Impedance matching for MIPI CSI-2 on a flex PCB Help  - Page 1
Forum section: electronics / general

author: EricNava
author_rank: Contributor
timestamp: October 25, 2024, 07:26:48 pm
content: Hey, I am in need of some help figuring out how to calculate the impedance of some traces on a flex PCB i'm working on.Background:I am trying to design a custom raspberrry Pi camera cable since I want a cable that turns 90 degrees like this:Raspberry Pi cameras use MIPI CSI-2 which uses multiple "lanes" of differential pairs. From what i've read, the impedance needs to be 50 ohm single ended and 100 ohm differential.So I need to make sure the impedance of my cable is correct and that the traces are length matched.The stock camera cables from raspberry pi use a single copper layer. The differential pairs are routed in a way such that there is a GND track, diff signal+, diff signal-, GND. There is is no GND copper under the tracks, so this is like a differential coplanar waveguide with no bottom reference.CalculatingI am hoping to use a cheap Chineese PCB manufacturer, however PCB way and JLC both only allow one core thickness for their flex PCBs, 25um.First I have tried calculating the impedance using a microstrip differential pair configuration with a bottom ground reference.Since 25um is very thin, this leads to trace widths that are too small to be manufactured.So I think I should use the same setup the stock cables have, a differential coplanar waveguide with no bottom reference.Using this configuration, I get some acceptable values for the differential impedance.The problem is, I need to also make sure the track widths I just calculated give me the 50 ohm single ended impedance I need.There are calculators for single ended coplanar impedance with no ground reference, but I don't know if they would work in my case because they assume the reference trace is a equal distance on both sides to the signal trace.In my case each signal trace has a ground reference close on one side and the other side is the other signal trace and then ground, so ground is not an equal distance on both sides.Here is an image trying to explain what I mean.ConclusionSo with that all said, am I missing somthing here? I don't see how to calculate the single ended impedance of trace in a diff pair the coplanar waveguide.Hopefully that rambling makes sense.

author: jduncan
author_rank: Regular Contributor
timestamp: October 25, 2024, 10:57:41 pm
content: Page 182:https://gctjaipur.wordpress.com/wp-content/uploads/2015/08/coplanar-waveguide-circuits-components-and-systems.pdfGet ready to calculate some hyperbolic trig functions.I would just YOLO it.

author: EricNava
author_rank: Contributor
timestamp: October 26, 2024, 03:30:29 am
content: Oh lord math was never my strongest subject :'(Thank you for the link though that document is impressive!I did get a suggestion from another forum to actually try using a different layout.Instead of GND,D,D,GND, I could add another guard rail to make each track in the differential pair it's own coplanar waveguide.Like this: GND,D,GND,D,GND.Since the differential impedance is 100 ohms the two data lines should be coupled as little as possible, meaning just aiming for the 50 ohm single ended impedance with as little coupling as possible should be fine (™).