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AD8302 Datasheet(Fiches technique) 17 Page  Analog Devices 

17 page REV. 0 AD8302 –17– Interfacing to the Input Channels The singleended input interfaces for both channels are identical and each consists of a driving pin, INPA and INPB, and an ac grounding pin, OFSA and OFSB. All four pins are internally dc biased at about 100 mV from the positive supply and should be externally accoupled to the input signals and to ground. For the signal pins, the coupling capacitor should offer negligible imped ance at the signal frequency. For the grounding pins, the coupling capacitor has two functions: it provides ac grounding and sets the highpass corner frequency for the internal offset compensation loop. There is an internal 10 pF capacitor to ground that sets the maximum corner to approximately 200 MHz. The corner can be lowered according the formula fHP (MHz) = 2/CC(nF), where CC is the total capacitance from OFSA or OFSB to ground, including the internal 10 pF. The input impedance to INPA and INPB is a function of frequency, the offset compensation capacitor and package parasitics. At moderate frequencies above fHP, the input network can be approximated by a shunt 3 k Ω resistor in parallel with a 2 pF capacitor. At higher frequencies, the shunt resistance decreases to approximately 500 Ω. The Smith chart in Figure 6 shows the input impedance over the frequency range 100 MHz to 3 GHz. 2.2GHz 2.7GHz 3.0GHz 1.8GHz 900MHz 100MHz Figure 6. Smith Chart Showing the Input Impedance of a Single Channel from 100 MHz to 3 GHz A broadband resistive termination on the signal side of the coupling capacitors can be used to match to a given source impedance. The value of the termination resistor, RT, is deter mined by, RT = RIN RS/(RIN – RS) (10) where RIN is the input resistance and RS the source impedance. At higher frequencies, a reactive, narrowband match might be desirable to tune out the reactive portion of the input imped ance. An important attribute of the twologamp architecture is that if both channels are at the same frequency and have the same input network, then impedance mismatches and reflection losses become essentially commonmode and hence do not impact the relative gain and phase measurement. However, mismatches in these external components can result in measurement errors. Dynamic Range The maximum measurement range for the gain subsystem is limited to a total of 60 dB distributed from –30 dB to +30 dB. This means that both gain and attenuation can be measured. The limits are determined by the minimum and maximum levels that each individual log amp can detect. In the AD8302, each log amp can detect inputs ranging from –73 dBV (223 µV, –60 dBm re: 50 Ω to –13 dBV (223 mV, 0 dBm re: 50 Ω). Note that log amps respond to voltages and not power. An equivalent power can be inferred given an impedance level, e.g., to convert from dBV to dBm in a 50 Ω system, simply add 13 dB. To cover the entire range, it is necessary to apply a reference level to one log amp that corresponds precisely to its midrange. In the AD8302, this level is at –43 dBV, which corresponds to –30 dBm in a 50 Ω environment. The other channel can now sweep from its low end, 30 dB below midrange, to its high end, 30 dB above midrange. If the reference is displaced from midrange, some measurement range will be lost at the extremes. This can occur either if the log amps run out of range or if the rails at ground or 1.8 V are reached. Figure 7 illustrates the effect of the reference channel level placement. If the reference is chosen lower than midrange by 10 dB, then the lower limit will be at –20 dB rather than –30 dB. If the reference chosen is higher by 10 dB, the upper limit will be 20 dB rather than 30 dB. GAIN MEASUREMENT RANGE – dB –30 0 +30 1.80 0.90 MAX RANGE FOR VREF = VREF OPT VREF > VREF OPT VREF < VREF OPT Figure 7. The Effect of Offsetting the Reference Level is to Reduce the Maximum Dynamic Range. The phase measurement range is of 0 to 180 °. For phase differ ences of 0 ° to –180°, the transfer characteristics are mirrored as shown in Figure 5, with a slope of the opposite sign. The phase detector responds to the relative position of the zero crossings between the two input channels. At higher frequencies, the finite rise and fall times of the amplitude limited inputs create an ambiguous situation that leads to inaccessible dead zones at the 0 ° and 180° limits. For maximum phase difference coverage, the reference phase difference should be set to 90 °. 
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