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AD8310ARM-REEL7 Fiches technique(PDF) 9 Page - Analog Devices |
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AD8310ARM-REEL7 Fiches technique(HTML) 9 Page - Analog Devices |
9 / 24 page AD8310 Rev. D | Page 9 of 24 THEORY OF OPERATION Logarithmic amplifiers perform a more complex operation than classical linear amplifiers, and their circuitry is significantly different. A good grasp of what log amps do and how they do it can help users avoid many pitfalls in their applications. For a complete discussion of the theory, see the AD8307 data sheet. The essential purpose of a log amp is not to amplify (though amplification is needed internally), but to compress a signal of wide dynamic range to its decibel equivalent. It is, therefore, a measurement device. An even better term might be logarithmic converter, because the function is to convert a signal from one domain of representation to another via a precise nonlinear transformation: ⎟⎟ ⎠ ⎞ ⎜⎜ ⎝ ⎛ = X IN Y OUT V V V V log (1) where: VOUT is the output voltage. VY is the slope voltage. The logarithm is usually taken to base ten, in which case VY is also the volts-per-decade. VIN is the input voltage. VX is the intercept voltage. Log amps implicitly require two references (here VX and VY) that determine the scaling of the circuit. The accuracy of a log amp cannot be any better than the accuracy of its scaling references. In the AD8310, these are provided by a band gap reference. VOUT 5VY 4VY 3VY 2VY VY –2VY VOUT =0 LOG VIN VSHIFT LOWER INTERCEPT VIN =10–2VX –40dBc VIN =102VX +40dBc VIN =104VX +80dBc VIN =VX 0dBc Figure 21. General Form of the Logarithmic Function While Equation 1, plotted in Figure 21, is fundamentally correct, a different formula is appropriate for specifying the calibration attributes or demodulating log amps like the AD8310, operating in RF applications with a sine wave input. ( ) O IN SLOPE OUT P P V V − = (2) where: e demodulated and filtered baseband (video or RSSI) o the e in RF systems is dB above 1 mW in 50 Ω, a level of 0 dBm. Note that the quantity (P – P ) is dB. e o ps use a cascade of n as y mpression log amps either provide a baseband he nal e VOUT is th output. VSLOPE is the logarithmic slope, now expressed in V/dB (25 mV/dB for the AD8310). PIN is the input power, expressed in dB relative to some reference power level. PO is the logarithmic intercept, expressed in dB relative t same reference level. A widely used referenc IN O The logarithmic function disappears from the formula, becaus the conversion has already been implicitly performed in stating the input in decibels. This is strictly a concession to popular convention. Log amps manifestly do not respond to power (tacitly, power absorbed at the input), but rather to input voltage. The input is specified in dBV (decibels with respect t 1 V rms) throughout this data sheet. This is more precise, although still incomplete, because the signal waveform is also involved. Many users specify RF signals in terms of power (usually in dBm/50 Ω) and this convention is used in this data sheet when specifying the performance of the AD8310. PROGRESSIVE COMPRESSION High speed, high dynamic-range log am nonlinear amplifier cells to generate the logarithmic functio a series of contiguous segments, a type of piecewise linear technique. The AD8310 employs six cells in its main signal path, each having a small-signal gain of 14.3 dB (×5.2) and a −3 dB bandwidth of about 900 MHz. The overall gain is about 20,000 (86 dB) and the overall bandwidth of the chain is approximatel 500 MHz, resulting in a gain-bandwidth product (GBW) of 10,000 GHz, about a million times that of a typical op amp. This very high GBW is essential to accurate operation under small- signal conditions and at high frequencies. The AD8310 exhibits a logarithmic response down to inputs as small as 40 µV at 440 MHz. Progressive co video response or accept an RF input and demodulate this signal to develop an output that is essentially the envelope of t input represented on a logarithmic or decibel scale. The AD8310 is the latter kind. Demodulation is performed in a total of nine detector cells. Six are associated with the amplifier stages, and three are passive detectors that receive a progres- sively attenuated fraction of the full input. The maximum sig frequency can be 440 MHz, but, because all the gain stages ar dc-coupled, operation at very low frequencies is possible. |
Numéro de pièce similaire - AD8310ARM-REEL7 |
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Description similaire - AD8310ARM-REEL7 |
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