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Yeah toner transfer drove me up the bend.
Just a note on grounding, a ground must never have a high-impedance connection, whichever paper said that stop reading it. I have a bit of history in mixed signal and low noise design.
The principles of high-impedance links are designed to prevent signals from flowing between different grounds. Unfortunately what happens is if a signal does flow over this link it raises the potential of the ground from the side it's coming from. Your digital ground may suddenly float a few volts above the analogue ground leading to very screwed up responses in mixed signal devices like digital to analogue converters.
Where high *resistance* connections are useful is if the ground current will only flow in case of a fault condition and the fault condition needs to be detected. e.g. in my power amp I have a 10ohm resistor connecting my signal ground to my chassis as there should not be any current flowing unless something goes bang, but none the less I need them connected to prevent the floating ground from causing it's own noise, and if something goes bang I want it to trip the earth fault detector in the house.
The solution is to filter out the signal of interest at the point where grounds connect. In this case you could for instance put capacitor banks on either side of the resistor, or substitute the resistor with an inductor (high impedance at higher frequencies, but low impedance at DC).
In mixed signal design routing the ground plane creatively is the best way of keeping signals and noises separate. It is all 100% about looking at the impedance loops. Return signals travel along the path of least inductance, which is often as close as possible to the original signal which went out. The picture I've attached below is a classic mixed signal device like a DAC with digital on one side, and analogue on the other.
The idea is to route slots down to the point where the signals get mixed (straight through the chip) and connect the grounds there, and also connect the powersupplies there. Keep all the digital signals on the digital side and the analogue on the analogue side. If ever they need to cross via some device they should either be isolated or cross at the one point (kind of like your star ground).
If you do this you effectively limit any mixing of the signals on the plane as you can see by the cream lines. If you do however cross then the signals need to travel back via the wrong side of the plane which is bad and will lead to contamination for want of a better word. When circuits get large it becomes very hard to route, even harder if you can't dedicate an entire plane to ground.
It's more of a black art than anything else. But if all else fails just remember the word star ground. Return all your grounds to one big very low impedance point and you're about 90% of the way there. Chasing that last 10% will drive you mad.
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