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ADE7757ARN Datenblatt(PDF) 11 Page - Analog Devices |
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ADE7757ARN Datenblatt(HTML) 11 Page - Analog Devices |
11 / 14 page –11– PRELIMINARY TECHNICAL DATA REV. PrC. ADE7757 DIGITAL-TO-FREQUENCY CONVERSION As previously described, the digital output of the low-pass filter after multiplication contains the real power information. How- ever, since this LPF is not an ideal “brick wall” filter imple- mentation, the output signal also contains attenuated components at the line frequency and its harmonics, i.e., cos(h ωt) where h = 1, 2, 3, . . . etc. The magnitude response of the filter is given by: 2 2 9 8 1 1 . ) ( f f H + = (5) For a line frequency of 50 Hz this would give an attenua- tion of the 2 ω (100 Hz) component of approximately – 22 dB. The dominating harmonic will be at twice the line frequency (2 ω) due to the instantaneous power calculation. Figure 21 shows the instantaneous real power signal at the output of the LPF which still contains a significant amount of instantaneous power information, i.e., cos (2 ωt). This signal is then passed to the digital-to-frequency converter where it is integrated (accumulated) over time in order to produce an output frequency. The accumulation of the signal will suppress or average out any non-dc components in the instantaneous real power signal. The average value of a sinusoidal signal is zero. Hence the frequency gener- ated by the ADE7757 is proportional to the average real power. Figure 21 shows the digital-to-frequency conver- sion for steady load conditions, i.e., constant voltage and current. ω FREQUENCY (RAD/S) LPF DIGITAL-TO- FREQUENCY F1 F2 DIGITAL-TO- FREQUENCY CF INSTANTANEOUS REAL POWER SIGNAL (FREQUENCY DOMAIN) MULTIPLIER TIME F1 CF TIME V I 0 LPF TO EXTRACT REAL POWER (DC TERM) 2 I V × ∑ ∑ ) t 2 ( cos ω LPF BY ATTENUATED ω 2 Figure 21. Real Power-to-Frequency Conversion As can be seen in the diagram, the frequency output CF is seen to vary over time, even under steady load conditions. This frequency variation is primarily due to the cos (2 ωt) component in the instantaneous real power signal. The output frequency on CF can be up to 2048 times higher than the frequency on F1 and F2. This higher output fre- quency is generated by accumulating the instantaneous real power signal over a much shorter time while convert- ing it to a frequency. This shorter accumulation period means less averaging of the cos (2 ωt) component. Conse- quently, some of this instantaneous power signal passes through the digital-to-frequency conversion. This will not be a problem in the application. Where CF is used for calibration purposes, the frequency should be averaged by the frequency counter which will remove any ripple. If CF is being used to measure energy; for example, in a micro- processor-based application, the CF output should also be averaged to calculate power. Because the outputs F1 and F2 operate at a much lower frequency, a lot more averaging of the instantaneous real power signal is carried out. The result is a greatly attenu- ated sinusoidal content and a virtually ripple-free fre- quency output. Interfacing the ADE7757 to a Microcontroller for Energy Measurement The easiest way to interface the ADE7757 to a microcontroller is to use the CF high frequency output with the output frequency scaling set to 2048 x F1, F2. This is done by setting SCF = 0 and S0 = S1 = 1, see Table III. With full-scale ac signals on the analog inputs, the output frequency on CF will be approximately 2.867 kHz. Figure 22 illustrates one scheme which could be used to digitize the output frequency and carry out the necessary averaging mentioned in the previous section. TIME ±10% AVERAGE FREQUENCY CF FREQUENCY RIPPLE MCU COUNTER TIMER CF ADE7757 Figure 22. Interfacing the ADE7757 to an MCU As shown, the frequency output CF is connected to an MCU counter or port. This will count the number of pulses in a given integration time which is determined by an MCU internal timer. The average power is propor- tional to the average frequency is given by: Time Counter Power Average Frequency Average = = The energy consumed during an integration period is given by: Counter Time Time Counter Time Power Average Energy = × = × = |
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