FAQ: AGC Response Time of AD8368 VGA  

Document created by enash Employee on Oct 26, 2009Last modified by AndyR on Jan 31, 2012
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How do you control the response time of the AD8368 VGA's integrated AGC controller?





The AGC response time of the AD8368 is dependent on the Cdeto capacitor shown in Figure 34 of the data sheet.  As a general rule, the bigger the Cdeto cap, the slower the AGC response time; or the smaller the Cdeto cap, the faster the response time. The RC (or Tau) response time can be expressed by the equation:



t (seconds) = 9000 x CDETO (F)




So a 1000nF capacitor will give a rise time of  9 ms.



You must take care not to make the CDETO cap too small because the fast response time could be too fast for your application.  For example, if the CDETO were too small, the AD8368 in AGC mode could start tracking and leveling any amplitude envelope (like in a QAM signal).  This would then corrupt your modulation envelope and degrade EVM.  So if you want to go for the fastest possible response time, you will have to make a compromise between EVM and response time.This behavior is discussed on page 16 of the data sheet. The best approach is to experimentally make the capacitor smaller and smaller until you start to see the EVM degrade. Note that the ADI evaluation board ships with a 1nF capacitor installed.