Seperating power busses: Really that important?

[DigiTan]

I have a question regarding digital and analog power busses on ICs. I'm using a voice recorder chip (ISD2590 Chipcorder) and in the datasheet, they mention that the voltage inputs, Vcca and Vccd use seperate pins to reduce noise. Can someone explain in general how this avoids noise and maybe explain the consequences of tying both pins together?

 

[GraveYard_Killer]

i remembered before, i used CMX860 IC which requires separate digital and analog power. what I did is use a single supply but has separate inductor and filter capacitor on both the analog and digital voltage pin. it didn't cause me any problems.

 

[Optikon]

I have a question regarding digital and analog power busses on ICs. I'm using a voice recorder chip (ISD2590 Chipcorder) and in the datasheet, they mention that the voltage inputs, Vcca and Vccd use seperate pins to reduce noise. Can someone explain in general how this avoids noise and maybe explain the consequences of tying both pins together?

The reason they are seperate is likely because the internal design returns switching digital currents through a different path as to keep the high frequency content out of the analog supply and ground so your analog portion remains quite. If you tie them together, you will force this noise on the analog supply and performance _will_ be degraded. Now, whether or not it bothers you in particular, well that's for you to decide but the MFG of the chip thought it was bad enough to provide seperate supplies.

Generally digital supplies have alot more "hash" on them anyways due t all the switching of logic gates. Alot of it is low frequency (audio range) as well. Feed this to an analog circuit at you own peril. If you can keep your supply quiet enough and use generous filtering, you might be ok with it.


If you tie them together, extra filtering might help but won't eliminate the problem.

 

[bmcculla]

Digital components create really nasty voltage spikes. Digital things don't draw current consistently; when a clock edge arrives all the transistors switch which draws a lot of current for a short time. This transient can couple into your power supply and show up in your analog signals.

Like optikon said If your analog stuff isn't that precice you might be able to get away with using the same power supply bus but its sort of asking for trouble. A low value resistor and a big capacitor (as an RC filter) will provide some isolation if you want something simple.