[SI-LIST] Re: Analog/Digital Gnd
- From: "hariharan" <hariharan@xxxxxxxxxxx>
- To: <weirsp@xxxxxxxxxx>, <shane.san.miguel@xxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Wed, 12 Mar 2003 11:16:59 +0530
Does Frame Ground (FGND) in a product follows the same rules for AGND/DGND?
Hariharan
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir
Sent: Wednesday, March 12, 2003 4:03 AM
To: shane.san.miguel@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Analog/Digital Gnd
Shane,
PLL's usually make up a special case. The corollary to: "Follow the
money." is: "Follow the current."
The supply for a PLL can be effectively isolated because significant high
speed currents do not need to flow between the PLL and the digital world
outside of the chip. By isolating the connection on the outside, the PLL
rides on the digital ground potential inside the chip where both grounds
are most definitely connected, and fast digital currents do not traverse
the PLL ground node inducing noise.
Now, contrast that to the VDAC problem just mentioned. There, high speed
currents definitely flow between the analog and digital. By inserting
impedance between the two "grounds", significant common mode voltages
develop showing up as the bounce reported. That's not a good thing.
The key here is to control where currents flow, and keep currents from
taking unwanted paths and disturbing victims we need to protect. Often,
this is a matter of placement and routing. In other cases, moating can
prevent closure of unwanted secondary current loops.
Regards,
Steve.
At 01:30 PM 3/11/2003 -0800, San Miguel, Shane wrote:
>I am for sure not the expert on this but in my experience, you want =
>analog isolated to take advantage of extra filtering. Usually I run =
>into a seperate analog gnd when there is a PLL involved. You have to =
>keep the digital supplies away from this so there is nothing to confuse =
>the PLL.=20
>
>Shane San Miguel=20
>
>
>-----Original Message-----
>From: Parthasarathy Sampath [mailto:parthsv@xxxxxxxxx]
>Sent: Tuesday, March 11, 2003 12:44 PM
>To: si-list@xxxxxxxxxxxxx
>Subject: [SI-LIST] Analog/Digital Gnd
>
>
>
>Hi All,
> Why is that Analog ground and Digital ground are
>kept seperate during Layout? I have even noticed in
>some cases, seperate power rails are provided for
>analog and digital block even though they require same
>VDD.
>
>Thanks in Advance,
>Partha!
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