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illegal feedback error from double-pole MUX

Question asked by KJBob on Nov 24, 2013
Latest reply on Nov 25, 2013 by KJBob

     Hello all,

 

     The circuit below uses a stereo, three-input mux as a DP3T switch where the "left" section chooses between two DC sources for the filter's Q input, and the "right" section selects filter or bypass.  The idea is to have three switch positions:  High-Q, Low-Q, and OFF.  However it won't compile, throwing an "illegal feedback" error instead:

Combined-mux-NG_pic.jpg

 

     If the output and Q functions are brought out to separate muxes as shown below, it compiles fine:

Separate-mux-OK_pic.jpg

     In neither circuit does the filter's Q input ever come from its BP output; these signals are handled by separate mux sections.  The two circuits are functionally identical -- the only difference is the button choice presented to the user.  Thus, I don't understand the illegal feedback error that the top circuit generates.

 

 

     Best regards,

 

     Bob

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