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AD736ARZ-RL 데이터시트(PDF) 11 Page - Analog Devices |
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AD736ARZ-RL 데이터시트(HTML) 11 Page - Analog Devices |
11 / 20 page Data Sheet AD736 Rev. I | Page 11 of 20 Mathematically, the rms value of a voltage is defined (using a simplified equation) as ( )2 rms V Avg V = This involves squaring the signal, taking the average, and then obtaining the square root. True rms converters are smart rectifiers; 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 depends on the type of waveform being measured. For 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 has a computational error 11% (of reading) higher than the true rms value (see Table 5). CALCULATING SETTLING TIME USING FIGURE 16 Figure 16 can be used to closely approximate the time required for the AD736 to settle when its input level is reduced in amplitude. The net time required for the rms converter to settle is the difference between two times extracted from the graph (the initial time minus the final settling time). As an example, consider the following conditions: a 33 µF averaging capacitor, a 100 mV initial rms input level, and a final (reduced) 1 mV input level. From Figure 16, the initial settling time (where the 100 mV line intersects the 33 µF line) is approximately 80 ms. The settling time corresponding to the new or final input level of 1 mV is approximately 8 seconds. Therefore, the net time for the circuit to settle to its new value is 8 seconds minus 80 ms, which is 7.92 seconds. Note that because of the smooth decay characteristic inherent with a capacitor/diode combination, this is the total settling time to the final value (that is, not the settling time to 1%, 0.1%, and so on, of the final value). In addition, this graph provides the worst-case settling time because the AD736 settles very quickly with increasing input levels. RMS MEASUREMENT—CHOOSING THE OPTIMUM VALUE FOR CAV Because the external averaging capacitor, CAV, holds the rectified input signal during rms computation, its value directly affects the accuracy of the rms measurement, especially at low frequencies. Furthermore, because the averaging capacitor appears across a diode in the rms core, the averaging time constant increases exponentially as the input signal is reduced. This means that as the input level decreases, errors due to nonideal averaging decrease, and the time required for the circuit to settle to the new rms level increases. Therefore, lower input levels allow the circuit to perform better (due to increased averaging) but increase the waiting time between measurements. Obviously, when selecting CAV, a trade-off between computational accuracy and settling time is required. Table 5. Error Introduced by an Average Responding Circuit when Measuring Common Waveforms Waveform Type 1 V Peak Amplitude Crest Factor (VPEAK/V rms) True RMS Value (V) Average Responding Circuit Calibrated to Read RMS Value of Sine Waves (V) % of Reading Error Using Average Responding Circuit Undistorted Sine Wave 1.414 0.707 0.707 0 Symmetrical Square Wave 1.00 1.00 1.11 +11.0 Undistorted Triangle Wave 1.73 0.577 0.555 −3.8 Gaussian Noise (98% of Peaks <1 V) 3 0.333 0.295 −11.4 Rectangular 2 0.5 0.278 −44 Pulse Train 10 0.1 0.011 −89 SCR Waveforms 50% Duty Cycle 2 0.495 0.354 −28 25% Duty Cycle 4.7 0.212 0.150 −30 |
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