FPGA Central

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 (AT) marvell (DOT) com> wrote:


From: T.K. Jeon <tkjeon (AT) marvell (DOT) com>
Subject: RE: [SI-LIST] AC coupling of SATA and SAS I/O
To: "Alief" <saifj_m (AT) yahoo (DOT) com>, "si-list (AT) freelists (DOT) org" <si-list (AT) freelists (DOT) .org>
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 (AT) freelists (DOT) org [mailto:si-list-bounce (AT) freelists (DOT) org] On Behalf Of Alief
Sent: Thursday, May 07, 2009 10:46 AM
To: si-list (AT) freelists (DOT) org
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

* * *
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Hi Alief,

My view is a little different from that below.

AC caps aren't used to set the Rx bias, but to allow the Rx to set it's own bias point. This is especially useful for rack-to-rack systems where the common mode cannot be well controlled. It also has the advantages of:

- allowing VTT referenced and GND referenced systems to work together
- allowing a single Serdes to cover multiple standards
- allowing newer (restricted supply) devices to work with legacy devices
- ... (I'm sure someone on this list has written a book or two on this...)

The hot-swap ability doesn't really demand dc blocking caps, but they give advantages. "hot-swap" is a bit of a nebulous term too, often.
One of the risks of hot-swap is shorting the connections (signal-signal or signal-supply). Some devices will be designed to withstand the short-term current only (not long-term lifetime degradation). Having the caps means a long-term effect, actually becomes a short-term one, therefore EM considerations are less restrictive on the circuit design side.
I believe PCIE defined them at the Tx for this (shorting) reason (whereas a majority of non-pluggable applications put them at the Rx side, why? well it depends).

Regards,
Richard

-----Original Message-----
From: si-list-bounce (AT) freelists (DOT) org [mailto:si-list-bounce (AT) freelists (DOT) org] On Behalf Of Alief
Sent: 07 May 2009 12:41
To: si-list (AT) freelists (DOT) org; 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 (AT) marvell (DOT) com> wrote:


From: T.K. Jeon <tkjeon (AT) marvell (DOT) com>
Subject: RE: [SI-LIST] AC coupling of SATA and SAS I/O
To: "Alief" <saifj_m (AT) yahoo (DOT) com>, "si-list (AT) freelists (DOT) org" <si-list (AT) freelists (DOT) .org>
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 (AT) freelists (DOT) org [mailto:si-list-bounce (AT) freelists (DOT) org] On Behalf Of Alief
Sent: Thursday, May 07, 2009 10:46 AM
To: si-list (AT) freelists (DOT) org
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

* * *
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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 (AT) freelists (DOT) org [mailto:si-list-bounce (AT) freelists (DOT) org] On Behalf Of Alief
Sent: Thursday, May 07, 2009 12:41 PM
To: si-list (AT) freelists (DOT) org; 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 (AT) marvell (DOT) com> wrote:


From: T.K. Jeon <tkjeon (AT) marvell (DOT) com>
Subject: RE: [SI-LIST] AC coupling of SATA and SAS I/O
To: "Alief" <saifj_m (AT) yahoo (DOT) com>, "si-list (AT) freelists (DOT) org" <si-list (AT) freelists (DOT) .org>
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 (AT) freelists (DOT) org [mailto:si-list-bounce (AT) freelists (DOT) org] On Behalf Of Alief
Sent: Thursday, May 07, 2009 10:46 AM
To: si-list (AT) freelists (DOT) org
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

* * *
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*





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