Datasheet AD8314 (Analog Devices) - 10

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AD8314 Data Sheet THEORY OF OPERATION
The AD8314 is a logarithmic amplifier (log amp) similar in However, in using this part, it must be understood that log design to the AD8313; further details about the structure and amps do not fundamentally respond to power. It is for this function can be found in the AD8313 data sheet and other log reason the dBV is used (decibels above 1 V rms) rather than the amps produced by ADI. Figure 28 shows the main features of commonly used metric of dBm. While the dBV scaling is fixed, the AD8314 in block schematic form. independent of termination impedance, the corresponding The AD8314 combines two key functions needed for the power level is not. For example, 224 mV rms is always −13 dBV measurement of signal level over a moderately wide dynamic (with one further condition of an assumed sinusoidal waveform; range. First, it provides the amplification needed to respond to see the Applications section for more information on the effect small signals, in a chain of four amplifier/limiter cells, each of waveform on logarithmic intercept), and it corresponds to a having a small signal gain of 10 dB and a bandwidth of power of 0 dBm when the net impedance at the input is 50 Ω. approximately 3.5 GHz. At the output of each of these amplifier When this impedance is altered to 200 Ω, the same voltage stages is a full-wave rectifier, essentially a square-law detector clearly represents a power level that is four times smaller cell, that converts the RF signal voltages to a fluctuating current (P = V2/R), that is, −6 dBm. Note that dBV can be converted to having an average value that increases with signal level. A dBm for the special case of a 50 Ω system by simply adding further passive detector stage is added prior to the first stage. 13 dB (0 dBV is equivalent to +13 dBm). Therefore, there are five detectors, each separated by 10 dB, Therefore, the external termination added prior to the AD8314 spanning some 50 dB of dynamic range. The overall accuracy at determines the effective power scaling. This often takes the the extremes of this total range, viewed as the deviation from an form of a simple resistor (52.3 Ω provides a net 50 Ω input), ideal logarithmic response, that is, the law-conformance error, but more elaborate matching networks can be used. This can be judged by reference to Figure 7, which shows that errors impedance determines the logarithmic intercept, the input across the central 40 dB are moderate. Figure 5, Figure 6, Figure 8 power for which the output would cross the baseline (V_UP = through Figure 11, Figure 13, and Figure 14 show how the zero) if the function were continuous for al values of input. conformance to an ideal logarithmic function varies with Because this is never the case for a practical log amp, the supply voltage, temperature, and frequency. intercept refers to the value obtained by the minimum-error The output of these detector cells is in the form of a differential straight-line fit to the actual graph of V_UP vs. PIN (more current, making their summation a simple matter. It can easily generally, VIN). Again, keep in mind that the quoted values be shown that such summation closely approximates a logarithmic assume a sinusoidal (CW) signal. Where there is complex function. This result is then converted to a voltage, at Pin V_UP, modulation, as in CDMA, the calibration of the power response through a high-gain stage. In measurement modes, this output needs to be adjusted accordingly. Where a true power (waveform- is connected back to a voltage-to-current (V-I) stage, in such a independent) response is needed, the use of an rms-responding manner that V_UP is a logarithmic measure of the RF input detector, such as the AD8361, should be considered. voltage, with a slope and intercept controlled by the design. For a fixed termination resistance at the input of the AD8314, a given voltage corresponds to a certain power level.
FLTR V-I VSET – + I-V V_UP DET DET DET DET DET – X2 V_DN RFIN + 10dB 10dB 10dB 10dB VPOS BAND GAP OFFSET AD8314 REFERENCE ENBL COMPENSATION
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COMM (PADDLE)
01086- Figure 28. Block Schematic Rev. C | Page 10 of 20 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION FUNCTIONAL BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION INVERTED OUTPUT APPLICATIONS BASIC CONNECTIONS TRANSFER FUNCTION IN TERMS OF SLOPE AND INTERCEPT dBV VS. dBm FILTER CAPACITOR OPERATING IN CONTROLLER MODE POWER-ON AND ENABLE GLITCH INPUT COUPLING OPTIONS INCREASING THE LOGARITHMIC SLOPE IN MEASUREMENT MODE EFFECT OF WAVEFORM TYPE ON INTERCEPT MOBILE HANDSET POWER CONTROL EXAMPLES OPERATION AT 2.7 GHz USING THE LFCSP PACKAGE EVALUATION BOARD OUTLINE DIMENSIONS ORDERING GUIDE