[SI-LIST] Re: Analog/Digital Gnd
- From: steve weir <weirsp@xxxxxxxxxx>
- To: <hariharan@xxxxxxxxxxx>, <shane.san.miguel@xxxxxxxxx>,<si-list@xxxxxxxxxxxxx>
- Date: Tue, 11 Mar 2003 22:23:21 -0800
Hariharan,
Frame ground is a different beast. Depending on the approach taken to
powering the device, frame ground will often, ( as in PC's and most telecom
equipment ) be bonded to the signal ground in multiple locations in order
to minimize EMI radiation. Exceptions to this rule occur when frame and
signal ground are DC isolated as in some telecom gear due to safety
considerations. However, in these cases, the manufacturer will usually
still do their best to achieve tight AC coupling for EMI suppression.
Regards,
Steve.
At 11:16 AM 3/12/2003 +0530, hariharan wrote:
>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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