[SI-LIST] Re: LVPECL termination and offset crossing
- From: bala <balaseven@xxxxxxxxx>
- To: tom@xxxxxxxxxxxxx, "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
- Date: Thu, 17 Oct 2013 00:37:30 +0530
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.
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> tom@xxxxxxxxxxxxx
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>
> -----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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