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AD8311 Datasheet(PDF) 11 Page - Analog Devices |
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AD8311 Datasheet(HTML) 11 Page - Analog Devices |
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11 / 24 page  AD8311 Rev. 0 | Page 11 of 24 THEORY OF OPERATION The AD8311 is a wideband logarithmic amplifier (log amp) similar in design to the AD8313, AD8314, and AD8315. Like the AD8315, it is strictly optimized for use in power control applications rather than as a measurement device. Figure 25 shows the main features in block schematic form. The output (Pin 2, 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 a 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 Z IN SLP OUT V V V V 10 log = (1) where: 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. 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 AD8311, a nominal (low frequency) slope of 24 mV/dB was chosen, and the intercept VZ was placed at −59 dBm for a sine wave input (251 µV rms). However, both the slope and the intercept are dependent on frequency. 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 ( ) Z IN DB OUT P P V V − = (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 in mV/dB. For a log amp having a slope VDB of 24 mV/dB and an intercept at −59 dBm, the output voltage for an input power of – 30 dBm is 0.024 [−30 − (−57)] = 0.696 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 the 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. 10dB INTERCEPT POSITIONING 10dB 10dB 10dB DET DET DET DET DET (CURRENT- NULLING MODE) RFIN (WEAK GM STAGE) (CURRENT-MODE SIGNAL) ⋅ 1.35 HI-Z LOW NOISE RAIL-TO-RAIL BUFFER (CURRENT-MODE FEEDBACK) VSET 23mV/dB 250mV TO 1.4V = 50dB (SMALL INTERNAL FILTER CAPACITOR FOR GHz RIPPLE) FLTR VAPC LOW NOISE BAND GAP REFERENCE LOW NOISE GAIN BIAS COM VPOS (PRECISE GAIN CONTROL) (PRECISE SLOPE CONTROL) V I OFFSET COMPENSATION Figure 25. Block Schematic |
Número de pieza similar - AD8311 |
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Descripción similar - AD8311 |
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