EDE1144 Resonater center lead - + or - BOTH ok?

xanatos

While I have apparently verified that it is OK to use either Vdd or Vss on the center lead of the 4mHz resonator I am using with my EDE1144 keyboard encoder chip, before I commit this to PC Board immortalization, I wanted to check with the folks on here to be SURE there is no issue with using Vdd instead of Vss for the center lead on the resonator. Yes, it is working on my breadboard setup... but I never trust things until I have multiple verifications... :-)

Thanks for the info,

Dave

Phil Pilgrim (PhiPi)

Both Vdd and Vss operate as "AC ground", so you should be okay. I've done the same thing in SX circuits without issue.

-Phil

xanatos

Thanks, Phil. Much appreciated. I have now commited this fact to PC Board layout immortalization! :-)

Dave

Zoot

I had no idea this was possible, and boy oh boy, would it have simplified some of my physical circuit layouts in the past. Phil -- if you have the time and inclination, can you explain in a bit more detail why this is OK?

Phil Pilgrim (PhiPi)

The reason it works is that the connection to the center pin is purely capacitive, so no DC component is transferred to the outer pins. That capacitance is very small compared to what exists between Vdd and Vss. So you're looking at a small capacitance in series with a very large capacitance to ground. The net capacitance to ground is then nearly equal to that present in the resonator.

That said, it's always a good idea to have a bypass cap nearby to guarantee a localized low impedance AC path to ground.

-Phil

Zoot

Thanks, Phil. Most informative. I generally cap near my resonators regardless; certainly helps with stability.