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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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AD8436JCPZ-RL Datenblatt(PDF) 10 Page - Analog Devices |
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AD8436JCPZ-RL Datenblatt(HTML) 10 Page - Analog Devices |
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10 / 20 page  AD8436 Rev. 0 | Page 10 of 20 THEORY OF OPERATION OVERVIEW The AD8436 is an implicit function rms-to-dc converter that renders a dc voltage dependent on the rms (heating value) of an ac voltage. In addition to the basic converter, this highly integrated functional circuit block includes two fully independent, optional amplifiers, a standalone FET input buffer amplifier and a precision dc output buffer amplifier (see Figure 1). The rms core includes a precision current responding full-wave rectifier and a log-antilog transistor array for current squaring and square rooting to imple- ment the classic expression for rms (see Equation 1). For basic applications, the converter requires only an external capacitor, for averaging (see Figure 30). The optional on-board amplifiers offer utility and flexibility in a variety of applications without incurring additional circuit board footprint. For lowest power, the amplifier supply pins are left unconnected. Why RMS? The rms value of an ac voltage waveform is equal to the dc voltage providing the same heating power to a load. A common measure- ment technique for ac waveforms is to rectify the signal in a straightforward way using a diode array of some sort, resulting in the average value. The average value of various waveforms (sine, square, and triangular, for example) varies widely; true rms is the only metric that achieves equivalency for all ac waveforms. See Table 5 for non-rms-responding circuit errors. The acronym “rms” means root-mean-square and reads as follows: “the square root of the average of the sum of the squares” of the peak values of any waveform. RMS is shown in the following equation: (1) For additional information, select Section I of the 2nd edition of the Analog Devices RMS-to-DC Applications Guide. RMS Core The core consists of a voltage-to-current converter (precision resistor), absolute value, and translinear sections. The translinear section exploits the properties of the bipolar transistor junctions for squaring and root extraction (see Figure 23). The external capacitor (CAVG) provides for averaging the product. Figure 19 shows that there is no effect of signal input on the transition times, as seen in the dc output. Although the rms core responds to input voltages, the conversion process is current sensitive. If the rms input is ac-coupled, as recommended, there is no output offset voltage, as reflected in Table 1. If the rms input is dc-coupled, the input offset voltage is reflected in the output and can be calibrated as with any fixed error. V– AC IN V+ OUT + V+ 5kΩ CAVG ABSOLUTE VALUE CIRCUIT V-TO-I 16kΩ Figure 23. RMS Core Block Diagram Table 5. General AC Parameters Waveform Type (1 V p-p) Crest Factor RMS Value Reading of an Average Value Circuit Calibrated to an RMS Sine Wave Error (%) Sine 1.414 0.707 0.707 0 Square 1.00 1.00 1.11 11.0 Triangle 1.73 0.577 0.555 −3.8 Noise 3 0.333 0.295 Rectangular 2 0.5 0.278 −11.4 Pulse 10 0.1 0.011 −44 SCR −89 DC = 50% 2 0.495 0.354 −28 DC = 25% 4.7 0.212 0.150 −30 |
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