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ADE7768AR-REF Datasheet(PDF) 14 Page - Analog Devices |
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ADE7768AR-REF Datasheet(HTML) 14 Page - Analog Devices |
14 / 20 page ADE7768 Rev. A | Page 14 of 20 CF TIME ±10% FREQUENCY RIPPLE AVERAGE FREQUENCY ADE7768 COUNTER TIMER MCU CF Figure 27. Interfacing the ADE7768 to an MCU As shown in Figure 27, the frequency output CF is connected to an MCU counter or port. This counts the number of pulses in a given integration time, which is determined by an MCU internal timer. The average power proportional to the average frequency is given by Time Counter Power Average Frequency Average = = (8) The energy consumed during an integration period is given by ) 9 ( Counter Time Time Counter Time Power Average Energy = × = × = For the purpose of calibration, this integration time could be 10 seconds to 20 seconds, to accumulate enough pulses to ensure correct averaging of the frequency. In normal operation, the integration time could be reduced to 1 second or 2 seconds, depending, for example, on the required update rate of a display. With shorter integration times on the MCU, the amount of energy in each update may still have some small amount of ripple, even under steady load conditions. However, over a minute or more the measured energy has no ripple. Power Measurement Considerations Calculating and displaying power information always has some associated ripple, which depends on the integration period used in the MCU to determine average power and also on the load. For example, at light loads, the output frequency may be 10 Hz. With an integration period of 2 seconds, only about 20 pulses are counted. The possibility of missing one pulse always exists, because the ADE7768 output frequency is running asynchro- nously to the MCU timer. This results in a 1-in-20 or 5% error in the power measurement. When REVP is logic high, the ADE7768 does not generate any pulse on F1, F2, and CF. INTERNAL OSCILLATOR (OSC) The nominal internal oscillator frequency is 450 kHz when used with RCLKIN, with a nominal value of 6.2 kΩ. The frequency outputs are directly proportional to the oscillator frequency, thus RCLKIN must have low tolerance and low temperature drift to ensure stability and linearity of the chip. The oscillator frequency is inversely proportional to the RCLKIN, as shown in Figure 28. Although the internal oscillator operates when used with RCLKIN values between 5.5 kΩ and 20 kΩ, choosing a value within the range of the nominal value, as shown in Figure 28, is recommended. RESISTANCE (k Ω) 5.8 5.9 6.1 6.3 6.7 420 430 440 450 460 480 470 490 6.0 6.2 6.4 6.5 6.6 410 400 Figure 28. Effect of RCLKIN on Internal Oscillator Frequency (OSC) TRANSFER FUNCTION Frequency Outputs F1 and F2 The ADE7768 calculates the product of two voltage signals (on Channel V1 and Channel V2) and then low-pass filters this product to extract positive-only real power information. This positive-only 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—for example, 0.175 Hz maximum for ac signals with S0 = S1 = 0 (see Table 6). This means that the frequency at these outputs is generated from positive-only real power information accumulated over a relatively long period of time. The result is an output frequency that is proportional to the average positive-only real power. The averaging of the positive- only 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: 2 75 . 494 REF 4 1 rms rms V F V2 V1 Freq − × × × = (10) where: Freq is the output frequency on F1 and F2 (Hz). V1rms is the differential rms voltage signal on Channel V1 (V). V2rms is the differential rms voltage signal on Channel V2 (V). VREF is the reference voltage (2.45 V ± 200 mV) (V). F1–4 are one of four possible frequencies selected by using the S0 and S1 logic inputs (see Table 5). |
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