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uncle_bob
I hate to say this, but at some point in circuit density, a multi layer board is indeed the way to go.
+1, although I don't think it's necessary to go to a multi-layer board for the RAMPS just yet. The main things to control are edge rates and current loops. Note the ringing in the traces above with the 1-2 uS turn-off time and the much worse ringing with the well under 1 uS turn-off. Faster is NOT always better! Exactly the same thing happens with the signal traces having high edge rates. If signal edge rate is a problem on the RAMPS, a few well placed series termination resistors will probably fix things up (nothing on the shield needs to have nS scale timing accuracy anyway). For current loops, there are the obvious high-current paths with the heater FETs, and also the "phantom" high-speed current return paths for the signal traces. It should be possible to arrange connectors such that the heater currents don't loop through the Due grounds but instead flow out the main power connector. A ground plane would help most with the "phantom" return paths for the high-speed signals, but IMHO series termination should generally reduce the current spikes enough that a more "meandering" ground return on a 2-layer board would work OK and avoid serious cross-talk to the ADC lines.