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ADE7757
Table V. F1 and F2 Frequency with Half-Scale AC Inputs
CF pulse width is set to half the period. For example, if the CF
S1
0
0
1
1
S0
0
1
0
1
Frequency on F1 and F2–
F 1–4 (Hz) * CH1 and CH2 Half-Scale AC Input *
0.86 0.051 × F 1 0.044 Hz
1.72 0.051 × F 2 0.088 Hz
3.44 0.051 × F 3 0.176 Hz
6.86 0.051 × F 4 0.352 Hz
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 μ s. Therefore,
t 4 will always be 35 μ s, regardless of output frequency on CF.
NO LOAD THRESHOLD
The ADE7757 also includes a no-load threshold and start-up
* Values are generated using the nominal frequency of 450 kHz.
When selecting a suitable F 1–4 frequency for a meter design,
the frequency output at I MAX (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 (F 1–4 ). 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 (t 1 )
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 (t 4 ) 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 (t 5 ) falls below 346 ms, the
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 F 1–4 frequency selections (see Table I). For example, for an
energy meter with a meter constant of 100 imp/kWh on F1, F2
using F 3 (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).
–14 –
REV. A
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