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AD536AKD Datasheet(PDF) 9 Page  Analog Devices 

AD536AKD Datasheet(HTML) 9 Page  Analog Devices 
9 / 17 page AD536A Data Sheet Rev. E  Page 8 of 16 THEORY OF OPERATION The AD536A embodies an implicit solution of the rms equation that overcomes the dynamic range as well as other limitations inherent in a straightforward computation of rms. The actual computation performed by the AD536A follows the equation = rms V V Avg rms V IN 2 Figure 6 is a simplified schematic of the AD536A. Note that it is subdivided into four major sections: absolute value circuit (active rectifier), squarer/divider, current mirror, and buffer amplifier. The input voltage (VIN), which can be ac or dc, is converted to a unipolar current (I1) by the active rectifiers (A1, A2). I1 drives one input of the squarer/divider, which has the transfer function I4 = II2/I3 The output current, I4, of the squarer/divider drives the current mirror through a lowpass filter formed by R1 and the exter nally connected capacitor, CAV. If the R1 CAV time constant is much greater than the longest period of the input signal, then I4 is effectively averaged. The current mirror returns a current I3, which equals Avg[I4], back to the squarer/divider to complete the implicit rms computation. Thus, I4 = Avg[II2/I4] = II rms 14 +VS –VS I3 I2 I1 IOUT RL VIN VINR–1 ABSOLUTE VALUE; VOLTAGECURRENT CONVERTER ONEQUADRANT SQUARER/DIVIDER CURRENT MIRROR Q1 Q2 Q3 Q4 Q5 COM 4 9 dB OUT 5 BUF OUT 6 3 8 1 BUF IN BUFFER 7 10 0.4mA FS A3 NOTES 1. PINOUTS ARE FOR 14LEAD DIP. 0.2mA FS R1 25kΩ R2 25kΩ 12kΩ 25kΩ 12kΩ R4 50kΩ R3 25kΩ 80kΩ A4 A2 A1 Figure 6. Simplified Schematic The current mirror also produces the output current, IOUT, which equals 2I4. IOUT can be used directly or can be converted to a voltage with R2 and buffered by A4 to provide a low impedance voltage output. The transfer function of the AD536A results in the following: VOUT = 2R2 × I rms = VIN rms The dB output is derived from the emitter of Q3 because the voltage at this point is proportional to –log VIN. The emitter follower, Q5, buffers and level shifts this voltage so that the dB output voltage is zero when the externally supplied emitter current (IREF) to Q5 approximates I3. CONNECTIONS FOR dB OPERATION The logarithmic (or decibel) output of the AD536A is one of its most powerful features. The internal circuit computing dB works accurately over a 60 dB range. The connections for dB measurements are shown in Figure 7. Select the 0 dB level by adjusting R1 for the proper 0 dB reference current (which is set to cancel the log output current from the squarer/divider at the desired 0 dB point). The external op amp provides a more convenient scale and allows compensation of the +0.33%/°C scale factor drift of the dB output pin. The temperaturecompensating resistor, R2, is available online in several styles from Precision Resistor Company, Inc., (Part Number AT35 and Part Number ST35). The average temperature coefficients of R2 and R3 result in the +3300 ppm required to compensate for the dB output. The linear rms output is available at Pin 8 on the DIP or Pin 10 on the header device with an output impedance of 25 kΩ. Some applications require an additional buffer amplifier if this output is desired. For dB calibration, 1. Set VIN = 1.00 V dc or 1.00 V rms. 2. Adjust R1 for dB output = 0.00 V. 3. Set VIN = +0.1 V dc or 0.10 V rms. 4. Adjust R5 for dB output = −2.00 V. Any other desired 0 dB reference level can be used by setting VIN and adjusting R1 accordingly. Note that adjusting R5 for the proper gain automatically provides the correct temperature compensation. 
