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AD736AR-REEL-7 Folha de dados(PDF) 6 Page - Analog Devices |
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AD736AR-REEL-7 Folha de dados(HTML) 6 Page - Analog Devices |
6 / 8 page AD736 REV. C –6– Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms Waveform Type Crest Factor True rms Value Average Responding % of Reading Error* 1 Volt Peak (VPEAK/V rms) Circuit Calibrated to Using Average Amplitude Read rms Value of Responding Circuit Sine Waves Will Read Undistorted 1.414 0.707 V 0.707 V 0% Sine Wave Symmetrical Square Wave 1.00 1.00 V 1.11 V +11.0% Undistorted Triangle Wave 1.73 0.577 V 0.555 V –3.8% Gaussian Noise (98% of Peaks <1 V) 3 0.333 V 0.295 V –11.4% Rectangular 2 0.5 V 0.278 V –44% Pulse Train 10 0.1 V 0.011 V –89% SCR Waveforms 50% Duty Cycle 2 0.495 V 0.354 V –28% 25% Duty Cycle 4.7 0.212 V 0.150 V –30% *% of Reading Error = Average Responding Value – True rmsValue True rmsValue ×100% TYPES OF AC MEASUREMENT The AD736 is capable of measuring ac signals by operating as either an average responding or a true rms-to-dc converter. As its name implies, an average responding converter computes the average absolute value of an ac (or ac and dc) voltage or current by full wave rectifying and low-pass filtering the input signal; this will approximate the average. The resulting output, a dc “average” level, is then scaled by adding (or reducing) gain; this scale factor converts the dc average reading to an rms equivalent value for the waveform being measured. For example, the aver- age absolute value of a sine-wave voltage is 0.636 that of VPEAK; the corresponding rms value is 0.707 times VPEAK. Therefore, for sine-wave voltages, the required scale factor is 1.11 (0.707 divided by 0.636). In contrast to measuring the “average” value, true rms measure- ment is a “universal language” among waveforms, allowing the magnitudes of all types of voltage (or current) waveforms to be compared to one another and to dc. RMS is a direct measure of the power or heating value of an ac voltage compared to that of dc: an ac signal of 1 volt rms will produce the same amount of heat in a resistor as a 1 volt dc signal. Mathematically, the rms value of a voltage is defined (using a simplified equation) as: V rms = Avg.(V 2) This involves squaring the signal, taking the average, and then obtaining the square root. True rms converters are “smart recti- fiers”: they provide an accurate rms reading regardless of the type of waveform being measured. However, average responding converters can exhibit very high errors when their input signals deviate from their precalibrated waveform; the magnitude of the error will depend upon the type of waveform being measured. As an example, if an average responding converter is calibrated to measure the rms value of sine-wave voltages, and then is used to measure either symmetrical square waves or dc voltages, the converter will have a computational error 11% (of reading) higher than the true rms value (see Table I). AD736 THEORY OF OPERATION As shown by Figure 16, the AD736 has five functional subsec- tions: input amplifier, full-wave rectifier, rms core, output am- plifier and bias sections. The FET input amplifier allows both a high impedance, buffered input (Pin 2) or a low imped- ance, wide-dynamic-range input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. The output of the input amplifier drives a full wave precision rectifier, which in turn, drives the rms core. It is in the core that the essential rms operations of squaring, averaging and square rooting are performed, using an external averaging capacitor, CAV. Without CAV, the rectified input signal travels through the core unprocessed, as is done with the average responding con- nection (Figure 17). A final subsection, an output amplifier, buffers the output from the core and also allows optional low-pass filtering to be per- formed via external capacitor, CF, connected across the feed- back path of the amplifier. In the average responding connection, this is where all of the averaging is carried out. In the rms circuit, this additional filtering stage helps reduce any output ripple which was not removed by the averaging capaci- tor, CAV. Figure 16. AD736 True RMS Circuit |
Nº de peça semelhante - AD736AR-REEL-7 |
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Descrição semelhante - AD736AR-REEL-7 |
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