# Differential trace characteristic impedance question

B

#### billcalley

Jan 1, 1970
0
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

P

#### Phil Allison

Jan 1, 1970
0
"billcalley = Shit For Brains Cunthead "

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.

** What sort of " differential circuits" are you on about ?

Kindly supply a link to an example.

BTW

Please be more vague in future.

****.

....... Phil

To clarify: If I design

P

#### Phil Allison

Jan 1, 1970
0
"Bob Wanker Congenital Anencephalic "

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

P

#### Phil Allison

Jan 1, 1970
0
"Terry Given ASD Fucked Pile of Kiwi Scum "

** Same as all the others.

....... Phil

D

#### David L. Jones

Jan 1, 1970
0
billcalley said:
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

It ultimately depends upon the receiver topology used, but differential
impedance usually does not matter. It's single ended that matters.
See here:
http://www.speedingedge.com/PDF-Files/diffsig.pdf

However, it is possible (but sometimes tricky) to design your tracks to meet
both 100ohms differential and 50ohm SE if you are that way inclined.

If you look at some app notes for the high speed diff interfaces on various
top end FPGA's (like Xilinx), you'll find similar guidelines. 50ohm SE is
what is needed.

Dave.

F

#### Fred Kruger

Jan 1, 1970
0
"Bob Wanker Congenital Anencephalic "

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

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.

P

#### Phil Allison

Jan 1, 1970
0
Fred Kruger said:
"Bob Wanker Congenital Anencephalic "

** My god, the vilest scumbags on the planet and all their pathetic
sock-puppets are coming out of the cracks now.

Phewwweeee - what a stench !!

....... Phil

B

#### billcalley

Jan 1, 1970
0
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 -

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

F

#### Fred Kruger

Jan 1, 1970
0
** My god, the vilest scumbags on the planet and all their pathetic
sock-puppets are coming out of the cracks now.

Go **** 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 !!

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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