[SI-LIST] Re: What is the intention of this application of ferrite beads?
- From: steve weir <weirsi@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 14 Apr 2014 08:38:53 -0700
Amen.
On 4/14/2014 8:18 AM, Todd Hubing wrote:
> Carson,
>
> App notes are generally written by people whose main concern is making an
> individual component function properly on a test board. On a well laid-out
> board, the ferrite bead between the grounds serves no useful purpose, and it
> can actually create significant EMC problems.
>
> High-frequency currents will not spread out randomly looking for other
> circuits to interfere with. Their paths are readily controlled by proper
> routing of the power and signal traces.
>
> High-frequency isolation of the analog return conductor will be a problem if
> any signals leaving the board use this conductor as their ground reference.
> All signals leaving the board need to have the same high-frequency ground
> reference.
>
> Todd
>
> Todd Hubing
> LearnEMC
> http://www.LearnEMC.com
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of Carson Au
> Sent: Monday, April 14, 2014 12:43 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] What is the intention of this application of ferrite beads?
>
> Hi,
> My first post on the reflector! :)
>
> I was looking through some documentation for the Xilinx FPGAs and came across
> this schematic diagram (page 72 - UG480 -
> http://www.xilinx.com/support/documentation/user_guides/ug480_7Series_XADC.pdf
> )
>
> This is meant to be a circuit to filter or prevent high frequency noise from
> coupling onto the inbuilt ADC of the FPGA.
>
> I do not quite understand the intention of Xilinx to recommend the ferrite
> bead on the low-side. Xilinx says it is to provide high frequency isolation.
> However, as I see it, the 470nF capacitor as shown will provide an AC path
> for the high frequency noise of the ground to couple into VCCADC anyway??
>
> Regards,
> Carson Au
> [image: Inline image 1]
>
>
>
>
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