ADE7760ARS datasheet(20/24 Pages) AD

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ADE7760

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

F1-4

Frequency on F1 and F2,

CH1 and CH2,

Half-Scale AC Inputs

1.72

0.085 Hz

3.44

0.17 Hz

6.86

0.34 Hz

13.5

0.68 Hz

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 ADE7760 also includes a no-load threshold and startup

current feature that eliminates any creep effects in the

meter. The ADE7760 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 minimum 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 would be 0.68 Hz

and 43.52 Hz on CF. The minimum output frequency at F1 or

F2 would be 0.0045% of 0.68 Hz or 3.06 × 10–5 Hz. This would

be 1.96 × 10–3 Hz at CF (64 × F1 Hz). In this example, the no-

load threshold would be 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 ADE7760 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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