AGC implementation of AD8367 with AD8361

Question

Hi! I want to use the AD8367 to realize the AGC function together with the AD8361. The reference circuit shown in Fig.38 shows the coupling between the output of AD8367 and the input to AD8361 is simply based on the series resistor R1(200kR). I am wondering due to the input impedance of AD8361's RFIN port, will the voltage at RFIN pin be attenuated and rather small since the resistor divider? Can you explain the circuit adopted here? And what's the best and most propriate way to realize the coupling between main signal path and the path for RMS detection? Thanks for your help! 
 Best, 
 Haojie Wu

Bruce_H · Accepted Answer

It&rsquo;s likely that the 200k is a typo. The tap resistor value of 200 Ohms is much more practical. The basic coupling factor due to the resistive tap is easily calculated from voltage divider formulas or with the help of LTspice. 
 Resistive tap such as we see here is practical for lower frequencies and lower power levels. At higher frequencies, the method is not recommended because of the drastic effect of parasitic capacitance of the tap resistor. Alternative coupling means would be a conventional power splitter circuit or a directional coupler.

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AGC implementation of AD8367 with AD8361

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