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ADE7768
When selecting a suitable F 1–4 frequency for a meter design, the
frequency output at I MAX (maximum load) with a meter constant
of 100 imp/kWh should be compared with Column 4 of
Table 9. The closest frequency in Table 9 determines the best
choice of frequency (F 1–4 ). For example, if a meter with a max-
imum current of 25 A is being designed, the output frequency
on F1 and F2 with a meter constant of 100 imp/kWh is 0.153 Hz
at 25 A and 220 V (from Table 8). In Table 9, the closest fre-
quency to 0.153 Hz in Column 4 is 0.176 Hz. Therefore, as
shown in Table 5, F3 (3.43 Hz) is selected for this design.
Frequency Outputs
Figure 2 shows a timing diagram for the various frequency
outputs. The F1 and F2 outputs are the low frequency outputs
that can be used to directly drive a stepper motor or electro-
mechanical impulse counter. The F1 and F2 outputs provide
two alternating low frequency pulses. The F1 and F2 pulse
widths (t 1 ) are set such that if they fall below 240 ms (0.24 Hz),
they are set to half of their period. The maximum output
frequencies for F1 and F2 are shown in Table 6.
The high frequency CF output is intended for communications
and calibration purposes. CF produces a 90-ms-wide active
high pulse (t 4 ) at a frequency proportional 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. For example, if the CF frequency is 20 Hz,
the CF pulse width is 25 ms.
When high frequency mode is selected (SCF = 0, S1 = S0 = 1),
the CF pulse width is fixed at 35 μs. Therefore, t 4 is always 35 μs,
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 would be 0.00244% of 3.43 Hz or
8.38 × 10 –5 Hz. This would be 2.68 × 10 –3 Hz at CF (32 × F1 Hz)
when SCF = S0 = 1, S1 = 0. In this example, the no-load
threshold would be 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 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 ADE7768 detects when the current and voltage channels
have a phase shift greater than 90°. This mechanism can detect
an incorrect meter connection or the generation of negative
power. The REVP pin output goes 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.
EVALUATION BOARD AND REFERENCE DESIGN
BOARD
An evaluation board can be used to verify the functionality and
the performance of the ADE7768. Download documentation
for the board from http://www.analog.com/ADE7768.
In addition, the reference design board ADE7768ARN-REF
and Application Note AN-679 can be used in the design
of a low cost watt-hour meter that surpasses IEC62053-21
accuracy specifications. Download the application note from
http://www.analog.com/ADE7768.
regardless of output frequency on CF.
NO-LOAD THRESHOLD
The ADE7768 includes a no-load threshold and start-up
current feature that eliminates any creep effects in the meter.
The ADE7768 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).
Rev. A | Page 16 of 20
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