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AD7751 Datasheet(PDF) 14 Page - Analog Devices |
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AD7751 Datasheet(HTML) 14 Page - Analog Devices |
14 / 16 page REV. A AD7751 –14– Fault with Active Input Greater than Inactive Input If V1A is the active current input (i.e., is being used for billing), and the signal on V1B (inactive input) falls by more than 12.5% of V1A, the fault indicator will go 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 one second on the logic output FAULT after the fault event. The FAULT logic output is independent of any activ- ity on outputs F1 or F2. Figure 13 illustrates one condition under which FAULT becomes active. Since V1A is the active input and it is still greater than V1B, billing is maintained on VIA, i.e., no swap to the V1B input will occur. V1A remains the active input. V1B < 87.5% OF V1A 0V V1A V1B V1A V1A V1N AGND V1B V1B FILTER AND COMPARE TO MULTIPLIER FAULT A B Figure 13. Fault Conditions for Inactive Input Less than Active Input Fault with V1B Greater than V1A Figure 14 illustrates another fault condition. If V1A is the active input (i.e., is being used for billing), and the voltage signal on V1B (inactive input) becomes greater than 114% of V1A, the FAULT indicator goes active and there is also a swap over to the V1B input. The analog input V1B has now become the active input. Again there is a time delay of about 1.2 second associated with this swap. V1A will not swap back to being the active channel until V1A becomes greater than 114% of V1B. However, the FAULT indicator will become inactive as soon as V1A is within 12.5% of V1B. This threshold eliminates poten- tial chatter between V1A and V1B. V1A < 87.5% OF V1B OR V1B > 114% OF V1A 0V V1A V1B V1A V1A V1N AGND V1B V1B FILTER AND COMPARE TO MULTIPLIER FAULT A B Figure 14. Fault Conditions for Inactive Input Greater than Active Input Calibration Concerns Typically, when a meter is being calibrated, the voltage and current circuits are separated as shown in Figure 15. This means that current will only pass through the phase or neutral circuit. Figure 15 shows current being passed through the phase circuit. This is the preferred option since the AD7751 starts billing on the input V1A on power-up. The phase circuit CT is connected to V1A in the diagram. Since there is no current in the neutral circuit the FAULT indicator will come on under these conditions. However, this does not affect the accuracy of the calibration and can be used as a means to test the functionality of the fault detection. Ib V 240Vrms NOTE: Ra Rf; Rb + VR = Rf Rb Rb V1A 0V V1B CT V1A AGND Rf CT NEUTRAL PHASE V1N Cf Cf Rf Rb VR V2P Rf V2N Cf Cf TEST CURRENT Ib Ra Figure 15. Fault Conditions for Inactive Input Greater than Active Input If the neutral circuit is chosen for the current circuit in the arrange- ment shown in Figure 15, it may have implications for the calibration accuracy. The AD7751 will power up with the V1A input active as normal. However, since there is no current in the phase circuit, the signal on V1A is zero. This will cause a FAULT to be flagged and the active input to be swapped to V1B (Neutral). The meter may be calibrated in this mode but the phase and neutral CTs may differ slightly. Since under no-fault condi- tions all billing is carried out using the phase CT, the meter should be calibrated using the phase circuit. Of course, both phase and neutral circuits may be calibrated. TRANSFER FUNCTION Frequency Outputs F1 and F2 The AD7751 calculates the product of two voltage signals (on Channel 1 and Channel 2) and then low-pass filters this product to extract real-power information. This real-power information is then converted to a frequency. The frequency information is output on F1 and F2 in the form of active low pulses. The pulse rate at these outputs is relatively low, e.g., 0.34 Hz maximum for ac signals with S0 = S1 = 0 (see Table III). This means that the frequency at these outputs is generated from real-power informa- tion accumulated over a relatively long period of time. The result is an output frequency that is proportional to the average real power. The averaging of the real-power signal is implicit to the digital-to-frequency conversion. The output frequency or pulse rate is related to the input voltage signals by the following equation. Freq V V Gain F VREF = ×× × × 574 1 2 14 2 . – (7) |
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