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Questions relating to designing PCBs
By Bergamot
#32395
My board has a ground plane fill on unused parts of the back.

Are there any real disadvantages to doing the same on the front, to carry 3.3v current?

Both planes are contiguous.
By cfinger
#32432
I can't think of any disadvantages other than taking up space. If you have the room, go for it. Its probably not necessary unless theres a lot of current passing through the trace, and there are components that need exactly 3.3v.

The reason you would include one would be to avoid voltage drops throughout the trace. I'm guessing you don't need it, but who knows.

Colin
By daemondust
#32434
I agree with cfinger that there shouldn't be any problem with it.

It's especially useful if you're using through-hole parts that need 3.3v so you effectively don't have to route power and ground to those chips. It makes it easier for surface mount parts too since you just have to push a via next to the pad (under the chip works well) and route a short trace.
By Bergamot
#32441
Yeah, I forgot to mention that the design is completely through-hole. and that the main reason I wanted to use a power plane was design simplicity.

It seemed to me that it should work from a digital perspective, since it shouldn't matter much what shape the chunk of metal is in as long as it isn't too small, but I wanted to make sure there weren't any analog factors i was overlooking.
By daemondust
#32442
There may be some odd capacitance and inductance that I don't know about between the layers.
By busonerd
#32450
There is capacitance, and thats a good thing [bonus high frequency decoupling. You still need decoupling caps, but planes help]!

Inductance can actually be lower, then trace routed, which is also a good thing. [Low impedance power supplies are good for digital].

Cheers,

--David Carne
User avatar
By bigglez
#32460
Bergamot wrote:Yeah, I forgot to mention that the design is completely through-hole. and that the main reason I wanted to use a power plane was design simplicity.

It seemed to me that it should work from a digital perspective, since it shouldn't matter much what shape the chunk of metal is in as long as it isn't too small, but I wanted to make sure there weren't any analog factors i was overlooking.
Greetings Bergamot,

Ground and power planes can create problems, more so in high speed digital circuits and many analog designs. The issue is adding capacitance that slows down digital edges. The drivers have to work harder to overcome the capacitance, and this increases the required input power (while doing nothing for the overall performance).

Planes can also allow coupling of strong signals into sensitive analog low power nodes. Op Amps and Oscillators in particular are susceptible to ground plane abuse. Here's a snip it form an earlier thread:
bigglez wrote:The most important thing is not to add too much capacitance (with large ground planes) as it may stall the oscillator. Such a problem is very hard to fix (spin the PCB design...)

Next on the list is to keep all other signals away from the RTC Xtal pins for fear of coupling in noise.

Finally, if you are looking for accuracy add a small trimmer cap across the crystal. Once the pads are in place you have the option of deleting the trimmer, but adding one after the fact is not so easy.

Take a look at the Dallas/Maxim app. note number 58, here.

Here's a layout I'm using for the DS1307 (8SOT) version (Battery back up and 1PPS led, I2C bus pull up and matching resistors):
Image

On a double-sided 0.063 thick PCB (BatchPCB for example) the ground plane on the bottom layer will be okay. Thinner boards (or multi-layer with internal ground planes) are a problem.
Read the whole thread here.

Comments Welcome!
By busonerd
#32470
Just wanted to add - ground planes are pretty much standard on high speed digital designs, even though they do add capacitance. The issue is that on higher impedance power supplies [aka, those with no ground plane], you tend to get higher ground bounce, requiring much more local decoupling.

Cheers,

--David Carne
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