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AD736AR データシート(PDF) 7 Page - Analog Devices |
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AD736AR データシート(HTML) 7 Page - Analog Devices |
7 / 8 page AD736 REV. C –7– As shown, the dc error is the difference between the average of the output signal (when all the ripple in the output has been removed by external filtering) and the ideal dc output. The dc error component is therefore set solely by the value of averaging capacitor used-no amount of post filtering (i.e., using a very large CF) will allow the output voltage to equal its ideal value. The ac error component, an output ripple, may be easily re- moved by using a large enough post filtering capacitor, CF. In most cases, the combined magnitudes of both the dc and ac error components need to be considered when selecting appro- priate values for capacitors CAV and CF. This combined error, representing the maximum uncertainty of the measurement is termed the “averaging error” and is equal to the peak value of the output ripple plus the dc error. As the input frequency increases, both error components de- crease rapidly: if the input frequency doubles, the dc error and ripple reduce to 1/4 and 1/2 their original values, respectively, and rapidly become insignificant. AC MEASUREMENT ACCURACY AND CREST FACTOR The crest factor of the input waveform is often overlooked when determining the accuracy of an ac measurement. Crest factor is defined as the ratio of the peak signal amplitude to the rms am- plitude (C.F. = VPEAK/V rms). Many common waveforms, such as sine and triangle waves, have relatively low crest factors ( ≤2). Other waveforms, such as low duty cycle pulse trains and SCR waveforms, have high crest factors. These types of waveforms require a long averaging time constant (to average out the long time periods between pulses). Figure 6 shows the additional error vs. crest factor of the AD736 for various values of CAV. SELECTING PRACTICAL VALUES FOR INPUT COUPLING (CC), AVERAGING (CAV) AND FILTERING (CF) CAPACITORS Table II provides practical values of CAV and CF for several common applications. Table II. AD737 Capacitor Selection Chart Application rms Low Max CAV CF Settling Input Frequency Crest Time* Level Cutoff Factor to 1% (–3dB) General Purpose 0–1 V 20 Hz 5 150 µF 10 µF 360 ms rms Computation 200 Hz 5 15 µF1 µF 36 ms 0–200 mV 20 Hz 5 33 µF 10 µF 360 ms 200 Hz 5 3.3 µF1 µF 36 ms General Purpose 0–1 V 20 Hz None 33 µF 1.2 sec Average 200 Hz None 3.3 µF 120 ms Responding 0–200 mV 20 Hz None 33 µF 1.2 sec 200 Hz None 3.3 µF 120 ms SCR Waveform 0–200 mV 50 Hz 5 100 µF 33 µF 1.2 sec Measurement 60 Hz 5 82 µF 27 µF 1.0 sec 0–100 mV 50 Hz 5 50 µF 33 µF 1.2 sec 60 Hz 5 47 µF 27 µF 1.0 sec Audio Applications Speech 0–200 mV 300 Hz 3 1.5 µF 0.5 µF 18 ms Music 0–100 mV 20 Hz 10 100 µF 68 µF 2.4 sec *Settling time is specified over the stated rms input level with the input signal increasing from zero. Settling times will be greater for decreasing amplitude input signals. RMS MEASUREMENT – CHOOSING THE OPTIMUM VALUE FOR CAV Since the external averaging capacitor, CAV, “holds” the recti- fied input signal during rms computation, its value directly af- fects the accuracy of the rms measurement, especially at low frequencies. Furthermore, because the averaging capacitor ap- pears across a diode in the rms core, the averaging time constant will increase exponentially as the input signal is reduced. This means that as the input level decreases, errors due to nonideal averaging will reduce while the time it takes for the circuit to settle to the new rms level will increase. Therefore, lower input levels allow the circuit to perform better (due to increased aver- aging) but increase the waiting time between measurements. Obviously, when selecting CAV, a trade-off between computa- tional accuracy and settling time is required. Figure 17. AD736 Average Responding Circuit RAPID SETTLING TIMES VIA THE AVERAGE RESPONDING CONNECTION (FIGURE 17) Because the average responding connection does not use the CAV averaging capacitor, its settling time does not vary with in- put signal level; it is determined solely by the RC time constant of CF and the internal 8 k Ω resistor in the output amplifier’s feedback path. DC ERROR, OUTPUT RIPPLE, AND AVERAGING ERROR Figure 18 shows the typical output waveform of the AD736 with a sine-wave input applied. As with all real-world devices, the ideal output of VOUT = VIN is never exactly achieved; instead, the output contains both a dc and an ac error component. Figure 18. Output Waveform for Sine-Wave Input Voltage |
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