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AD8315 Fiches technique(PDF) 8 Page - Analog Devices |
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AD8315 Fiches technique(HTML) 8 Page - Analog Devices |
8 / 20 page REV. B AD8315 –8– AVERAGE = 16 SAMPLES VAPC VS AND VENBL 2 s PER HORIZONTAL DIVISION 1V PER VERTICAL DIVISION 200mV PER VERTICAL DIVISION GND GND TPC 25. Power-On and -Off Response with VSET Grounded NC = NO CONNECT RFIN ENBL VSET VPOS VAPC NC COMM FLTR AD8315 1 2 3 4 5 6 7 8 220pF 52.3 TEK TDS694C SCOPE TRIG STANFORD DS345 PULSE GENERATOR PULSE OUT TRIG OUT R AND S SMT03 SIGNAL GENERATOR RF OUT 10MHz REF OUTPUT EXT TRIG TEK P6205 FET PROBE AD811 732 49.9 TEK P6205 FET PROBE TPC 26. Test Setup for Power-On and -Off Response with VSET Grounded GND GND VAPC 1V PER VERTICAL DIVISION 500mV PER VERTICAL DIVISION 2 s PER HORIZONTAL DIVISION VS AVERAGE = 16 SAMPLES TPC 27. Power-On and -Off Response with VSET and ENBL Grounded NC = NO CONNECT RFIN ENBL VSET VPOS VAPC NC COMM FLTR AD8315 1 2 3 4 5 6 7 8 220pF 52.3 TEK TDS694C SCOPE TRIG STANFORD DS345 PULSE GENERATOR PULSE OUT TRIG OUT R AND S SMT03 SIGNAL GENERATOR RF OUT 10MHz REF OUTPUT EXT TRIG TEK P6205 FET PROBE AD811 732 49.9 TEK P6205 FET PROBE TPC 28. Test Setup for Power-On and -Off Response with VSET and ENBL Grounded GENERAL DESCRIPTION AND THEORY The AD8315 is a wideband logarithmic amplifier (log amp) similar in design to the AD8313 and AD8314. However, it is strictly optimized for use in power control applications rather than as a measurement device. Figure 1 shows the main features in block schematic form. The output (Pin 7, VAPC) is intended to be applied directly to the automatic power-control (APC) pin of a power amplifier module. Basic Theory Logarithmic amplifiers provide a type of compression in which a signal having a large range of amplitudes is converted to one of smaller range. The use of the logarithmic function uniquely results in the output representing the decibel value of the input. The fundamental mathematical form is: VV V V OUT SLP IN Z = log10 (1) Here VIN is the input voltage, VZ is called the intercept (voltage) because when VIN = VZ the argument of the logarithm is unity and thus the result is zero, and VSLP is called the slope (voltage), which is the amount by which the output changes for a certain change in the ratio (VIN/VZ). When BASE-10 logarithms are used, denoted by the function log10, VSLP represents the “volts/decade,” and since a decade corresponds to 20 dB, VSLP/20 represents the “volts/dB.” For the AD8315, a nominal (low frequency) slope of 24 mV/dB was chosen, and the intercept VZ was placed at the equivalent of –70 dBV for a sine wave input (316 mV rms). This corresponds to a power level of –57 dBm when the net resistive part of the input impedance of the log amp is 50 W. However, both the slope and the intercept are dependent on frequency (see TPC 13 and TPC 16). Keeping in mind that log amps do not respond to power but only to voltages and that the calibration of the intercept is waveform dependent and is only quoted for a sine wave signal, the equivalent power response can be written as: VV P P OUT DB IN Z = (– ) (2) where the input power PIN and the equivalent intercept PZ are both expressed in dBm (thus, the quantity in parentheses is simply a number of decibels), and VDB is the slope expressed as so many mV/dB. For a log amp having a slope VDB of 24 mV/dB and an intercept at –57 dBm, the output voltage for an input power of –30 dBm is 0.024 [–30 – (–57)] = 0.648 V. Further details about the structure and function of log amps can be found in data sheets for other log amps produced by Analog Devices. Refer to data sheets for the AD640 and AD8307, both of which include a detailed discussion of the basic principles of operation and explain why the intercept depends on waveform, an important consideration when complex modulation is imposed on an RF carrier. |
Numéro de pièce similaire - AD8315 |
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Description similaire - AD8315 |
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