ADE7761ARS datasheet(24/28 Pages) AD

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ADE7761

Rev. A | Page 24 of 28

Table 9. F1 and F2 Frequency with Half-Scale AC Inputs

F1–4

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 9. The frequency that is closest in Table 9 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/kWh is 0.266 Hz at 40 A and 240 V (from

Table 8). Looking at Table 9, the closest frequency to 0.266 Hz

in Column 4 is 0.17 Hz. Therefore, F2 (3.4 Hz; see Table 5) is

selected for this design.

Frequency Outputs

Figure 2 shows a timing diagram for the various frequency

outputs. 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 that is propor-

tional to active power. The CF output frequencies are given in

Table 7. 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.

No-Load Threshold

The ADE7761 includes a no-load threshold and startup current

feature that eliminates creep effects in the meter. The ADE7761

is designed to issue a minimum output frequency. Any load

generating a frequency lower than this minimum frequency

does not cause a pulse to be issued on F1, F2, or CF. The mini-

mum output frequency is given as 0.0045% of the full-scale

output frequency. (See Table 7 for maximum output frequencies

for ac signals).

For example, an energy meter with a meter constant of

100 impulses/kWh on F1, F2 using SCF = 1, S1 = 0, and S0 = 1,

the maximum output frequency at F1 or F2 is 0.68 Hz and 43.52

Hz on CF. The minimum output frequency at F1 or F2 is

0.0045% of 0.68 Hz or 3.06 × 10–5 Hz. This is 1.96 × 10–3 Hz at

CF (64 × F1 Hz).

In this example, the no-load threshold is equivalent to 1.1 W of

load or a startup current of 4.6 mA at 240 V. Compare this value

to the IEC61036 specification, which states that the meter must

start up with a load equal to or less than 0.4% IB. For a 5 A (IB)

meter, 0.4% of IB is equivalent to 20 mA.

Note that the no-load threshold is not enabled when using the

high CF frequency mode: SCF = 0, S1 = S0 = 1.

NEGATIVE POWER INFORMATION

The ADE7761 detects when the current and voltage channels

have a phase shift greater than 90°. This mechanism can detect

wrong connection of the meter or generation of negative power.

The REVP pin output goes active high when negative power is

detected, and active low when positive power is detected. The

REVP pin output changes state as a pulse is issued on CF.

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