Best method to start routing

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1

I am thinking about the best method to start routing for my design.

I have a schematic with digital ground (GND), analog ground (GNDA), +5V, -5V and +3.3V.
Because the schematic is a large one (for my experience) I want to use autorouting.

I know it is good practice to separate digital and analog ground and connect them only at one point near the ground source from the power supply and to use ground planes (polygons) instead of single lines.
For power supply (+5V, -5V, 3.3V) the autorouter shall generate wide tracks (at least 1mm), but signal tracks shall be thin enough to fit between pins, etc.

Is it possible to tell the autorouter these conditions for digital/analog ground and for different track widths?
(Remark: ERC of the schematic warns that GND and GNDA are connected and only net GND will be used.)

2

Perhaps you need a Net Tie ?

3

That depends on the autorouter you find that can do work. Kicad does not have an autorouter.

4

If analog and digital parts have noting to do with each other than it can be good solution.
But if there are signal lines going from analog to digital or in the opposite direction than if return current for such connection would have to travel to that one point and back then you create a circle that thanks to its large area is sensitive to interference and if there are digital signals then it can emit disturbances better than if it had the return current path just under it.

Currently no autorouter is better then human, I think. But AI is coming…

5

You don’t start very well.
This for example:

Is a 60+ year old fallacy. It’s been deprecated for at least 20 years but it is still stubborn. On itself, sure it works, but it only works properly when no other signals cross the gap between the GND plane, and that is nearly always impossible. In real life a single good quality GND plane (or even two of them on a 4 or multi-layer PCB) is often the better option, and then combine it with physical separation distance between “noisy” and “clean” parts of the design.

For the track width. You can set these up in the net classes.

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6

You might not want to do that! In almost all scenarios, its much better to have a single solid ground plane and proper power delivery.

The idea is that all your signals are coupled closely to their return paths, such that they don’t suddenly see a mismatch and start coupling elsewhere in your system.

Eg. A ground plane and wide good tracks for power. If you have more layers, then ground and power close to your components that need them. (Using decoupling and vĂ­as to reduce impedance)

So for analog, its much better to simply group it together and place it at a distance from the switched, digital stuff. But all in the same plane. That way you can manage crosstalk.

A continuous ground plane also tends to help with EMC.

As far as I remember (I might be wrong) only below 20kHz is where it might not be critical or maybe you have other requirements or specific isolation cases, etc.