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AD737BQ Datasheet(PDF) 6 Page - Analog Devices |
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AD737BQ Datasheet(HTML) 6 Page - Analog Devices |
6 / 8 page AD737 REV. C –6– 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% input (Pin 1). The high impedance input, with its low input bias current, is well suited for use with high impedance input attenuators. The input signal may be either dc or ac coupled to the input amplifier. Unlike other rms converters, the AD737 permits both direct and indirect ac coupling of the inputs. AC coupling is provided by placing a series capacitor between the input signal and Pin 2 (or Pin 1) for direct coupling and between Pin 1 and ground (while driving Pin 2) for indirect coupling. 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, the bias section, permits a “power down” function. This reduces the idle current of the AD737 from 160 µA down to a mere 30 µA. This feature is selected by tying Pin 3 to the +VS terminal. In the average responding connection, all of the averaging is carried out by an RC post filter consisting of an 8 k Ω internal scale-factor resistor connected between Pins 6 and 8 and an external averaging capacitor, CF. In the rms cir- cuit, this additional filtering stage helps reduce any output ripple which was not removed by the averaging capacitor, CAV. 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 con- stant 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 cir- cuit to settle to the new rms level will increase. Therefore, lower input levels allow the circuit to perform better (due to increased averaging) but increase the waiting time between measure- ments. Obviously, when selecting CAV, a trade-off between computational accuracy and settling time is required. 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 de voltages, the converter will have a computational error 11% (of reading) higher than the true rms value (see Table I). AD737 THEORY OF OPERATION As shown by Figure 16, the AD737 has four functional subsec- tions: input amplifier, full-wave rectifier, rms core and bias sec- tions. The FET input amplifier allows both a high impedance, buffered input (Pin 2) or a low impedance, wide-dynamic-range FULL WAVE RECTIFIER INPUT AMPLIFIER RMS CORE 8k AD737 BIAS SECTION 8k 8 7 6 5 1 2 3 4 COM +VS OUTPUT CAV –VS POWER DOWN VIN CC 0.1 F 0.1 F –VS +VS POSITIVE SUPPLY COMMON NEGATIVE SUPPLY CAV 33 F CF 10 F (OPTIONAL) CC 10 F (OPTIONAL VOUT Figure 16. AD737 True RMS Circuit Table I. Error Introduced by an Average Responding Circuit When Measuring Common Waveforms |
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