ADE7761B datasheet(17/24 Pages) AD

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ADE7761B

Rev. 0 | Page 17 of 24

Table 6. f1–4 Frequency Selection

f1–4 (Hz)1

f1−4 = OSC/2n2

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

Logic Input S0, Logic Input S1, and Logic Input 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

Channel V1 and Channel V2, 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 may vary from device to device due to a reference

tolerance of ±8%.

367

Frequency

CF Frequency

= F1, F2 × 64 = 23.5 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),

1/t2

CF Maximum

Frequency (Hz),

1/t5

CF-to-F1

Ratio

0.37

46.98

128

0.37

23.49

0.73

46.98

0.73

23.49

1.47

46.98

1.47

23.49

2.94

46.98

2.94

6013

2048

FAULT DETECTION

The ADE7761B incorporates a novel fault detection scheme

that warns of fault conditions and allows the ADE7761B to

continue accurate billing during a fault event. The ADE7761B

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 ADE7761B 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 ADE7761B is propor-

tional to the product of the voltage signals on V1A and Channel V2.

If the difference between V1A and V1B on power-up is greater than

6.25%, the fault indicator (FAULT) becomes active after about

1 second. In addition, if V1B is greater than V1A, the ADE7761B

selects V1B as the input. Fault detection is automatically disabled

when the voltage signal on Channel V1 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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