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Observer
Observer
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Registered: ‎07-16-2018

Ferrite bead for 7-series (XC7A15T-FTG256)

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Hello, the PCB design guide contains detailed information about proper bypassing but I didn't see anything about possible ferrite bead selection. I'm not concerned about the operation of the FPGA, but the noise it might inject into my PCB and disrupt sensitive analog components. Are there any recommended ferrites for this that would avoid undesired LC resonance with the bypass caps? I've followed the bypassing recommendations exactly as layed out in the PCB design guide.

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199 Views
Registered: ‎07-23-2019

Re: Ferrite bead for 7-series (XC7A15T-FTG256)

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The best decoupling for FPGAs is based on low impedance ceramic caps and good power and ground plane. 

Ferrites are good for a clock generator, sensitive ADC, etc. but for an FPGA drawing amps, first of all you will need the 'power' ones, then you will have the problem of the series resistance and V drop. You will have to add the different voltage drop (low current - high current) to the noise and fit it into the rail voltage tolerance.

Yet more problems, even if ferrites are not inductors, they have an inductive component and can resonate with capacitors (that you will have anyways). So forget about the beautiful curve of the ferrite impedance on its datasheet, you will have frequencies where the impedance will be minimum (just the series R, typically less than 1 ohm) and any noise will circulate freely from and into the FPGA.

In short: Do not do that.

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Moderator
Moderator
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Registered: ‎07-23-2015

Re: Ferrite bead for 7-series (XC7A15T-FTG256)

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@matthuszagh  Which rail are you looking at for using at ferrite bead? I don't recollect any rail in Artix-7 needing a ferrite bead. 

When not specifically needed, the usage of ferrites can bring more problems than they solve. So I would be wary of how and where you use them especially when it is not recommended in the user guide. 

- Giri
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Highlighted
200 Views
Registered: ‎07-23-2019

Re: Ferrite bead for 7-series (XC7A15T-FTG256)

Jump to solution

 

The best decoupling for FPGAs is based on low impedance ceramic caps and good power and ground plane. 

Ferrites are good for a clock generator, sensitive ADC, etc. but for an FPGA drawing amps, first of all you will need the 'power' ones, then you will have the problem of the series resistance and V drop. You will have to add the different voltage drop (low current - high current) to the noise and fit it into the rail voltage tolerance.

Yet more problems, even if ferrites are not inductors, they have an inductive component and can resonate with capacitors (that you will have anyways). So forget about the beautiful curve of the ferrite impedance on its datasheet, you will have frequencies where the impedance will be minimum (just the series R, typically less than 1 ohm) and any noise will circulate freely from and into the FPGA.

In short: Do not do that.

View solution in original post