ADE7757_15 datasheet(14/16 Pages) AD

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REV. A

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ADE7757

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

Frequency on F1 and F2–

F1–4 (Hz)* CH1 and CH2 Half-Scale AC Input*

0.86

0.051

× F

0.044 Hz

1.72

0.051

× F

0.088 Hz

3.44

0.051

× F3

0.176 Hz

6.86

0.051

× F

0.352 Hz

*Values are generated using the nominal frequency of 450 kHz.

When selecting a suitable F1–4 frequency for a meter design,

the frequency output at IMAX (maximum load) with a meter con-

stant of 100 imp/kWh should be compared with column four of

Table V. The closest frequency in Table V will determine the

best choice of frequency (F1–4). For example, if a meter with a

maximum current of 25 A is being designed, the output fre-

quency on F1 and F2 with a meter constant of 100 imp/kWh is

0.153 Hz at 25 A and 220 V (from Table IV). Looking at Table V,

the closest frequency to 0.153 Hz in column four is 0.176 Hz.

Therefore, F3 (3.44 Hz—see Table I) is selected for this design.

Frequency Outputs

Figure 1 shows a timing diagram for the various frequency out-

puts. The outputs F1 and F2 are the low frequency outputs that

can be used to directly drive a stepper motor or electromechanical

impulse counter. The F1 and F2 outputs provide two alter-

nating low frequency pulses. The F1 and F2 pulse widths (t1)

are set such that if they fall below 1062 ms (0.942 Hz) they are

set to half of their period. The maximum output frequencies for

F1 and F2 are shown in Table II.

The high frequency CF output is intended to be used for com-

munications and calibration purposes. CF produces a 173 ms wide

active high pulse (t4) at a frequency proportional to active power.

The CF output frequencies are given in Table III. As in the case

of F1 and F2, if the period of CF (t5) falls below 346 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.

NOTE: When the high frequency mode is selected (i.e., SCF =

0, S1 = S0 = 1), the CF pulse width is fixed at 35

µs. Therefore,

t4 will always be 35

µs, regardless of output frequency on CF.

NO LOAD THRESHOLD

The ADE7757 also includes a no-load threshold and start-up

current feature that will eliminate any creep effects in the meter.

The ADE7757 is designed to issue a minimum output frequency.

Any load generating a frequency lower than this minimum fre-

quency will not cause a pulse to be issued on F1, F2, or CF.

The minimum output frequency is given as 0.0014% for each of

the F1–4 frequency selections (see Table I). For example, for an

energy meter with a meter constant of 100 imp/kWh on F1, F2

using F3 (3.44 Hz), the minimum output frequency at F1 or F2

would be 0.0014% of 3.44 Hz or 4.81

× 10–5 Hz. This would be

3.08

× 10–3 Hz at CF (64 × F1 Hz) when SCF = S0 = 1, S1 = 0.

In this example, the no-load threshold would be equivalent to

1.7 W of load or a start-up current of 8 mA at 220 V. Compare

this value to the IEC 1036 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.

Negative Power Information

The ADE7757 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 will go active high when negative power

is detected and active low if positive power is detected. The

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

REVP pin is not functional in the current version and will only

work in the A version (ADE7757A).

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