Datasheet AD8310 (Analog Devices) - 9

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AD8310 THEORY OF OPERATION
Logarithmic amplifiers perform a more complex operation than = − OUT V VSLOPE ( IN P O P ) (2) classical linear amplifiers, and their circuitry is significantly different. A good grasp of what log amps do and how they do it where: can help users avoid many pitfalls in their applications. For a VOUT is the demodulated and filtered baseband (video or RSSI) complete discussion of the theory, see the AD8307 data sheet. output. VSLOPE is the logarithmic slope, now expressed in V/dB The essential purpose of a log amp is not to amplify (though (25 mV/dB for the AD8310). amplification is needed internally), but to compress a signal of PIN is the input power, expressed in dB relative to some wide dynamic range to its decibel equivalent. It is, therefore, a reference power level. measurement device. An even better term might be logarithmic PO is the logarithmic intercept, expressed in dB relative to the converter, because the function is to convert a signal from one same reference level. domain of representation to another via a precise nonlinear transformation: A widely used reference in RF systems is dB above 1 mW in 50 Ω, a level of 0 dBm. Note that the quantity (PIN − PO) is dB. ⎛ V ⎞ V = IN The logarithmic function disappears from the formula, because OUT Y V log ⎜⎜ ⎟⎟ (1) ⎝ VX ⎠ the conversion has already been implicitly performed in stating the input in decibels. This is strictly a concession to popular where: convention. Log amps manifestly do not respond to power VOUT is the output voltage. (tacitly, power absorbed at the input), but rather to input VY is the slope voltage. The logarithm is usually taken to voltage. The input is specified in dBV (decibels with respect to base ten, in which case VY is also the volts-per-decade. 1 V rms) throughout this data sheet. This is more precise, VIN is the input voltage. although still incomplete, because the signal waveform is also VX is the intercept voltage. involved. Many users specify RF signals in terms of power (usually in dBm/50 Ω), and this convention is used in this data Log amps implicitly require two references (here VX and VY) sheet when specifying the performance of the AD8310. that determine the scaling of the circuit. The accuracy of a log amp cannot be any better than the accuracy of its scaling
PROGRESSIVE COMPRESSION
references. In the AD8310, these are provided by a band gap High speed, high dynamic-range log amps use a cascade of reference. nonlinear amplifier cells to generate the logarithmic function
VOUT
as a series of contiguous segments, a type of piecewise linear
5VY
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
4VY V
−3 dB bandwidth of about 900 MHz. The overall gain is about
SHIFT 3V
20,000 (86 dB), and the overall bandwidth of the chain is
Y LOWER INTERCEPT
approximately 500 MHz, resulting in a gain-bandwidth product
2VY
(GBW) of 10,000 GHz, about a million times that of a typical op amp. This very high GBW is essential to accurate operation
VY
under small-signal conditions and at high frequencies. The
LOG VIN V
AD8310 exhibits a logarithmic response down to inputs as
OUT = 0 VIN = 10–2VX V V IN = VX IN = 102VX VIN = 104VX
small as 40 μV at 440 MHz.
–40dBc 0dBc +40dBc +80dBc
Progressive compression log amps either provide a baseband
–2VY
video response or accept an RF input and demodulate this 01084-021 signal to develop an output that is essentially the envelope of the Figure 21. General Form of the Logarithmic Function input represented on a logarithmic or decibel scale. The While Equation 1, plotted in Figure 21, is fundamentally AD8310 is the latter kind. Demodulation is performed in a total correct, a different formula is appropriate for specifying the of nine detector cells. Six are associated with the amplifier calibration attributes or demodulating log amps like the stages, and three are passive detectors that receive a progres- AD8310, operating in RF applications with a sine wave input. sively attenuated fraction of the full input. The maximum signal frequency can be 440 MHz, but, because all the gain stages are dc-coupled, operation at very low frequencies is possible. Rev. F | Page 9 of 24 Document Outline FEATURES APPLICATIONS FUNCTIONAL BLOCK DIAGRAM GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATION AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION PROGRESSIVE COMPRESSION SLOPE AND INTERCEPT CALIBRATION OFFSET CONTROL PRODUCT OVERVIEW ENABLE INTERFACE INPUT INTERFACE OFFSET INTERFACE OUTPUT INTERFACE USING THE AD8310 BASIC CONNECTIONS TRANSFER FUNCTION IN TERMS OF SLOPE AND INTERCEPT dBV vs. dBm INPUT MATCHING NARROW-BAND MATCHING GENERAL MATCHING PROCEDURE Step 1: Tune Out CIN Step 2: Calculate CO and LO Step 3: Split CO into Two Parts Step 4: Calculate LM SLOPE AND INTERCEPT ADJUSTMENTS INCREASING THE SLOPE TO A FIXED VALUE OUTPUT FILTERING LOWERING THE HIGH-PASS CORNER FREQUENCY OF THE OFFSET COMPENSATION LOOP APPLICATIONS INFORMATION CABLE-DRIVING DC-COUPLED INPUT EVALUATION BOARD DIE INFORMATION OUTLINE DIMENSIONS ORDERING GUIDE