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ADE7755 Datasheet(PDF) 12 Page  Analog Devices 

ADE7755 Datasheet(HTML) 12 Page  Analog Devices 
12 / 20 page ADE7755 Rev. A  Page 12 of 20 THEORY OF OPERATION The two ADCs of the ADE7755 digitize the voltage signals from the current and voltage transducers. These ADCs are 16bit, secondorder ΣΔ with an oversampling rate of 900 kHz. This analog input structure greatly simplifies transducer interfacing by providing a wide dynamic range for direct connection to the transducer and also by simplifying the antialiasing filter design. A programmable gain stage in the current channel further facilitates easy transducer interfacing. A highpass filter in the current channel removes any dc components from the current signal. This removal eliminates any inaccuracies in the active power calculation due to offsets in the voltage or current signals (see the HPF and Offset Effects section). The active 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. To extract the active power component (that is, the dc component), the instantaneous power signal is lowpass filtered. Figure 22 illustrates the instantaneous active power signal and shows how the active power information can be extracted by lowpass filtering the instantaneous power signal. This scheme correctly calculates active 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. TIME ADC PGA ADC CH1 CH2 MULTIPLIER F1 F2 DIGITALTO FREQUENCY CF DIGITALTO FREQUENCY INSTANTANEOUS ACTIVE POWER SIGNAL INSTANTANEOUS POWER SIGNAL {p(t)} LPF HPF V × I 2 V × I 2 V × I p(t) = i(t) × v(t) WHERE: v(t) = V × cos(ωt) i(t) = I × cos(ωt) p(t) = V × I {1+cos (2ωt)} 2 Figure 22. Signal Processing Block Diagram The low frequency output of the ADE7755 is generated by accumulating this active power information. This low frequency inherently means a long accumulation time between output pulses. The output frequency is therefore proportional to the average active power. This average active power information can, in turn, be accumulated (for example, by a counter) to generate active energy information. Because of its high output frequency and shorter integration time, the calibration frequency (CF) output is proportional to the instantaneous active power. This is useful for system calibration purposes that take place under steady load conditions. POWER FACTOR CONSIDERATIONS The method used to extract the active power information from the instantaneous power signal (that is, by lowpass filtering) is valid even when the voltage and current signals are not in phase. Figure 23 displays the unity power factor condition and a displacement power factor (DPF) = 0.5, that is, current signal lagging the voltage by 60°. Assuming that the voltage and current waveforms are sinusoidal, the active power component of the instantaneous power signal (that is, the dc term) is given by () ° × ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ × 60 cos 2 I V This is the correct active power calculation. V× I 2 0V CURRENT VOLTAGE VOLTAGE CURRENT V× I 2 cos(60°) 0V INSTANTANEOUS POWER SIGNAL INSTANTANEOUS ACTIVE POWER SIGNAL INSTANTANEOUS POWER SIGNAL INSTANTANEOUS ACTIVE POWER SIGNAL 60° Figure 23. DC Component of Instantaneous Power Signal Conveys Active Power Information PF < 1 
