Re: gEDA-user: General ground/power plane design questions

[Mark Rages <markrages@mlug.missouri.edu>]

On Wed, Oct 20, 2004 at 09:41:40AM -0400, Randall Nortman wrote:
> Unlike my last plane question, this has nothing in particular to do
> with geda or pcb -- I'm soliciting advice about PCB design in general.
> I've been following this list for a while, and there seem to be quite
> a few knowledgeable people here, which is what makes me think this is
> a good place to ask.  However, feel free to tell me to take it to
> another forum (suggestions welcome), or simply ignore me.
> 
> I'm designing a simple microcontroller board -- 4-layers, with ground
> and power (+5V) planes, to reduce noise.  The board will have a 16MHz
> AVR microcontroller, which is by far the highest frequency that ought
> to be going over any traces.  There are also analog inputs feeding
> into peripheral ADC's, connected to the AVR via SPI (running at less
> than 1MHz; probably 125kHz), some other peripherals on the I2C bus
> (running at 100kHz), and two serial interfaces (one RS232 and one
> RS485) running at less than 100kHz.  Almost everything will be surface
> mount, with the exception of connectors, jumper blocks, and some
> low-power solid state relays (kept well away from noise-sensitive
> stuff).
> 
> The good news is that I'm measuring what ought to be very
> low-frequency signals on the analog inputs (I don't care about
> anything over 10Hz, really), so I'm going to fake a low-pass filter by
> averaging multiple samples together.  (I want to keep the number of
> discrete components down, so I think this is better than an actual RC
> low-pass filter, though perhaps I'm missing something important.)  So
> what really concerns me is not noise on the analog portions, but noise
> on all the digital lines -- SPI, I2C, and the two UARTs.

How much resolution do you need from the ADC? If it's just 8 or 10 bits,
don't worry about the layout so much... you're not building a satellite
radio here, if you punch in "speed of light / 16e6 hz" to google you
will find the shortest wavelength is about 18 meters.  If your board has
traces approaching 1/10 of that length, you might start to care about
things like radiation and distributed capacitance.

You will have the most problems with ground loops I expect.  Setting up 
a start ground system might be better than a ground plane. Put 10ohm 
resistors in series with power to any chip you think might be noisy.  
This will clean up the power bus.

About ADC grounding, I think Bob Pease discusses this in 
"Troubleshooting Analog Circuits."  I'll look at my copy tonight if I 
have time, and post his suggestions.

Regards,
Mark
markrages@mlug
-- 
You think that it is a secret, but it never has been one.