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AD637BR Scheda tecnica(PDF) 4 Page - Analog Devices |
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AD637BR Scheda tecnica(HTML) 4 Page - Analog Devices |
4 / 10 page AD637 REV. E –4– the AD637 can be ac coupled through the addition of a non- polar capacitor in series with the input as shown in Figure 2. BUFFER AD637 ABSOLUTE VALUE SQUARER/DIVIDER BIAS SECTION FILTER 25k 25k 1 2 3 4 5 6 7 14 13 12 11 10 9 8 CAV –VS +VS NC VIN NC OPTIONAL AC COUPLING CAPACITOR VO = VIN3 Figure 2. Standard RMS Connection The performance of the AD637 is tolerant of minor variations in the power supply voltages, however, if the supplies being used exhibit a considerable amount of high frequency ripple it is advisable to bypass both supplies to ground through a 0.1 µF ceramic disc capacitor placed as close to the device as possible. The output signal range of the AD637 is a function of the sup- ply voltages, as shown in Figure 3. The output signal can be used buffered or nonbuffered depending on the characteristics of the load. If no buffer is needed, tie buffer input (Pin 1) to common. The output of the AD637 is capable of driving 5 mA into a 2 k Ω load without degrading the accuracy of the device. SUPPLY VOLTAGE – DUAL SUPPLY – Volts 20 15 0 0 18 5 10 10 5 15 3 Figure 3. AD637 Max VOUT vs. Supply Voltage CHIP SELECT The AD637 includes a chip select feature which allows the user to decrease the quiescent current of the device from 2.2 mA to 350 µA. This is done by driving the CS, Pin 5, to below 0.2 V dc. Under these conditions, the output will go into a high im- pedance state. In addition to lowering power consumption, this feature permits bussing the outputs of a number of AD637s to form a wide bandwidth rms multiplexer. If the chip select is not being used, Pin 5 should be tied high. FUNCTIONAL DESCRIPTION The AD637 embodies an implicit solution of the rms equation that overcomes the inherent limitations of straightforward rms computation. The actual computation performed by the AD637 follows the equation V rms = Avg V IN 2 V rms Figure 1 is a simplified schematic of the AD637, it is subdivided into four major sections; absolute value circuit (active rectifier), square/divider, filter circuit and buffer amplifier. The input volt- age 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 of the squarer/divider, I4 drives A4 which forms a low-pass filter with the external averaging capacitor. If the RC time constant of the filter is much greater than the long- est period of the input signal than A4s output will be propor- tional to the average of I4. The output of this filter amplifier is used by A3 to provide the denominator current I3 which equals Avg. I4 and is returned to the squarer/divider to complete the implicit rms computation. I4 = Avg I1 2 I4 = I 1 rms and VOUT = VIN rms If the averaging capacitor is omitted, the AD637 will compute the absolute value of the input signal. A nominal 5 pF capacitor should be used to insure stability. The circuit operates identically to that of the rms configuration except that I3 is now equal to I4 giving I4 = I1 2 I4 I4 = I1 The denominator current can also be supplied externally by pro- viding a reference voltage, VREF, to Pin 6. The circuit operates identically to the rms case except that I3 is now proportional to VREF. Thus: I4 = Avg I1 2 I3 and VO = V IN 2 V DEN This is the mean square of the input signal. STANDARD CONNECTION The AD637 is simple to connect for a majority of rms measure- ments. In the standard rms connection shown in Figure 2, only a single external capacitor is required to set the averaging time constant. In this configuration, the AD637 will compute the true rms of any input signal. An averaging error, the magnitude of which will be dependent on the value of the averaging capaci- tor, will be present at low frequencies. For example, if the filter capacitor CAV, is 4 µF this error will be 0.1% at 10 Hz and in- creases to 1% at 3 Hz. If it is desired to measure only ac signals, |
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Descrizione simile - AD637BR |
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