ADE7761AARSZ datasheet(17/24 Pages) AD

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ADE7761A

Rev. 0 | Page 17 of 24

Table 6. F1–4 Frequency Selection

F1–4 (Hz)1

F1−4 = OSC/2n 2

1.72

OSC/218

3.44

OSC/217

6.86

OSC/216

13.7

OSC/215

1 Values are generated using the nominal frequency of 450 kHz.

2 F1–4 are a binary fraction of the master clock and, therefore, vary with the

internal oscillator frequency (OSC).

Frequency Output CF

The pulse output calibration frequency (CF) is intended for use

during calibration. The output pulse rate on CF can be up to

2048 times the pulse rate on F1 and F2. The lower the F1–4

frequency selected, the higher the CF scaling. Table 7 shows

how the two frequencies are related, depending on the states of

the logic inputs S0, S1, and SCF. Because of its relatively high

pulse rate, the frequency at this logic output is proportional to

the instantaneous active power. As with F1 and F2, the

frequency is derived from the output of the low-pass filter after

multiplication. However, because the output frequency is high,

this active power information is accumulated over a much

shorter time. Therefore, less averaging is carried out in the

digital-to-frequency conversion. With much less averaging of

the active power signal, the CF output is much more responsive

to power fluctuations (see Figure 22).

Table 7. Relationship Between CF and F1, F2 Frequency

Outputs

SCF

F1–4 (Hz)

CF Frequency Output

1.72

128 × F1, F2

1.72

64 × F1, F2

3.44

64 × F1, F2

3.44

32 × F1, F2

6.86

32 × F1, F2

6.86

16 × F1, F2

13.7

16 × F1, F2

13.7

2048 × F1, F2

Example

In this example, if ac voltages of ±660 mV peak are applied to

V1 and V2, then the expected output frequency on CF, F1, and

F2 is calculated as

Gain = 1, PGA = 0

F1–4 = 1.7 Hz, SCF = S1 = S0 = 0

V1rms = rms of 660 mV peak ac = 0.66/√2 V

V2rms = rms of 660 mV peak ac = 0.66/√2 V

VREF = 2.5 V (nominal reference value)

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

frequencies can vary from device to device due to a reference

tolerance of ±8%.

− Frequency

CF Frequency = F1 − F2 × 64 = 22.0 Hz

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 8 shows a complete listing of all

maximum output frequencies for ac signals.

Table 8. Maximum Output Frequencies on CF, F1, and F2 for

AC Inputs

SCF

F1, F2 Maximum

Frequency (Hz)

CF Maximum

Frequency (Hz)

CF-to-

F1 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 ADE7761A incorporates a novel fault detection scheme

that warns of fault conditions and allows the ADE7761A to

continue accurate billing during a fault event. The ADE7761A

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 ADE7761A 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 ADE7761A is

proportional to the product of the voltage signals on V1A and

Channel 2. If the difference between V1A and V1B on power-up is

greater than 6.25%, the fault indicator (FAULT) becomes active

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

ADE7761A 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.

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