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ADE7760 Datasheet(PDF) 17 Page - Analog Devices |
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ADE7760 Datasheet(HTML) 17 Page - Analog Devices |
17 / 24 page 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%. Hz 0 . 22 64 Hz 34 . 0 5 . 2 2 2 Hz 72 . 1 66 . 0 66 . 0 70 . 5 2 = × − = = × × × × × = − 2 1 2 1 F F Frequency CF Frequency F F 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 S1 S0 F1, F2 Maximum Frequency (Hz) CF Maximum Frequency (Hz) CF to F1 Ratio 1 0 0 0.34 43.52 128 0 0 0 0.34 21.76 64 1 0 1 0.68 43.52 64 0 0 1 0.68 21.76 32 1 1 0 1.36 43.52 32 0 1 0 1.36 21.76 16 1 1 1 2.72 43.52 16 0 1 1 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 TO MULTIPLIER FAULT A B V1A V1B V1B < 93.75% OF V1A >0 <0 ACTIVE POINT – INACTIVE INPUT 6.25% OF ACTIVE INPUT 0V 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 TO MULTIPLIER FAULT A B V1A V1B V1A < 93.75% OF V1B >0 <0 ACTIVE POINT – INACTIVE INPUT 6.25% OF INACTIVE INPUT 0V FAULT + SWAP V1A V1B Figure 24. Fault Conditions for Inactive Input Greater than Active Input |
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