pulse Output
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADE7755 Datasheet(HTML) 18 Page - Analog Devices

 
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ADE7755
Rev. A | Page 18 of 20
Table 8. Maximum Output Frequency on CF
SCF
S1
S0
f1, f2, f3, and f4 (Hz)
CF Maximum for AC Signals
1
0
0
f1 = 1.7
128 × F1, F2 = 43.52 Hz
0
0
0
f1 = 1.7
64 × F1, F2 = 21.76 Hz
1
0
1
f2 = 3.4
64 × F1, F2 = 43.52 Hz
0
0
1
f2 = 3.4
32 × F1, F2 = 21.76 Hz
1
1
0
f3 = 6.8
32 × F1, F2 = 43.52 Hz
0
1
0
f3 = 6.8
16 × F1, F2 = 21.76 Hz
1
1
1
f4 = 13.6
16 × F1, F2 = 43.52 Hz
0
1
1
f4 = 13.6
2048 × F1, F2 = 5.57 kHz
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 has been optimized for a meter constant of
100 imp/kWh with a maximum current between 10 A and 120 A.
Table 9 shows the output frequency for several maximum currents
(IMAX) with a line voltage of 220 V. In all cases, the meter
constant is 100 imp/kWh.
Table 9. F1 and F2 Frequency at 100 imp/kWh
IMAX (A)
F1 and F2 (Hz)
12.5
0.076
25
0.153
40
0.244
60
0.367
80
0.489
120
0.733
The fi frequencies (i = 1, 2, 3, or 4) 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. This allows overcurrent
signals and signals with high crest factors to be accommodated.
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
S1
S0
f1, f2, f3, and f4 (Hz)
F1 and F2 Frequency on CH1 and
CH2 Half-Scale AC Inputs (Hz)
0
0
f1 = 1.7
0.085
0
1
f2 = 3.4
0.17
1
0
f3 = 6.8
0.34
1
1
f4 = 13.6
0.68
When selecting a suitable fi frequency (i = 1, 2, 3, or 4) for a
meter design, the frequency output at IMAX (maximum load)
with a meter constant of 100 imp/kWh should be compared with
Column 4 of Table 10. The frequency that is closest in Table 10
determines the best choice of fi frequency (i = 1, 2, 3, or 4). For
example, if a meter with a maximum current of 25 A is being
designed, the output frequency on F1 and F2 with a meter constant
of 100 imp/kWh is 0.153 Hz at 25 A and 220 V (from Table 9).
Table 10, the closest frequency to 0.153 Hz in Column 4, is 0.17 Hz.
Therefore, f2 (3.4 Hz, see Table 6) is selected for this design.
FREQUENCY OUTPUTS
Figure 2 shows a timing diagram for the various frequency
outputs. The F1 and F2 outputs are the low frequency outputs
that can be used to directly drive a stepper motor or electro-
mechanical impulse counter. The F1 and F2 outputs provide
two alternating low going pulses. The pulse width (t1) is set at
275 ms, and the time between the falling edges of F1 and F2 (t3)
is approximately half the period of F1 (t2). If, however, the period of
F1 and F2 falls below 550 ms (1.81 Hz), the pulse width of F1 and
F2 is set to half of their period. The maximum output frequencies
for F1 and F2 are shown in Table 7.
The high frequency CF output is intended to be used for
communications and calibration purposes. CF produces a
90 ms wide active high pulse (t4) at a frequency proportional
to active power. The CF output frequencies are listed in Table 8.
As in the case of F1 and F2, if the period of CF (t5) falls below
180 ms, the CF pulse width is set to half the period. For example,
if the CF frequency is 20 Hz, the CF pulse width is 25 ms.
When the high frequency mode is selected (that is, SCF = 0,
S1 = S0 = 1), the CF pulse width is fixed at 18 μs. Therefore, t4
is always 18 μs, regardless of the output frequency on CF.


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