[sdiy] Mixed-signal problems (dsPIC digital delay project)

[jays at aracnet.com]

Maybe there is something I don't get but I don't see how the analog 
section of this works at all. Don't under stand why the analog section 
is ran at 3.3V. Couldn't find any minimum voltage for the TL07X chips in 
the data sheets but the web site says 7 volts. Check the min volts in 
the parametric section.

http://www.ti.com/product/TL072

Next issue with the analog section is Vref at 4.5 volts and running the 
opamps at 3.3V. If low voltage, rail to rail, non-latching/non-inverting 
opamps are used on the input I might see it working but not the output. 
Just doesn't seem right having a 3.3V opamp with Vref 4.5V and 3.3V 
signals. Output doesn't seem to be AC coupled either. Don't know if that 
is a problem or not.

Grab the Microchip AN682 app note and take a look at Fig 7.

Personally would not recommend doing a ground plane fill. This should be 
left to people that understand ground planes and current flow over a 
circuit board.

Other things mentioned that are a good idea are more bypass caps, 
ferrites and make sure the digital and analog traces don't run parallel 
to each other.

Jay S.