[SI-LIST] Re: Ferrite bead in ground path
- From: "Istvan Nagy" <buenos@xxxxxxxxxxx>
- To: <hitheshn@xxxxxxxxx>, "si-list" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 21 Dec 2012 18:51:51 -0800
Hi,
"adding inductance to return current paths"
That pin is a signal return path ONLY IF it is used for I/O circitry on the
chip.
If the supply rail is used for an on-chip PLL (which is not I/O) then it
should provide the cleanest power supply for the PLL or oscillator.
Otherwise if the PLL and the IO and core logic share the same GND
conenctions, then all voltage drop on the GND pin connection (caused by DC
and AC spiking I/O currents) would be added to the supply voltage seen by
the PLL circuitry on chip. And that could make the clock signal jitter
performance worse. This should depend on the chip design. Maybe I'm wrong? I
used these GND-FBI connections few times.
Regards,
Istvan Nagy
Fortinet, CA
-----Original Message-----
From: Lucci, Christopher
Sent: Friday, December 21, 2012 5:05 PM
To: lihuizju@xxxxxxxxx ; prashant.jaiswar@xxxxxxxxx
Cc: hitheshn@xxxxxxxxx ; garys@xxxxxxxxxxx ; SI-List
Subject: [SI-LIST] Re: Ferrite bead in ground path
This was Joe's point earlier in the email chain and I'd probably have to
agree that this might be some attempt to "contain" a "HF" energy source
instead of trying to track it down and eliminate it at the source. Perhaps
looking at the properties of the ferrite could help Hithesh identify the
intent of the ferrite and perhaps the source of energy.
Regardless it is encouraging that so many people are so vocal about the
obvious issues associated with adding inductance to return current paths.
Now if all management and PCA designers felt the same way...
Happy Holidays to all!
C.S. Lucci
Christopher S. Lucci
Philips Home Healthcare Solutions
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of ??
Sent: Friday, December 21, 2012 7:44 PM
To: prashant.jaiswar@xxxxxxxxx
Cc: hitheshn@xxxxxxxxx; garys@xxxxxxxxxxx; SI-List
Subject: [SI-LIST] Re: Ferrite bead in ground path
In my opinion, the FB is used to isolate different power supply and ground,
such as analog power and digital one.
Prashant Jaiswar <prashant.jaiswar@xxxxxxxxx>编写：
>I think the first question is to be shot to your MCU vendor,in the first
>place why the FB has been introduced onto the power supply? For any PSU
>design onboard, the basic requirement is to have minimised power plane
>impedance for the required frequency decoupling. Having FB onto the PSU
>line will introduce increased impedance for the desired frequency of
>operation.
>
>Sent from my iPad
>
>On 21-Dec-2012, at 11:31 PM, Hithesh <hitheshn@xxxxxxxxx> wrote:
>
>> Gary,
>> The ferrite is between the MCU analog ground(GNDA) and the circuit
>> ground(not chassis ground). Majority of the components on the board are
>> digital(MCU, SDRAM etc). There's also a ferrite in the power section.
>> The MCU has 1.8V for analog circuitry (not ADC), probably PLL or some
>> Vref
>> circuitry. I did a quick datasheet search, didn't get any details.
>>
>> -Hithesh
>>
>>
>> On Fri, Dec 21, 2012 at 11:16 PM, Gary Stein <garys@xxxxxxxxxxx> wrote:
>>
>>> Ground path between what? Beads between chassis ground and signal
>>> ground
>>> are frequently used for emi/esd suppression, ie keep external high
>>> frequency
>>> transients out of signal ground and to keep any ground noise from
>>> radiating
>>> from connectors shells and such. In such applications, chassis ground
>>> is
>>> not a signal nor power return path.
>>>
>>> -----Original Message-----
>>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>>> On
>>> Behalf Of Ken Wyatt
>>> Sent: Friday, December 21, 2012 11:43 AM
>>> To: hitheshn@xxxxxxxxx
>>> Cc: SI-List
>>> Subject: [SI-LIST] Re: Ferrite bead in ground path
>>>
>>> I've seen ferrites in series with the green wire safety ground wire.
>>> Otherwise, I wouldn't think you'd want an impedance in the signal OR
>>> power
>>> return path, unless you'd want to keep us consultants employed!
>>>
>>> Cheers, Ken
>>>
>>> Kenneth Wyatt
>>> Wyatt Technical Services LLC
>>> Woodland Park, CO
>>> ken@xxxxxxxxxxxxxxxx
>>> www.emc-seminars.com
>>> (Sent from my iPad)
>>>
>>> On Dec 21, 2012, at 10:14 AM, Hithesh <hitheshn@xxxxxxxxx> wrote:
>>>
>>>> I came across an old schematic that has ferrite bead in the ground
>>>> path. Is there any advantage of using ferrite bead in the ground path.
>>>> The general rule is to keep the ground path at low impedance.
>>>> -Hithesh
>>>>
>>>>
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