ADE7769
Rev. A | Page 16 of 20
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 maximum
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 frequency to
0.153 Hz in Column 4 is 0.176 Hz. Therefore, as shown in
Tab le
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
electromechanical 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 to be used 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 (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 output frequency on CF.
NO-LOAD THRESHOLD
The ADE7769 includes a no-load threshold and start-up
current feature, which eliminates any creep effects in the meter.
The ADE7769 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
or F2. 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 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.
The no-load condition is indicated with CF output pulse
remaining logic high, as shown in
Figure 28.
CF
0W
NO-LOAD
THRESHOLD
MAGNITUDE
TIME
ACTIVE POWER
CF FREQUENCY PROPORTIONAL TO POWER
05332-028
Figure 28. No-Load Indication Using ADE7769
NEGATIVE POWER INFORMATION
The ADE7769 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 ADE7769. Download the documenta-
tion for the board from http://www.analog.com/ADE7769.
In addition, the reference design board ADE7769ARN-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. The application note can be
downloaded from http://www.analog.com/ADE7769.
