ADE7760ARS datasheet(17/24 Pages) AD

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ADE7760

Rev. 0 | Page 17 of 24

Note that if the on-chip reference is used, actual output

frequencies may vary from device to device due to reference

tolerance of ±8%.

−

Frequency

As can be seen from these two example calculations, the

maximum output frequency for ac inputs is always half of that

for dc input signals. Table 7 shows a complete listing of all

maximum output frequencies for ac signals.

Table 7. Maximum Output Frequency on CF, F1, and F2 for

AC inputs

SCF

F1, F2 Maximum

Frequency

(Hz)

CF Maximum

Frequency

(Hz)

CF to

Ratio

0.34

43.52

128

0.34

21.76

0.68

43.52

0.68

21.76

1.36

43.52

1.36

21.76

2.72

43.52

2.72

5570

2048

FAULT DETECTION

The ADE7760 incorporates a novel fault detection scheme that

warns of fault conditions and allows the ADE7760 to continue

accurate billing during a fault event. The ADE7760 does this by

continuously monitoring both the phase and neutral (return)

currents. A fault is indicated when these currents differ by more

than 6.25%. However, even during a fault, the output pulse rate

on F1 and F2 is generated using the larger of the two currents.

Because the ADE7760 looks for a difference between the voltage

signals on V1A and V1B, it is important that both current

transducers be closely matched.

On power-up, the output pulse rate of the ADE7760 is pro-

portional to the product of the voltage signals on V1A and

Channel 2. If there is a difference of greater than 6.25% between

V1A and V1B on power-up, the fault indicator (FAULT) becomes

active after about 1 s. In addition, if V1B is greater than V1A, the

ADE7760 selects V1B as the input. The fault detection is

automatically disabled when the voltage signal on Channel 1 is

less than 0.3% of the full-scale input range. This eliminates false

detection of a fault due to noise at light loads.

Fault with Active Input Greater than Inactive Input

If V1A is the active current input (that is, is being used for

billing), and the voltage signal on V1B (inactive input) falls below

93.75% of V1A, the fault indicator becomes active. Both analog

inputs are filtered and averaged to prevent false triggering of

this logic output. As a consequence of the filtering, there is a

time delay of approximately 3 s on the logic output FAULT after

the fault event. The FAULT logic output is independent of any

activity on outputs F1 or F2. Figure 23 shows one condition

under which FAULT becomes active. Because V1A is the active

input and it is still greater than V1B, billing is maintained on V1A,

that is, no swap to the V1B input occurs. V1A remains the active

input.

V1B

V1N

V1A

AGND

FILTER

AND

COMPARE

MULTIPLIER

FAULT

V1A

V1B

V1B < 93.75% OF V1A

ACTIVE POINT – INACTIVE INPUT

6.25% OF ACTIVE INPUT

FAULT

V1A

V1B

Figure 23. Fault Conditions for Active Input Greater than Inactive Input

Fault with Inactive Input Greater than Active Input

Figure 24 illustrates another fault condition. If the difference

between V1B, the inactive input, and V1A, the active input (used

for billing), becomes greater than 6.25% of V1B, the FAULT

indicator goes active, and there is also a swap over to the V1B

input. The analog input V1B becomes the active input. Again,

there is a time constant of about 3 s associated with this swap.

V1A does not swap back to being the active channel until V1A is

greater than V1B and the difference between V1A and V1B—in this

order—becomes greater than 6.25% of V1A. The FAULT

indicator, however, becomes inactive as soon as V1A is within

6.25% of V1B. This threshold eliminates potential chatter

between V1A and V1B.

V1B

V1N

V1A

AGND

FILTER

AND

COMPARE

MULTIPLIER

FAULT

V1A

V1B

V1A < 93.75% OF V1B

ACTIVE POINT – INACTIVE INPUT

6.25% OF INACTIVE INPUT

FAULT + SWAP

V1A

V1B

Figure 24. Fault Conditions for Inactive Input Greater than Active Input

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