FAQ: Reducing the Response Time of an RMS Detector

Document created by Larry Employee on Oct 30, 2009Last modified by AndyR on Jan 31, 2012
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Q.

As I increase the value of the AD8362 filtering capacitor, the device's fall time after an RF burst has decayed becomes very long. How can I get around this?

 

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A.

As the CLPF capacitor is increased, the residual ripple on the output decreases at the cost of a slower response time. However, as the value of CLPF increases, the fall time increases at a faster rate than the rise time. The shape of the falling voltage becomes more linear than exponential (see the attached oscilloscope plot)

 

AD8362 Response Time 1.bmp

In applications where the fall time is critical, it is better to reduce residual ripple by placing an RC filter at the rms output of the AD8362 rather than further increasing CLPF. In the second oscilloscope plot below, such a scheme is implemented. A filtering capacitor of  390 pF is placed on CLPF and an RC filter (75 ohms, 0.15 uF) is placed at the rms output pin. With this configuration, the fall time reverts to a faster exponential decay while residual ripple is a little less than the previous case (150 mVpp vs 200 mVpp).

 

The attached file shows resoponse times for other filter component values and also includes response time plots for the AD8363  and AD8364 rms detectors.

AD8362 Response Time 2.bmp

 

 

 

The ADL5902 acts much like the other RMS detectors when a filter is put on its output. However the filter also causes some peaking in the detector response.  Placing a resistor in parallel with the CLPF capacitor, has the same affect as an output filter but without the peaking.  More information is included in the ADL5902 datasheet.   

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