[SI-LIST] Re: What is the intention of this application of ferrite beads?
- From: tayyabalikhan.pathan@xxxxxxx
- To: carson.au@xxxxxxxxx
- Date: Mon, 14 Apr 2014 11:28:50 +0530
I think the Ferrite in power supply rail with 470nF forms a LC low pass
filter. The 470nF is not for decoupling, its associated with supply
filter.
Regards,
Tayyab
From: Carson Au <carson.au@xxxxxxxxx>
To: "Wawoo, Nachiket" <Nachiket.Wawoo@xxxxxxxxxxxxxxxx>
Cc: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
Date: 04/14/2014 10:50 AM
Subject: [SI-LIST] Re: What is the intention of this application of
ferrite beads?
Sent by: si-list-bounce@xxxxxxxxxxxxx
Thanks for your replies.
@Nachiket: I understand that the bead prevents high frequency noise from
reaching to the ADC GND. However, why place the 470nF cap there at all? If
it is for purely decoupling purposes, why not place it just like the 100nF
cap (on the top right of schematic), and place the 470nF cap across VCCADC
and GNDADC (i.e. after the ferrie beads)?
@Steve: Point taken. I presume you are talking about the impedance of the
470nF capacitor. But the my question as to Nachiket still remains - why
place the 470nF there at all? Why not place it across VCCADC and GNDADC?
On Mon, Apr 14, 2014 at 3:08 PM, Wawoo, Nachiket <
Nachiket.Wawoo@xxxxxxxxxxxxxxxx> wrote:
> Hello Carson,
>
> 470nF capacitor is used as decoupling purpose and bead is for isolation
of
> GND.
> Consider the case that ferrite bead is not placed. Now when 470nF cap
will
> decouple high frequency noise, it will be spread in to GND of circuit
which
> may further affect GND of ADC due to common GND.
> Now if we place the bead then decoupled noise will be prevented to reach
> to ADC GND.
>
> Hope this helps.
>
> Best Regards / Mit freundlichen Grüßen,
> Nachiket Wawoo
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Carson Au
> Sent: Monday, April 14, 2014 10:13 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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