# FEDEVELPlatform forum

## About strange computation of 120 ohm impedance

mulfycrowh , 01-22-2021, 09:46 AM
Hi everyone,

My stackup has 4 standard layers.
I would like to add 120 ohm tracks.
Here attached the result.
I am little bit surprised by the result: 0.018 mm is very small.
I guess an error.
Any idea?

WhoKnewKnows , 01-22-2021, 07:09 PM
For controlled impedance, if the built in trace width calculator calculates that the traces need to be incredibly thin, the you need to change something else in the design, like the dielectric material or the dielectric thickness. Perhaps try sharing the entire layer stackup, not just the impedance calculation portion?
mulfycrowh , 01-23-2021, 04:18 AM
I tried to enter the attached stackup from my manufacturer.
Now it is no more possible to reach 50 Ohm impedance...

Any idea maybe?
Thanks a lot.
goncaloc , 01-23-2021, 04:38 AM
Hi Mulfycrowh

AD19 does no support multiple Dielectric between copper layers, remove one of the prepregs and change de other to 180um
You should have a 300um width for 50R

mulfycrowh , 01-23-2021, 07:18 AM
Seems OK with 50 Ohm impedance but always getting a very small track for 120 Ohm.
I should have something like 0.12 mm.
goncaloc , 01-23-2021, 07:29 AM
I don't think you will be able to have both 50R and 120R on the same layer
To increase the necessary width of a track you will have to increase the thickness of the prepreg, so when you have 0.12mm for the 120R your 50R will be so wide it won't be practical.

mulfycrowh , 01-23-2021, 07:46 AM
My manufacturer made the computation for both impedances with the stackup I shared.
goncaloc , 01-23-2021, 07:54 AM
I see...
The thing is, your 50R are single ended and your 120R are differential and that makes a world of difference.

In altium when calculating your impedances don't forget to set the correct type, single ended or differential

mulfycrowh , 01-23-2021, 08:05 AM
You are right!
mulfycrowh , 01-23-2021, 08:06 AM
But width remains wrong.
goncaloc , 01-23-2021, 08:13 AM
double check your values, I get 120,25R

mulfycrowh , 01-23-2021, 08:21 AM
Here what I've got for 120 Ohm impedance
goncaloc , 01-23-2021, 08:25 AM
You will need to select the layer ( either Top or Bottom ) and the properties will show up, as you already have the values you just need to insert the values on the right place
mulfycrowh , 01-23-2021, 08:37 AM
OK but why AD19 is not able to find the right values?
goncaloc , 01-23-2021, 08:44 AM
The values AD19 is giving you are right, they are values that will solve the equations needed for this calculations, the thing is they are not the only ones.