[SI-LIST] Re: AC coupling of SATA and SAS I/O

Alief,

Power up/down step function due to the hot plug(or other operations) will 
result in significant common mode voltage surge(Vcm). By simply putting a 
capacitor between the aggressor(this might be your tx or rx) and the 
victim(again, your tx or rx), you can reduce the common mode transient 
significantly. In other words, if you choose a capacitor(Cac) such that 
Cac*(Ragg+Rvic) become the coupling time constant, where Ragg and Rvic are the 
termination resistor at aggressor and victim, respectively, the resulted common 
mode voltage at the victim should be some fraction of Vcm. Therefore, with a 
proper coupling capacitor, you don't need to worry about breaking the victim 
side.

This time constant is also used for Rx detect function in PCI express, i.e., 
when Rx is open the time constant would be smaller than when Rx is terminated.

TK

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Alief
Sent: Thursday, May 07, 2009 12:41 PM
To: si-list@xxxxxxxxxxxxx; T.K. Jeon
Subject: [SI-LIST] Re: AC coupling of SATA and SAS I/O

Thank you for your response T.K.
 
Yes, a 0V DC bias would establish an automatic reference (in theory); thus it 
would be akin to DC coupling.
 
However, I am hesitant to make my common mode 0V; because of issues with ground 
loops, ground path parasitics, etc and also because the receiver would see 
negative voltages.
 
I realize that this is to first order a biasing issue and allows for 
interoperability.
 
However, are there other considerations ?  I have heard/read that it might 
permit hot swappability.  Is this true.  If I had a known common mode level 
doesn't that make it easier for hot swapping ?
 
Also, are there other issues which AC coupling helps with ?
 
Thanks a bunch in advance,
 
Alief
--- On Thu, 5/7/09, T.K. Jeon <tkjeon@xxxxxxxxxxx> wrote:


From: T.K. Jeon <tkjeon@xxxxxxxxxxx>
Subject: RE: [SI-LIST] AC coupling of SATA and SAS I/O
To: "Alief" <saifj_m@xxxxxxxxx>, "si-list@xxxxxxxxxxxxx" 
<si-list@xxxxxxxxxxxxxx>
Date: Thursday, May 7, 2009, 6:55 PM


Alief,

AC coupling is needed to maintain the correct DC bias for receivers. If your 
transmitter has 0V DC bias, then I guess you don't have to implement AC 
coupling.

TK

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On 
Behalf Of Alief
Sent: Thursday, May 07, 2009 10:46 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] AC coupling of SATA and SAS I/O

Greetings everyone,
 
I am currently looking at SATA and SAS SerDes designs.  It looks like the data 
transmission is AC coupled.  Is this a standards requirement ?  
 
Could anyone enlighten me on why SATA/SAS specifies only AC coupled links [if 
that is in fact the case]; i.e. what was the thinking/justification for 
restricting transmission to ac coupled only.
 
Also, do the other standards :  Gbit Ethernet, PCI Express, RapidIO, etc. have 
similar restrictions/requirements ?
 
Thanks v much in advance,
 
Alief

      
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
        http://www.freelists.org/archives/si-list
or at our remote archives:
        http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
        http://www.qsl.net/wb6tpu
  




      
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field

or to administer your membership from a web page, go to:
http://www.freelists.org/webpage/si-list

For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field


List technical documents are available at:
                http://www.si-list.net

List archives are viewable at:     
                http://www.freelists.org/archives/si-list
or at our remote archives:
                http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
                http://www.qsl.net/wb6tpu
  

Other related posts: