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 [EMAIL PROTECTED] -- You think that it is a secret, but it never has been one.
