Datasheet

AD71056
Rev. A | Page 17 of 20
to active power. The CF output frequencies are given in Table 7.
As with F1 and F2, if the period of CF (t
5
) falls below 180 ms,
the CF pulse width is set to half the period. If the CF frequency,
for example, 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 35 μs. Therefore, t
4
is always 35 μs, regardless of the output frequency on CF.
NO LOAD THRESHOLD
The AD71056 also includes a no load threshold and start-up
current feature that eliminates any creep effects in the meter.
The AD71056 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 0.00244%
for each of the f
1…4
frequency selections (see Table 5).
For example, for an energy meter with a meter constant of
100 imp/kWh on F1, F2 using f
3
(3.43 Hz), the minimum
output frequency at F1 or F2 equals 0.00244% of 3.43 Hz or
8.38 × 10
–5
Hz. This is 2.68 × 10
–3
Hz at CF (32 × F1 Hz) when
SCF = S0 = 1, S1 = 0. In this example, the no load threshold is
equivalent to 3 W of load or a start-up current of 13.72 mA at
220 V. Compare this value to the IEC62053-21 specification that
states 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 AD71056 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.