[sdiy] DG409 blues

Donald Tillman don at till.com
Sun Feb 7 15:27:13 CET 2010

On Feb 6, 2010, at 12:31 PM, David G. Dixon wrote:

> Does anybody have any experience with or insight into the DG408/9  
> analog
> multiplexers?  Specifically, I would like to know whether it is OK  
> to put
> +15V into the logical inputs.

See the design note in my Quadrature Trapezoid VCO article:


"There's nothing mentioned about this in the data sheet, but I found  
out the hard way that the DG409 behaves badly when large voltage  
swings (16.0 V in this case) are applied to the digital inputs.   
Specifically the multiplexer deposits a substantial negative charge on  
one of the capacitors during certain transistions.  So that's why the  
zeners are there.  As a nice side effect the zeners minimize the size  
of voltage transients in the circuit as well as minimizing current  
transients that might affect the power supply."

   -- Don

Don Tillman
Palo Alto, California
don at till.com

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