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Author Topic: Ground Plane Decisions  (Read 1604 times)

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Offline want2learnTopic starter

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Ground Plane Decisions
« on: January 11, 2009, 03:33:23 AM »
Hi,
 As I mentioned in another post I'm building my first board for use with I2C. This board is a 5V@ 8A +3.3V @ 1A power supply with battery switching from master battery to backup battery and battery meter (fuel gauge). I'll be using an at 45 or similar for the I2c comms and a couple other things (I haven't decided on the final uC at this point but I have most of the materials to begin with)

I have ground planes on the topside of my board around the high frequency lines and same on the bottom side of the board. I'm also using a ground plane on the bottom side for additional heatsinking.

My question is: Should I include more ground planes around the uController and smoothing caps for the outputs or will I be fine without them?
Quality of the output voltage and low crosstalk is whats more important to me, I don't care about wasting etchant etc.
I don't have a schematic for you at this point as I mainly do mine by hand, same for my pcb designs ;I find it much easier this way

EDIT:- I also want to make a shield to cover the topside when it's built to reduce radiated EMF, should I tie this to the ground planes or attach it straight to the -ve supply (via a diode)
« Last Edit: January 11, 2009, 03:42:48 AM by want2learn »
The question that drives me hazy:

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Offline dunk

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Re: Ground Plane Decisions
« Reply #1 on: January 11, 2009, 06:16:32 AM »
with the components you list there should be no particular problems with cross-talk or EMF.
just use the smoothing caps and i2c pull-up resistors recommended on the AVR datasheet and you should be fine.
leave a ground plane where it is convenient but don't put too much effort into it. the AVRs i2c implementation is quite forgiving and you don't appear to have any other high speed signals.

dunk.

Offline want2learnTopic starter

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Re: Ground Plane Decisions
« Reply #2 on: January 11, 2009, 12:08:17 PM »
Thanks dunk.
 I'm not 100% sure on ground planes and figured I'd best check before I finalise my design. Don't want to induce too much stray voltages but want to give adequate shielding
The question that drives me hazy:

Am I, or the others crazy?

Offline Admin

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Re: Ground Plane Decisions
« Reply #3 on: January 26, 2009, 01:50:11 AM »
For a one-off design its easier/better to play it safe and just add the filtering caps.

You don't want to have to redo the design or struggle debugging it later.

Intense mathematical design/optimization should be left for things you want to turn into a product ;D

Offline want2learnTopic starter

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Re: Ground Plane Decisions
« Reply #4 on: January 26, 2009, 04:04:05 PM »
For a one-off design its easier/better to play it safe and just add the filtering caps.

You don't want to have to redo the design or struggle debugging it later.

Intense mathematical design/optimization should be left for things you want to turn into a product ;D

Yeah I've already finished the prototype, thats the board I've been been having problems with the I2C on.
I omitted the ground planes around the filtering caps and thus far the output voltage has between 80-90mV ripple on it depending on the load. I didn't really know if ground planes around the caps would've helped here or not, I also wanted to eliminate as much noise as possible from the I2C lines between the fuel gauge and uC, and between the uC and the output I2C comms.

If I ever get the board to work as I had planned I'll post the files (still got to do them yet, as previously mentioned I do most of mine by hand).

The projected goal here was to make a power supply board bigger than most people needed with added functionality over a basic psu while trying to stick to the guidelines on dunk's google page. I had planned for a single board to stack onto this one to give battery charging facilities too (after someone checked my maths ;D). I've already done a couple of simple boards to the guidelines but think I bit off more than I could chew programming wise this time.

The only other problem I've had with this board is ALL the components are SMD, the guidelines ask for through-hole versions too. Can't find any :)
The question that drives me hazy:

Am I, or the others crazy?

Offline Admin

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Re: Ground Plane Decisions
« Reply #5 on: January 26, 2009, 09:59:33 PM »
80-90mV ripple? Thats pretty large . . . at approximately what frequency? Try adding a 1uF, a .1uF, and a 100nF. Higher the noise frequency, smaller the cap value. You can probably find a chart somewhere that matches capacitor to frequency if you google around.

Offline Soeren

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Re: Ground Plane Decisions
« Reply #6 on: January 27, 2009, 08:48:17 AM »
Hi,

[...] Try adding a 1uF, a .1uF, and a 100nF.

Eh?
0.1F is pretty darn close to 100nF in my book :P
Regards,
Sren

A rather fast and fairly heavy robot with quite large wheels needs what? A lot of power?
Please remember...
Engineering is based on numbers - not adjectives

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Re: Ground Plane Decisions
« Reply #7 on: February 07, 2009, 07:32:37 AM »
[...] Try adding a 1uF, a .1uF, and a 100nF.

Eh?
0.1F is pretty darn close to 100nF in my book :P

lol yea . . . oops . . . ignore the 100nF . . .

 


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