Differential trace characteristic impedance question

B

billcalley

Guest
Guys,

Since I have to deal with differential circuits now, and I am only
use to single-ended, I was wondering if the characteristic impedance
of a differential trace matters. To clarify: If I design
differential traces to terminate into say, 100-ohms differential, I
could use traces that are 90 mils wide with a trace-to-trace spacing
of 100 mils to accomplish this, or I could use traces that were 30
mils wide with a spacing of 5 mils. Is either OK, even though the
characteristic impedance of each individual trace is significantly
different in the two examples? Would it matter to the circuit at all,
as it does with the characteristic impedance of a microstrip trace in
a single-ended circuit -- or is it only the differential impedance
that matters in such applications?

Thanks!

-Bill
 
billcalley wrote:
Guys,

Since I have to deal with differential circuits now, and I am only
use to single-ended, I was wondering if the characteristic impedance
of a differential trace matters. To clarify: If I design
differential traces to terminate into say, 100-ohms differential, I
could use traces that are 90 mils wide with a trace-to-trace spacing
of 100 mils to accomplish this, or I could use traces that were 30
mils wide with a spacing of 5 mils. Is either OK, even though the
characteristic impedance of each individual trace is significantly
different in the two examples? Would it matter to the circuit at all,
as it does with the characteristic impedance of a microstrip trace in
a single-ended circuit -- or is it only the differential impedance
that matters in such applications?

Thanks!

-Bill
Click to expand...
http://www.icd.com.au/

have a differential impedance calculator.

I just did a 100Mb ethernet using 100R differential 50R single-ended
traces, terminated with a pair of 50Rs each to a capacitive shunt (IOW
50R to 0V, 100R diff).

** What sort of " differential circuits" ?

differential ones I guess :)

Cheers
Terry
 
On Wed, 9 Apr 2008 15:34:41 +1000 "Phil Allison" <philallison@tpg.com.au>
wrote in Message id: <6632rjF2ielepU1@mid.individual.net>:

"Bob Wanker Congenital Anencephalic "



** Puke, puke puke......
Click to expand...
Do the world and yourself a favor - walk into your local Walmart, lay a
franklin on the counter for a sturdy double-barrelled shotgun, load it,
then walk into the bathroom and decorate the ceiling with your skullcap.
 
On Tue, 8 Apr 2008 21:32:55 -0700 (PDT), billcalley
<billcalley@yahoo.com> wrote:

Guys,

Since I have to deal with differential circuits now, and I am only
use to single-ended, I was wondering if the characteristic impedance
of a differential trace matters. To clarify: If I design
differential traces to terminate into say, 100-ohms differential, I
could use traces that are 90 mils wide with a trace-to-trace spacing
of 100 mils to accomplish this, or I could use traces that were 30
mils wide with a spacing of 5 mils. Is either OK, even though the
characteristic impedance of each individual trace is significantly
different in the two examples? Would it matter to the circuit at all,
as it does with the characteristic impedance of a microstrip trace in
a single-ended circuit -- or is it only the differential impedance
that matters in such applications?

Thanks!

-Bill
Click to expand...
If you're using a balanced driver and a balanced termination/receiver,
like LVDS maybe, only the differential (odd mode) impedance matters.

You can make a 100 ohm diff pair from two independent (ie, uncoupled)
50 ohm traces which, as you note, will be fat traces far from one
another. Each will calculate as having 50 ohm odd-mode impedance and
50-ohm even mode, in other words just a 50 ohm microstrip.

To save room, make the traces skinnier and move them close, so that
they couple. Keep the odd mode z of each trace 50 ohms, so the diff
remains 100. Even mode impedance will go up as coupling increases.
There are calculators available to do the math, like TXLINE. Very
skinny traces will get lossy.

John
 
On Tue, 08 Apr 2008 21:32:55 -0700, billcalley wrote:

Guys,

Since I have to deal with differential circuits now, and I am only
use to single-ended, I was wondering if the characteristic impedance of
a differential trace matters. To clarify: If I design differential
traces to terminate into say, 100-ohms differential, I could use traces
that are 90 mils wide with a trace-to-trace spacing of 100 mils to
accomplish this, or I could use traces that were 30 mils wide with a
spacing of 5 mils. Is either OK, even though the characteristic
impedance of each individual trace is significantly different in the two
examples? Would it matter to the circuit at all, as it does with the
characteristic impedance of a microstrip trace in a single-ended circuit
-- or is it only the differential impedance that matters in such
applications?

Thanks!

-Bill
Click to expand...
If you're transmitting and receiving differentially then the trace-ground
impedance probably doesn't matter. It certainly won't matter as much as
the trace-trace impedance. If it's critical you could use a 3-resistor
termination to get trace-trace and trace-ground both terminated
correctly, but I doubt that it'll be critical until you're really pushing
the frequency/length envelope.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html
 
On Apr 9, 7:03 am, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Tue, 8 Apr 2008 21:32:55 -0700 (PDT), billcalley





billcal...@yahoo.com> wrote:
Guys,

  Since I have to deal withdifferentialcircuits now, and I am only
use to single-ended, I was wondering if the characteristic impedance
of adifferentialtrace matters.  To clarify:  If I design
differentialtraces to terminate into say, 100-ohmsdifferential, I
could use traces that are 90 mils wide with a trace-to-trace spacing
of 100 mils to accomplish this, or I could use traces that were 30
mils wide with a spacing of 5 mils.  Is either OK, even though the
characteristic impedance of each individual trace is significantly
different in the two examples?  Would it matter to the circuit at all,
as it does with the characteristic impedance of a microstrip trace in
a single-ended circuit -- or is it only thedifferentialimpedance
that matters in such applications?

Thanks!

-Bill

If you're using a balanced driver and a balanced termination/receiver,
like LVDS maybe, only thedifferential(odd mode) impedance matters.

You can make a 100 ohm diff pair from two independent (ie, uncoupled)
50 ohm traces which, as you note, will be fat traces far from one
another. Each will calculate as having 50 ohm odd-mode impedance and
50-ohm even mode, in other words just a 50 ohm microstrip.

To save room, make the traces skinnier and move them close, so that
they couple. Keep the odd mode z of each trace 50 ohms, so the diff
remains 100. Even mode impedance will go up as coupling increases.
There are calculators available to do the math, like TXLINE. Very
skinny traces will get lossy.

John- Hide quoted text -

- Show quoted text -
Click to expand...

Thanks guys for all those very clarifying answers! I feel I
understand much better how to use balanced microstrip now. So
basically the single-ended impedance doesn't really matter, just so
long as the differential impedance of the traces meets our specs
(because, as stated above, to place a 100-ohm balanced microstrip with
50-ohm single-ended characteristic impedance on, let's say, a 62mil
PCB, would take up a lot of space!).

Cheers,

-Bill
 
On Wed, 9 Apr 2008 19:47:18 +1000 "Phil "dicksplash" Allison"
<philallison@tpg.com.au> shot his load in Message id:
<663hl9F2i7478U1@mid.individual.net>:

"Fred Kruger" <sack@myhouse.cum

"Bob Wanker Congenital Anencephalic "



** Puke, puke puke......


** My god, the vilest scumbags on the planet and all their pathetic
sock-puppets are coming out of the cracks now.
Click to expand...
Go fuck yourself with an 18V cordless drill fitted with a 1/2" hole saw
bit, Phil. You should be brutally flogged using the cured, platted skin of
your own dead mother, be rolled in salt, sand & dog shit and then dipped
in bleach before being battered to death with a two foot black rubber
dildo with a nail through it.

Phewwweeee - what a stench !!
Click to expand...
Gee Phil, that's just your breath blowing back in your face. One would
have thought you'd be used to that by now.
 
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