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AD636JH Folha de dados(PDF) 11 Page - Analog Devices |
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AD636JH Folha de dados(HTML) 11 Page - Analog Devices |
11 / 16 page Data Sheet AD636 Rev. E | Page 11 of 16 slightly more restricted than in the dual supply connection. The load resistor, RL, is necessary to provide current sinking capability. C2 3.3µF AD636 ABSOLUTE VALUE SQUARER DIVIDER 10kΩ 10kΩ CURRENT MIRROR – CAV BUF + 20kΩ NONPOLARIZED 39kΩ 0.1µF 0.1µF +VS VOUT RL 1kΩ TO 10kΩ VIN 1 2 3 4 5 6 7 14 13 12 11 10 9 8 VIN NC –VS CAV dB BUF OUT BUF IN NC NC NC COM RL IOUT NC = NO CONNECT + – Figure 11. Single-Supply Connection (See Text) CHOOSING THE AVERAGING TIME CONSTANT The AD636 computes the rms of both ac and dc signals. If the input is a slowly varying dc voltage, the output of the AD636 tracks the input exactly. At higher frequencies, the average output of the AD636 approaches the rms value of the input signal. The actual output of the AD636 differs from the ideal output by a dc (or average) error and some amount of ripple, as demonstrated in Figure 12. TIME IDEAL EO DC ERROR = EO – EO (IDEAL) AVERAGE EO = EO DOUBLE-FREQUENCY RIPPLE EO Figure 12. Typical Output Waveform for Sinusoidal Input The dc error is dependent on the input signal frequency and the value of CAV. Figure 13 can be used to determine the minimum value of CAV, which yields a given % dc error above a given frequency using the standard rms connection. The ac component of the output signal is the ripple. There are two ways to reduce the ripple. The first method involves using a large value of CAV. Because the ripple is inversely proportional to CAV, a tenfold increase in this capacitance effects a tenfold reduction in ripple. When measuring waveforms with high crest factors (such as low duty cycle pulse trains), the averaging time constant should be at least ten times the signal period. For example, a 100 Hz pulse rate requires a 100 ms time constant, which corresponds to a 4 μF capacitor (time constant = 25 ms per μF). INPUT FREQUENCY (Hz) 100 0.01 1 10 0.1 1 10 100 0.1 0.01 *% dc ERROR + % RIPPLE (PEAK) 1 10 100 1k 10k 100k VALUES FOR CAV AND 1% SETTLING TIME FOR STATED % OF READING AVERAGING ERROR* ACCURACY ±20% DUE TO COMPONENT TOLERANCE Figure 13. 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 increased proportionately. Figure 13 shows 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 for increasing signals (the values in Figure 13 are for decreasing signals). Settling time also increases for low signal levels, as shown in Figure 14. rms INPUT LEVEL 10.0 7.5 0 10mV 100mV 1.0 5.0 2.5 1V 1mV Figure 14. Settling Time vs. Input Level A better method for reducing output ripple is the use of a post- filter. Figure 15 shows a suggested circuit. If a single-pole filter is used (C3 removed, RX shorted), and C2 is approximately 5 times the value of CAV, the ripple is reduced, as shown in Figure 16, and the settling time is increased. For example, with CAV = 1 µF and C2 = 4.7 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. The values of CAV and C2 can therefore be reduced to permit faster settling times while still providing substantial ripple reduction. |
Nº de peça semelhante - AD636JH |
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Descrição semelhante - AD636JH |
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