ADE7755 datasheet(18/20 Pages) AD | Energy Metering IC with Pulse Output

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ADE7755

Rev. A | Page 18 of 20

Table 8. Maximum Output Frequency on CF

SCF

f1, f2, f3, and f4 (Hz)

CF Maximum for AC Signals

f1 = 1.7

128 × F1, F2 = 43.52 Hz

f1 = 1.7

64 × F1, F2 = 21.76 Hz

f2 = 3.4

64 × F1, F2 = 43.52 Hz

f2 = 3.4

32 × F1, F2 = 21.76 Hz

f3 = 6.8

32 × F1, F2 = 43.52 Hz

f3 = 6.8

16 × F1, F2 = 21.76 Hz

f4 = 13.6

16 × F1, F2 = 43.52 Hz

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

0.153

0.244

0.367

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

f1, f2, f3, and f4 (Hz)

F1 and F2 Frequency on CH1 and

CH2 Half-Scale AC Inputs (Hz)

f1 = 1.7

0.085

f2 = 3.4

0.17

f3 = 6.8

0.34

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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