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Decoupling capacitors for separate Analog, Digital & Mixed supply rails (Read 732 times)
jdp
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Decoupling capacitors for separate Analog, Digital & Mixed supply rails
Oct 02nd, 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?
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deba
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Re: Decoupling capacitors for separate Analog, Digital & Mixed supply rails
Reply #1 - Oct 18th, 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.
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Sx_cut
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Re: Decoupling capacitors for separate Analog, Digital & Mixed supply rails
Reply #2 - Oct 19th, 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!
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