ADE7768

Rev. A | Page 16 of 20

of 100 imp/kWh should be compared with Column 4 of

Table 9. The closest frequency in Table 9 determines the best

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

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

CF output frequencies are given in Table 7. As with F1 and F2,

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),

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 example, for an energy meter with a meter constant of

output frequency at F1 or F2 would be 0.00244% of 3.43 Hz or

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

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.