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Category: Hardware
Product Number: AD8361

dear all,

i'm entering the AD8361 with a AM signal (1.5 upto 30MHz) 1kHz modulation tone 80% depth. Then i connect 1nF between the pins 1 to 6 because the input signal could have in general 200kHz bandwidth and i would have like to see the output signal  at VRMS pin (so could even not be an AM signal). I was expected to have at that pin (pin 7) something that should look like a  squared sinusoid, but what i have seen is a sinusoid (so the modulation tone) as i had an envelope detector.

Is this behaviour correct? If this were an rms true detector shouldn't i have seen a squared wave?


Stefano Casilli

RF Power Amplifier Designer 

  • Hi Stefano,

    Sorry I don't completely understand your question. However, based on these statements, sounds like the behavior is correct, as expected. The AD8361 is a rms detector, responding in volts-per-volt. So for AM input with sine-wave modulation, one would expect sine wave out, matching the AM input. In practice, you might consider trying various values of external filter capacitance to observe the behavior. Larger filter capacitor is needed to support lower RF input frequencies, but that will necessarily limit the demodulation bandwidth.   -Bruce H.

  • Hi Bruce,

    in true rms detector if you make a light filtering (or if you dont filter at all) i believe you should see the square of video signal. In the description of the AD8361 in fact (page 11) you can read: The AD8361 responds to the voltage, VIN, at its input by squaring this voltage to generate a current proportional to VIN squared.  That is my question: is it possible that i see a sinusoidal voltage and not a squared sinusoidal voltage?


  • Hi Stefano,  

    The normal application for these power detectors is to accurately measure total RF power, with or without analog or digital modulation. Customers normally have enough external capacitance (if necessary) to remove the modulation component, leaving only a relatively slow output DC voltage on VRMS pin that can be calibrated to what a RF power meter would read. In other words, the external filter capacitor is removing the modulation. However if you make the external capacitor smaller, the lower frequency AM components of the input waveform will be more observable. And if you make that external capacitor smaller yet, and if the RF input frequency is low enough, eventually you will start to see the RF carrier leaking through to the VRMS output (probably more 2*F than 1*F).  Hope that helps answer your question.   -Bruce H.