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ADE7752A Datasheet(PDF) 12 Page  Analog Devices 

ADE7752A Datasheet(HTML) 12 Page  Analog Devices 
12 / 24 page ADE7752/ADE7752A Rev. C  Page 12 of 24 THEORY OF OPERATION The six voltage signals from the current and voltage transducers are digitized with ADCs. These ADCs are 16bit secondorder ∑Δ with an oversampling rate of 833 kHz. This analog input structure greatly simplifies transducer interface by providing a wide dynamic range for direct connection to the transducer and also by simplifying the antialiasing filter design. A highpass filter in the current channel removes the dc component from the current signal. This eliminates any inaccuracies in the real power calculation due to offsets in the voltage or current signals. See the The low frequency output of the ADE7752 is generated by accumulating the total real power information. This low frequency inherently means a long accumulation time between output pulses. The output frequency is therefore proportional to the average real power. This average real power information can, in turn, be accumulated (by a counter, for example) to generate real energy information. Because of its high output frequency and therefore shorter integration time, the CF output is proportional to the instantaneous real power. This pulse is useful for system calibration purposes that would take place under steady load conditions. HPF and Offset Effects section. The real power calculation is derived from the instantaneous power signal. The instantaneous power signal is generated by a direct multiplication of the current and voltage signals of each phase. In order to extract the real power component (the dc component), the instantaneous power signal is lowpass filtered on each phase. POWER FACTOR CONSIDERATIONS Lowpass filtering, the method used to extract the real power information from the individual instantaneous power signal, is still valid when the voltage and current signals of each phase are not in phase. Figure 15 illustrates the instantaneous real power signal and shows how the real power information can be extracted by lowpass filtering the instantaneous power signal. This method is used to extract the real power information on each phase of the polyphase system. The total real power information is then obtained by adding the individual phase real power. This scheme correctly calculates real power for nonsinusoidal current and voltage waveforms at all power factors. All signal processing is carried out in the digital domain for superior stability over temperature and time. Figure 16 displays the unity power factor condition and a DPF (displacement power factor) = 0.5, or current signal lagging the voltage by 60°, for one phase of the polyphase. Assuming that the voltage and current waveforms are sinusoidal, the real power component of the instantaneous power signal, or the dc term, is given by () ° × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × 60 cos 2 1 V TIME IAP IAN VAP HPF LPF IBP IBN VBP ICP ICN VCP VN DIGITALTO FREQUENCY DIGITALTO FREQUENCY F1 F2 CF INSTANTANEOUS REAL POWER SIGNAL INSTANTANEOUS POWER SIGNAL  p(t) INSTANTANEOUS TOTAL POWER SIGNAL VA × IA + VB × IB + VC ×IC 2 ABS X Σ Σ Σ LPF LPF X X p(t) = i(t) × v(t) WHERE: V × I 2 {1+ cos (2 ωt)} v(t) = V × cos (ωt) i(t) = I × cos (ωt) p(t) = V × I 2 V × I V × I 2 MULTIPLIER MULTIPLIER MULTIPLIER HPF HPF ADC ADC ADC ADC ADC ADC Figure 15. Signal Processing Block Diagram 
