Ground?

bambino

I have an App that uses an ezLCD and I am laying out a board to backpack on the screen. The connector has 4·analog Ground·pins·and 1 Signal ground pin. The prop and the LCD will use the same Battery, but use different regulators. The Prop communicates with the LCD via· TTL rs232. What is the best way to tie in the Props ground lines.

I can post a schematic of what I have later tonight, but maybe someone knows already!

Mike Green

The logic ground has to connect with the Prop's ground. The analog ground and the logic ground need to be connected together, but only at one point, often at the power supply for the analog circuitry.

bambino

Mike is this close to what your saying or do the prop gnds need to connect at the same point as the signal(Logic) gnd?

bambino

Yes, I'm lousy in Paint. Here's the schematic with the ground traces in question.
Is this the proper oreintation for the logic and analog grounds?


Please ignore the missing passive components. I've yet to create component pads,because I'm not sure what kind I'm going to use yet!

Mike Green

Unfortunately, the routing of the ground wires on the schematic has little or nothing to do with the physical layout on the PCB which is what is important. I suggest you look at the physical layout of the parts on the board. The ground leads on the voltage regulators are really the reference for the device since the +5V and +3.3V are relative to that. Truthfully, you have a physically small device with only a few parts and the ground traces are likely to be short, maybe a few inches. It probably doesn't matter what you do. Ideally, you would run the power ground (all connected together) to the ground point of the regulators. You would run the logic ground to the Vss line of the Prop chip and on to the regulators and other parts needing logic ground. You run the analog ground separately to the ground point of the regulators and it has its own bypass capacitors.

bambino

Thanks Mike, I beleive it sunk in that time. I allready have a protoboard doing this task, but making a board gives me alot of options.

OwenS

With analog power, there are three main things to do:
1) Avoid ground loops
2) Avoid noise on the analog supplies (I.E: Nothing digital should be connected to it, and it should preferably both the digital and analog supplies should branch from the main supply directly after the filtering).
3) Minimize resistance between any two analog grounding points to as close to 0 as possible.

The main thing to consider is avoiding anything which causes the voltage of a supply rail to change.