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AD737JRZ5R7 Datasheet(PDF) 12 Page  Analog Devices 

AD737JRZ5R7 Datasheet(HTML) 12 Page  Analog Devices 
12 / 24 page AD737 Data Sheet Rev. I  Page 12 of 24 THEORY OF OPERATION As shown in Figure 23, the AD737 has four functional subsec tions: an input amplifier, a fullwave rectifier, an rms core, and a bias section. The FET input amplifier allows a high impedance, buffered input at Pin 2 or a low impedance, wide dynamic range input at Pin 1. The high impedance input, with its low input bias current, is ideal for use with high impedance input attenuators. The input signal can be either dccoupled or accoupled 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. RMS TRANSLINEAR CORE 8 COM +VS 7 6 OUTPUT 5 CAV CURRENT MODE ABSOLUTE VALUE 1 2 3 POWER DOWN 4 CA 33µF AC CC = 10µF CF 10µF (OPTIONAL LPF) VIN –VS –VS +VS VIN CC + OPTIONAL RETURN PATH 8kΩ + + DC BIAS SECTION FET OP AMP IB < 10pA 8kΩ 0.1µF 0.1µF COMMON POSITIVE SUPPLY NEGATIVE SUPPLY Figure 23. AD737 True RMS Circuit (Test Circuit) The output of the input amplifier drives a fullwave precision rectifier, which, in turn, drives the rms core. It is the core that provides the essential rms operations of squaring, averaging, and square rooting, using an external averaging capacitor, CAV. Without CAV, the rectified input signal passes through the core unprocessed, as is done with the average responding connection (see Figure 25). In the average responding mode, averaging is carried out by an RC post filter consisting of an 8 kΩ internal scale factor resistor connected between Pin 6 and Pin 8 and an external averaging capacitor, CF. In the rms circuit, this addi tional filtering stage reduces any output ripple that was not removed by the averaging capacitor. Finally, the bias subsection permits a powerdown function. This reduces the idle current of the AD737 from 160 µA to 30 µA. This feature is selected by connecting Pin 3 to Pin 7 (+VS). TYPES OF AC MEASUREMENT The AD737 is capable of measuring ac signals by operating as either an average responding converter or a true rmstodc con verter. As its name implies, an average responding converter computes the average absolute value of an ac (or ac and dc) voltage or current by fullwave rectifying and lowpass filtering the input signal; this approximates 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 average 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 a dc voltage; an ac signal of 1 V rms produces the same amount of heat in a resistor as a 1 V dc signal. Mathematically, the rms value of a voltage is defined (using a simplified equation) as ) ( 2 V Avg V rms = 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 depends on 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 has a computational error 11% (of reading) higher than the true rms value (see Table 5). The transfer function for the AD737 is ) ( 2 IN OUT V Avg V = 
