[SI-LIST] Re: SATA board to board connector question
- From: olaney@xxxxxxxx
- To: goister@xxxxxxxxx
- Date: Mon, 27 Aug 2007 08:25:33 -0700
The most fundamental thing you must understand is that high frequency
signals need their own ground returns. The fact that the grounds of two
boards are connected for power supply purposes is interesting, but has
little to do with adequate signal integrity. Power supply current and
power supply grounds should be routed together. That can be on a
separate connector from signal paths. But, signals and their ground
returns must also be kept together. The idea that one fat ground strap
between source and destination takes care of all ground return needs
works where signals are slow and S/N ratio is not an issue, for instance
the battery and alternator (DC) wiring in automobiles. However, this
approach is of no use for high frequency capable, constant impedance
transmission lines. Don't think like an ohmmeter.
If you think like a high frequency signal, e.g. a digital bit stream with
fast rise and fall times, you want a ground return path that is intimate
with the signal, and this need has nothing to do with the idea that other
signals, including power, already have ground return paths of their own.
This even applies to differential signals, which in theory have no
ground currents, but in practice always have some imbalance. Allocate
ground return paths on a signal basis for anything high frequency or that
requires better than average signal to noise ratio, unless you hate clean
edges and quiet grounds, and *want* to fail radiated EMI testing.
The physical reasoning has to do with the inductive loop area created by
the signal + associated ground return path, which needs to be minimized.
To understand why, you need to bone up on SI and EMC theory. I urge you
to hit the books and seminars, scrounge the internet, and otherwise do
your homework. This forum is not for teaching foundational knowledge,
but the forum is here to point you in the right direction, and I hope
that has been accomplished.
Regards,
Orin Laney
On Mon, 27 Aug 2007 18:03:41 +0800 "Sihan Goi" <goister@xxxxxxxxx>
writes:
Thanks. One more question. You mention to carry the ground pins together
with the signals on the same connector. Why is this so? I do have another
connector that carries the power signals. Does this mean I should not
route any other ground signal(s) with this power connector, and rely
solely on the signal connector to provide the ground plane for the
daughterboard?
On 8/24/07, olaney@xxxxxxxx <olaney@xxxxxxxx> wrote:
Separate power is the usual approach, and not a problem. Some of the
smaller Samtec connectors sound like a good choice for constant Z board
to board, among many others. Most formal backplane connectors probably
have way too many pins for your need. It looks like you have everything
on track for a successful design. Keep the list posted on the more
significant problems and progress, so that everybody can learn from your
experience. And make those app engineers take you out to lunch.
Regards,
Orin
On Fri, 24 Aug 2007 12:07:16 +0800 "Sihan Goi" <goister@xxxxxxxxx>
writes:
Thanks.
Basically the daughterboard acts simply as kind of an extender of the
SATA signal. So all the logic circuitry is on the main board, with the
SATA signals coming out of the SATA controller chip, and to the daughter
board via the high speed connector direct(no cables). These signals are
then routed to an actual SATA HDD 7pin connector on the daughterboard, to
be connected to a SATA HDD. SATA power to the HDD will most likely be
routed with another connector, probably a regular pin header/socket. That
shouldn't be a problem right?
Regarding the fat pads, I'll probably be using 0402 components as much as
possible to reduce this problem, particularly for the 10nF capacitors
required for the SATA signals. I believe this is part of the SATA
recommendations too.
On 8/24/07, olaney@xxxxxxxx < olaney@xxxxxxxx> wrote:
Yes, the connector manufacturer recommendations. Any connector with the
right impedance and adequate data rate specs will do. The same connector
family can be used for many purposes, so it is not a matter of the
signaling system used as much as the electrical performance and the
mechanical need. Extra pins can be ignored or used for other purposes.
I don't know if you are trying to connect your boards connector to
connector direct, or through a short cable. In any instance, keep the
I/O chip close to the connector, and avoid changes in trace widths other
than what might be recommended for dealing with proximity effects
(sometimes the signal traces are tapered where they run under connector
dielectric). One common signal integrity killer is where the traces are
carefully designed, yet run through coupling caps or other passives using
the fat SMT pads that production loves so much. Sometimes production
balks at the practice of using wider traces or narrower parts to minimize
the discontinuity in microstrip width. Given the choice between a design
that works and one that can be built without hand operations or other
accommodations, they have a tough time making up their minds. As an
engineer with proper test equipment and adequate time (That's all of us,
right? Right?), you can often meet both goals.
Anyway, you might as well check out formal SATA connectors to understand
them electrically before widening your search. The GSSGSSG layout is not
absolutely necessary for short distances, as crosstalk can be controlled
with adequate pair separation. I would not assume that a 7 pin connector
is the goal. Think of it as two impedance controlled pairs plus whatever
ground paths are designed in to ensure impedance and crosstalk control.
The ground need not be discrete pins on par with the signals, in the same
way that a coax shield is not carried by a pin like that of the center
conductor. However, make sure that the ground is carried with the
signals through the same connector!!! Any ground connections provided
elsewhere in the system might make an ohmmeter happy, but relying on them
= death at high frequencies.
Molex might be another vendor to include in your list. Don't be shy
about using vendor field app engineers -- that's what they're paid for,
and it's job security for them.
Orin
On Thu, 23 Aug 2007 23:07:46 +0800 "Sihan Goi" <goister@xxxxxxxxx>
writes:
Thanks. It's kinda confusing when I go to samtec, FCI or amphenol website
and they don't show what kind of applications the high speed connectors
are meant for, or their impedance rating or whatever...
Anyway, I'm also wondering about how the signals should be routed to the
connector. SATA signals on regular SATA connector are as follows
GND A+ A- GND B+ B- GND
Should I be following this topology or does it even matter? I'm guessing
most of these high speed connectors won't have exactly 7 pins. They're
usually spec'ed for 2/3/4 pairs or even more. I'm guessing I only need a
2 pair connector since I only have 2 differential SATA signal pairs. I'm
wondering if I even need to route the GND with the high speed connector
or can it be from another regular connector somewhere else...?
Lastly, when you say "pay attention to the manufacturer recommendations"
which manufacturer do you mean? The connector manufacturer?
Again, thanks for your reply!
On 8/23/07, olaney@xxxxxxxx < olaney@xxxxxxxx> wrote:
Basically, yes, but give your design some margin. For 3 Gb/s go for 5
GHz or more of frequency response. Biggest headache tends to be keeping
the impedance constant where the traces enter the connector launch area.
Pay attention to the manufacturer recommendations.
Orin Laney
On Thu, 23 Aug 2007 16:08:09 +0800 "Sihan Goi" <goister@xxxxxxxxx>
writes:
> Hi,
> I have a design where I have to route SATA signals from a main board
> to a
> daughter board. The daughter board will likely have nothing except
> for the 7
> SATA signals(4 data and 3 GND) connected to a regular SATA HDD
> connector
> (unless some passives are needed?).
>
> In my previous PATA design, I used a normal 44pin IDE type connector
> pair(pin header + socket), and this worked well for PATA. I'm
> guessing this
> will not work so well with SATA though. What kind of connectors
> would work
> for SATA1/2? I know the differential impedance is 100ohms, and that
> the
> trace length difference must be within 5 mils. I'm guessing I have
> to get a
> high speed connector that has the same impedance and is able to
> support 3GHz
> speeds? Is that all I need to be looking for?
>
> Thanks.
>
> --
> - Goi Sihan
> goister@xxxxxxxxx
>
>
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--
- Goi Sihan
goister@xxxxxxxxx
--
- Goi Sihan
goister@xxxxxxxxx
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- Goi Sihan
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