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AD636 데이터시트(PDF) 5 Page - Analog Devices |
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AD636 데이터시트(HTML) 5 Page - Analog Devices |
5 / 8 page AD636 REV. B –5– INPUT FREQUENCY – Hz 100 0.01 1 100k 1.0 10 100 1k 10k 10 0.1 1.0 10 100 0.1 0.01 VALUES FOR CAV AND 1% SETTLING TIME FOR STATED % OF READING AVERAGING ERROR* ACCURACY 20% DUE TO COMPONENT TOLERANCE *% dc ERROR + % RIPPLE (PEAK) Figure 5. Error/Settling Time Graph for Use with the Standard rms Connection The primary disadvantage in using a large CAV to remove ripple is that the settling time for a step change in input level is in- creased proportionately. Figure 5 shows the relationship be- tween CAV and 1% settling time is 115 milliseconds for each microfarad of CAV. The settling time is twice as great for de- creasing signals as for increasing signals (the values in Figure 5 are for decreasing signals). Settling time also increases for low signal levels, as shown in Figure 6. rms INPUT LEVEL 10.0 7.5 0 1mV 1V 10mV 100mV 1.0 5.0 2.5 Figure 6. Settling Time vs. Input Level A better method for reducing output ripple is the use of a “post-filter.” Figure 7 shows a suggested circuit. If a single pole filter is used (C3 removed, RX shorted), and C2 is approxi- mately 5 times the value of CAV, the ripple is reduced as shown in Figure 8, and settling time is increased. For example, with CAV = 1 µF and C2 = 4.7 µF, the ripple for a 60 Hz input is re- duced from 10% of reading to approximately 0.3% of reading. The settling time, however, is increased by approximately a factor of 3. The values of CAV and C2 can therefore be reduced to permit faster settling times while still providing substantial ripple reduction. The two-pole post-filter 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, since the dc error is dependent upon this value and is independent of the post filter. For a more detailed explanation of these topics refer to the RMS-to-DC Conversion Application Guide, 2nd Edition, available from Analog Devices. 1 2 3 4 5 6 7 AD636 14 13 12 11 10 9 8 ABSOLUTE VALUE SQUARER DIVIDER BUF 10k 10k CURRENT MIRROR Vrms OUT +VS VIN –VS CAV + – + – C2 C3 (FOR SINGLE POLE, SHORT Rx, REMOVE C3) Rx 10k Figure 7. 2 Pole ‘’Post’’ Filter FREQUENCY – Hz 10 0.1 10 10k 1 1k 100 p-p RIPPLE CAV = 1 F (FIG 1) p-p RIPPLE (ONE POLE) CAV = 1 F C2 = 4.7 F DC ERROR CAV = 1 F (ALL FILTERS) p-p RIPPLE (TWO POLE) CAV = 1 F, C2 = C3 = 4.7 F Figure 8. Performance Features of Various Filter Types RMS MEASUREMENTS AD636 PRINCIPLE OF OPERATION The AD636 embodies an implicit solution of the rms equation that overcomes the dynamic range as well as other limitations inherent in a straightforward computation of rms. The actual computation performed by the AD636 follows the equation: V rms = Avg. VIN 2 V rms Figure 9 is a simplified schematic of the AD636; it is subdivided into four major sections: absolute value circuit (active rectifier), squarer/divider, current mirror, and buffer amplifier. The input voltage, VIN, which can be ac or dc, is converted to a unipolar current I1, by the active rectifier A1, A2. I1 drives one input of the squarer/divider, which has the transfer function: I4 = I1 2 I3 The output current, I4, of the squarer/divider drives the current mirror through a low-pass filter formed by R1 and the externally connected capacitor, CAV. If the R1, CAV time constant is much greater than the longest period of the input signal, then I4 is effectively averaged. The current mirror returns a current I3, which equals Avg. [I4], back to the squarer/divider to complete the implicit rms computation. Thus: I4 = Avg. I1 2 I4 = I 1 rms |
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