[SI-LIST] Re: LVPECL termination and offset crossing
- From: "Tom Dagostino" <tom@xxxxxxxxxxxxx>
- To: "'bala'" <balaseven@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 16 Oct 2013 14:23:40 -0700
Bala
Can you define 100 Ohm parallel termination and what you mean by crossing
offset? And what part are you using? Pictures would be worth a 1000 words.
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire St.
Suite 102
Portland, OR 97225
USA
971-279-5325 Office
971-279-5326 FAX
503-430-1065 Cell
tom@xxxxxxxxxxxxx
www.teraspeed.com <http://www.teraspeed.com/>
Teraspeed Consulting Group LLC
16 Stormy Brook Road
Falmouth, ME 04105
401-284-1827
From: bala [mailto:balaseven@xxxxxxxxx]
Sent: Wednesday, October 16, 2013 12:08 PM
To: tom@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] LVPECL termination and offset crossing
Tom,
Yes its true LVPECL driver.I am talking about LVPECL to LVPECL topology.I
prefer thevenin termination as its gives good output.However,some vendor
suggest 100 ohm parallel termination instead of thevenin(which is strongly
recommended for PECL buffer).My question is,why i am getting crossing offset
for single ended waveform while using 100ohm,whereas,no offset while using
thevenin termination.
Orin,
These are 100 ohm differential LVPECL signals.I am aware of usual
termination techniques,but LVPECL demands some specific termination
schemes..
Regards
bala
On Wed, Oct 16, 2013 at 11:51 PM, Tom Dagostino <tom@xxxxxxxxxxxxx> wrote:
Bala
You are leaving out several key pieces of information. You call this a
LVPECL driver. Is it truly LVPECL or is it a driver that will produce
LVPECL levels but using an LVDS or other driver topology set up for LVPECL
levels?
If it is truly an LVPECL driver you need to have enough standing current in
the output stage to bias the transistors with enough current to operate at
their designed current. Too little current will slow the output down. Too
much current can also slow the output down and dissipate too much power.
The recommended 50 Ohms to Vcc - 2V provides this current. But Thevenin
equivalent circuits will work just as fine.
If this is one of the pseudo LVPECL drivers using an LVDS like driver then
treat it like a LVDS driver and terminate it with a 100 Ohm floating
differential load or with two 50 Ohm loads tied to Vtt for this driver.
If there is some other driver topology read the manual for the device,
understand where the currents need to flow and design the appropriate
termination. Some of these look like CMOS totem pole outputs and will need
a termination to a Vtt voltage.
Good luck
Tom Dagostino
Teraspeed Labs
9999 SW Wilshire St.
Suite 102
Portland, OR 97225
USA
971-279-5325 Office
971-279-5326 FAX
503-430-1065 Cell
tom@xxxxxxxxxxxxx
www.teraspeed.com
Teraspeed Consulting Group LLC
16 Stormy Brook Road
Falmouth, ME 04105
401-284-1827
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of bala
Sent: Wednesday, October 16, 2013 7:00 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] LVPECL termination and offset crossing
Hi all,
I have some questions on the LVPECL termination techniques.DC path to ground
is needed to enable my output transistor irrespective of my receiver logic.
So I am adding 150ohm resistor to ground at driver side .Every vendor
suggest different termination scheme for receiver side, some vendor
recommend thevenin termination and some vendor suggest 50 ohm to Vcc-2.I
understand that to avoid the addition of new voltage scheme for Vcc-2,we are
using thevenin termination scheme. Some vendors suggest 100 ohm parallel
termination. I did simulate 156Mhz differential LEPECL which has unbalanced
differential choke on each signal, with two different termination scheme,
one with 100ohm, another one with thevenin termination .Differential results
looks good for both cases, whereas ,my single ended waveform shows 'crossing
offset' when I use 100ohm termination. I did the simulation again with
thevein termination and crossings are reasonable. Why we are getting 'offset
crossing' while using 100ohm termination?
--
bala
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bala
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