The Designer's Guide Community Forum

The Designer's Guide Community Forum
https://designers-guide.org/forum/YaBB.pl
Design >> Mixed-Signal Design >> Decoupling capacitors for separate Analog, Digital & Mixed supply rails
https://designers-guide.org/forum/YaBB.pl?num=1475462173

Message started by jdp on Oct 2^nd, 2016, 7:36pm

Title: Decoupling capacitors for separate Analog, Digital & Mixed supply rails
Post by jdp on Oct 2^nd, 2016, 7:36pm

Hi.
I need to have separate Vdd and Ground pairs for Analog, Digital & Mixed-signal domains in a Delta Sigma modulator (e.g. AVdd & AGnd, DVdd & DGnd,...). A common practice is to use the “free space” in the layout of a chip to have the decoupling capacitors.

As I have space constraint in the layout, I need to know if I need to connect decoupling capacitors between each Vdd and the corresponding Gnd ? - or any particular pair (like AVdd & AGnd) is more prior in the need of decoupling capacitors?

Title: Re: Decoupling capacitors for separate Analog, Digital & Mixed supply rails
Post by deba on Oct 18^th, 2016, 3:01am

Yes you need to keep both Avdd and DVdd silent with respect to their grounds. In my opinion, more priority should be given to make Avdd cleaner as noise on this will effect you circuit more than the noise on Dvdd. So Avdd should have more decoupling capacitors.

Title: Re: Decoupling capacitors for separate Analog, Digital & Mixed supply rails
Post by Sx_cut on Oct 19^th, 2016, 2:05am

You need to keep your sensitive analog blocks and its VDD and GND away from the switching noise of digital block. If you are using SOI tech, you can separate grounds easily e.g. the substrate which is buried under a BOX layer is recommended to be connected to only one ground. You can connect it to digital ground while your analog ground can remain in BEOL. In bulk nodes substrate is the common ground and the only way is to keep digital and analog grounds far apart!