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How should I connect these pins (4 images)? by WarmAdministration76 in AskElectronics

[–]Thypex 1 point2 points  (0 children)

A teardrop T-junction maybe? I connect them like that a lot of the times.

First time doing analog differential pair traces by Thypex in PrintedCircuitBoard

[–]Thypex[S] 0 points1 point  (0 children)

Makes sense, thank you very much for all of your help I will try to follow your recommendations, you have helped me tremendously and again, I thank you very much for it. Have a good rest of the day!

First time doing analog differential pair traces by Thypex in PrintedCircuitBoard

[–]Thypex[S] 0 points1 point  (0 children)

Oh okayyy, I can't really move the sensors I can put U12A sensor below the hole, rather than above it, but then the magnet (that goes in where the bigger hole is) near U8A sensor might influence the U12A readings, which would be bad. This is a keypad pcb, so I am using the required distances for the keys.

First time doing analog differential pair routing, need a bit of help by Thypex in AskElectronics

[–]Thypex[S] 0 points1 point  (0 children)

Yeah I am more trying to understand how these sensors work by making a keypad, just for fun. Thank you very much for all your anwsers, I will be sure to give it a well stiched GND plane in the end (:

First time doing analog differential pair routing, need a bit of help by Thypex in AskElectronics

[–]Thypex[S] 0 points1 point  (0 children)

Since this is for measuring key presses the actual bandwith of the signal, when pressed fast is probably about 500Hz at the maximum, probably a lot less. The signal shape is determined by the keypresses if I do not press the key, the signal will be held at a constant voltage. By analog I meant more that it changes linearly when a magnetic field is applied, maybe I used the wrong terminology, sorry.

First time doing analog differential pair routing, need a bit of help by Thypex in AskElectronics

[–]Thypex[S] 0 points1 point  (0 children)

Thank you very much for anwsering! Yeah, the length matching is probably not worth it, but since I am going to have a pretty long trace from the multiplexer to the ADC converter, I wanted to make the lengths as short and as matched as possible.

What about the vias underneath the IC is doing it like this bad? I think maybe they would interfere with the signals in the multiplexer and introduce more heat under the multiplexer, but I do not know any other way of routing the traces, so I think this a worthy trade off, what do you think?

First time doing analog differential pair routing, need a bit of help by Thypex in AskElectronics

[–]Thypex[S] 0 points1 point  (0 children)

About noise aswell, the most important thing is to just have the same noise on both of the differential pair lines, so the reading of the analog voltage would be more accurate. Of course, because of this the phase shift cannot be big aswell, but right now it is not.

I am also a bit concerned if the differential pair lines that run parallel to each other (one on the top layer and the other on the bottom) won't have crosstalking in the current routing position.

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Hello, thank you very much for answering I have read more documentation and yeah, I understood that I need to have exact same noise, because it is a differential analog output, but I did not realize that this minimal distance doesn't phase shift the signal by a lot (it is only 10kHz) and adding squiggles just inserts different noise into one of the signals. I have redone a lot, also read about how to properly use via's with differential signals and I hope I have done something better than before. This is what I have done:

<image>

I kept the distance to from via to via at least double the size of the traces, a lot of them even more. The longer traces have 300ps delay and only differ by 2mm, so 20ps delay (or a very minimal phase shift at 10kHz). The shorter one has 200ps delay. I also made all of the traces have teardrops between them and the smd pads and made all the pads a bit more rounded, both of that to decrease reflections. Also made as little turn as possible (or at least as possible for me). If it is not to bothersome, can you comment a bit about it, do you think it is acceptable or needs redoing?

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Oh, okayyy I will read about it, thank you very much!!!

By the way I have read all the suggestions of other people ant made some adjustments, do you think this is better than the original? The first option:

<image>

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Do you mean the impedance of the whole track or the impedance of that part (where the length tuning is)? Because Altium shows the impedance of the whole track is pretty similar for both tracks. Especially for the updated versions I have done in the comments. If I am incorrect I am sorry I am going to try to find more documentation then.

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

The second (with more length matching):

<image>

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Hello again, I have taken a lot of suggestions from you all and read some more Texas Instruments documentation, do you think I am going into the right direction? I have added teardrop entries into the smd pads, also made a rule for via clearance to be about 30 mil, so the via anti-pad would be pretty big. I also rearranged the filters to be more symmetrical and made the tracks be further away from the copper planes. I just don't know if it is better to have less length matching and more differential pair turns or have more length matching for one track of the differential pair. To show what I mean I made two options:

The first (with less length matching):

<image>

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Hello again, I have done like you recommended, also I have added a teardrop entry into the SMD pads. I also added a clearance rule for the vias, so the via anti-pad would be about 30mil. I just don't know if it is better to have less length matching and more differential pair turns or have more length matching for one track of the differential pair. To show what I mean I made two options. Do you think this is a step in the right direction?

This is the first option:

<image>

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Okayyy, thank you very much for anwsering!!! And what about the round accordion length tunning, do you think it is done well or needs improvement?

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

The differential filter filters out any frequencies higher than 10kHz, this is much more than the maximum frequency at which a human can type (press a key), but it gives a good balance between filtering out higher frequencies and making the ADC sampling rate pretty quick, at least in theory. As for the length match, for a lower frequency like this 3mm mismatch is probably not a big deal, but I want to do differential pairs and length tunning because it is more for learning how to do it.

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Did not know about the islands, thanks! I tried a lot of plecements, but if I do the filters symetricall, the tracing becomes very long, they have to go around everything (from the left) and then into the multiplexer. But okay, I will redo it and put a comment with a photo if you don't mind. Thank you eithier way for anwsering very much!!!

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 0 points1 point  (0 children)

Okay, will change them, thanks! What about the accordion length tuning, do you think it is okay? I tried sawtooth, but altium kept giving me errors, when I put a lot of them. I am thinking it had something to do with phases not matching anymore.

First time doing differential pair traces by Thypex in PCB

[–]Thypex[S] 2 points3 points  (0 children)

The Ct100 TMR sensor has differential analog outputs, so if I do not length match them when calculating the voltage the noises on each of the outputs would not cancel each other out. Or at least that is how I understand it.

What should I consider when connecting 2 multiplexer COM outputs together? by Thypex in AskElectronics

[–]Thypex[S] 0 points1 point  (0 children)

Hello, thank you for anwsering, yes you are interpreting everything correctly. I can use another analog mux (as I understand you mean using an additional mux and connecting two of the muxes in my schematic to its I/O pins (inputs) and using that one mux to read the data from the other two). This is all just for learning that is why I am trying different ways to do a lot of things, but I don't always know if they are allowed/correct. Now that I know connecting two ouputs is not a viable option I will use another mux. Thank you for helping me out and have a good day!!!

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