PCB Grounding and Power Tracking Questions

Hi,

I have been reading up on grounding and power tracking in PCB design, and want to put a few dumb questions that I'm not sure on yet...

My questions are in context of relatively low DC voltages/current on a 2-sided PCB (say, 15V@3A max), connected with an arduino to supplying it with voltage and break out pins to connectors, with no radio or high-speed comms.

If I have a circuit that includes a DC supply and a Battery (with IC-controlled charging and powerpath control), their 0V/Ground terminals should/must be connected, right? I've been confused by talk of 'isolation', which I assume refers to using mosfets, diodes, etc to ensure their positive voltages are isolated.

I've also seen reference to 'separate power supply returns' (i.e. 0V). In a low voltage/current circuit as referred to above, is this needed for on-board step-down regulators, battery ground, etc? Does it mean having a star topology to a common point, or is just having a ground plane on each layer of the PCB, with these connected at the nearest point OK?

While I get the basic concept of ground loops, I'm confused about it within a circuit. Taking an arduino shield as an example, if I connected the shield to more than one arduino ground pin, would that potentially cause a loop? Looking at the ethernet shield, it appears to do this.

Also, how 'bad' is it to use vias to join ground planes together? I'm trying to avoid this when routing, but have a busy PCB and it's doing my head in.

I've seen talk of it being desirable to avoid crossing signal and power tracks. Is this really going to matter in this context?

Is it advisable to add a small resistor on the 0V DC connections from battery and DC supplies? I saw this suggested to 'prevent ground current flow'.

Lastly, if using a metal enclosure, or a plastic case with a metal chassis, is it good/bad/indifferent to connect the PCB's ground to the chassis (in one place only)?

Thanks in advance...

Cheers,

Lee

A single point of connection will also help with the ground current flow.

You may want to read Atmel application notes AVR-040 EMC design considerations and AVR-042 Hardware Design considerations. Of course they apply to Arduino. They might answer more questions than you stated You can download them from Atmel's website for free.

Udo

Thanks Udo,

That looks pretty good. Will read through tomorrow.

Lee

relatively low DC voltages/current on a 2-sided PCB (say, 15V@3A max),

3A is not a low current, it can cause a lot of problems and is in the medium range.

've been confused by talk of 'isolation', which I assume refers to using mosfets, diodes, etc to ensure their positive voltages are isolated.

No that is not what isolation means. Isolation is the complete separation of both positive, negative and signals between two circuits. Isolation is not normally a circuit requirement. Don't confuse isolation with decoupling.

if I connected the shield to more than one arduino ground pin, would that potentially cause a loop

It depends, a ground loop is only a problem if large currents flow round it (greater than half an amp). This ha the effect of making the ground different potentials at different points.

Is it advisable to add a small resistor on the 0V DC connections from battery and DC supplies?

No in digital circuits it is a very bad idea. It is sometimes used on the output side of audio circuits where it is known as a round lift resistor. Even then it can cause trouble when interconnecting equipment.

is it good/bad/indifferent to connect the PCB's ground to the chassis (in one place only)?

Yes,
it can be good, bad or indifferent depending on exactly what you are doing.

Thanks Mike, that all makes sense.

For anyone else with similar questions, the Atmel paper Udo linked is very useful, as is reading relevant parts of the Art of Electronics and the Circuit Designer's Companion.