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5962-89805012A Datasheet(PDF) 14 Page - Analog Devices |
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5962-89805012A Datasheet(HTML) 14 Page - Analog Devices |
14 / 17 page ![]() Data Sheet AD536A Rev. E | Page 13 of 16 The primary disadvantage in using a large CAV to remove ripple is that the settling time for a step change in input level is increased proportionately. Figure 19 illustrates that the relationship between CAV and 1% settling time is 115 ms for each microfarad of CAV. The settling time is twice as great for decreasing signals as it is for increasing signals. The values in Figure 19 are for decreasing signals. Settling time also increases for low signal levels, as shown in Figure 20. 10 100 1k 10k 0.1 1 10 100 0.01 1 100k INPUT FREQUENCY (Hz) 0.1 1 10 100 0.01 1PERCENT DC ERROR AND PERCENT RIPPLE (PEAK) VALUES FOR CAV AND 1% SETTLING TIME FOR STATED % OF READING AVERAGING ERROR1 ACCURACY ± 20% DUE TO COMPONENT TOLERANCE Figure 19. Error/Settling Time Graph for Use with the Standard RMS Connection (See Figure 13 Through Figure 15) 10m 100m 1 7.5 10.0 5.0 1m 10 rms INPUT LEVEL (V) 1.0 2.5 Figure 20. Settling Time vs. Input Level A better method to reduce output ripple is the use of a postfilter. Figure 21 shows a suggested circuit. If a single-pole filter is used (C3 removed, RX shorted) and C2 is approximately twice the value of CAV, the ripple is reduced, as shown in Figure 22, and settling time is increased. For example, with CAV = 1 µF and C2 = 2.2 μF, the ripple for a 60 Hz input is reduced from 10% of reading to approximately 0.3% of reading. The settling time, however, is increased by approximately a factor of 3. Therefore, the values of CAV and C2 can be reduced to permit faster settling times while still providing substantial ripple reduction. The two-pole postfilter uses an active filter stage to provide even greater ripple reduction without substantially increasing the settling times over a circuit with a one-pole filter. The values of CAV, C2, and C3 can then be reduced to allow extremely fast settling times for a constant amount of ripple. Caution should be exercised in choosing the value of CAV, because the dc error is dependent on this value and is independent of the postfilter. For a more detailed explanation of these topics, refer to the RMS to DC Conversion Application Guide, 2nd Edition, available online from Analog Devices, Inc., at www.analog.com. C2 VIN CAV +VS 14 13 12 11 10 9 8 1 2 3 4 5 6 7 AD536A 25kΩ ABSOLUTE VALUE SQUARER/ DIVIDER CURRENT MIRROR –VS Rx 24kΩ + – C31 Vrms OUT 1FOR SINGLE POLE, SHORT Rx, REMOVE C3. VIN NC –VS CAV +VS NC NC NC dB COM BUF OUT RL BUF IN IOUT BUF Figure 21. Two-Pole Postfilter 1 1k 100 10k 0.1 10 10 PEAK-TO-PEAK RIPPLE CAV = 1µF DC ERROR CAV = 1µF (ALL FILTERS) PEAK-TO-PEAK RIPPLE CAV = 1µF C2 = C3 = 2.2µF (TWO-POLE) Rx = 0Ω PEAK-TO-PEAK RIPPLE (ONE POLE) CAV = 1µF, C2 = 2.2µF FREQUENCY (Hz) Figure 22. Performance Features of Various Filter Types (See Figure 13 to Figure 15 for Standard RMS Connection) |
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