Design decisions for mixed Audio and MCU project - Page 1
Forum section: electronics / beginners

author: obiwanjacobi
author_rank: Super Contributor
timestamp: June 11, 2012, 10:20:39 am
content: Hi,Another beginners questionI'm doing a project that concerns a MCU and an audio circuit (opamps and switches). I've heard people talking about making a separation between the two in the board layout. Also talk about splitting the ground plane...? I (think I) "understand" the reasons why, I just don't know how that expresses into board layout / design.What would be the things to watch out for when laying out a board that has Audio and MCU (16 Mhz) components on them?Thanks,Marc

author: DrGeoff
author_rank: Frequent Contributor
timestamp: June 11, 2012, 12:24:16 pm
content: Separate the power supply rails, both power and ground rails, joining only at the psu itself, usually through a ferrite inductor. If you are using planes then have separate planes for the analogue stuff and separate ones for the digital stuff. This should prevent noisy switching currents from digital circuitry getting into your analogue supply rails.

author: obiwanjacobi
author_rank: Super Contributor
timestamp: June 14, 2012, 07:38:41 am
content: Thank you.I read that using ground planes for audio might not be a good idea. For the MCU stuff it IS a good idea.Is the power rail the only part I need to consider?Anything else?

author: T4P
author_rank: Super Contributor
timestamp: June 14, 2012, 08:45:50 am
content: What you have been answered probably had been from aaudiophoolagainAnyway, ground planes for audio is perfectly fine, if any problems there isn't any plus it helps to keep noise low

author: obiwanjacobi
author_rank: Super Contributor
timestamp: June 14, 2012, 08:56:02 am
content: From "When not to use a ground plane"Quote from: Rerouter on June 10, 2012, 05:05:16 amQuote from: free_electron on June 10, 2012, 06:49:16 pmQuote from: ejeffrey on June 10, 2012, 07:59:38 pm

author: codeboy2k
author_rank: Super Contributor
timestamp: June 14, 2012, 09:37:35 am
content: Minimize Noise in Audio Channels with Smart PCB Layouthttp://www.maxim-ic.com/app-notes/index.mvp/id/4079The key is that the digital section needs a ground plane, the distributed capacitance there will help reduce noise. The audio side should have it's own star ground system.The maxim app note also talks about a ground plane for the audio, with a single exit point returning ground currents to the star ground point, but I don't like it. I am with free_electron here.. a ground plane in the audio section can cause unwanted oscillations. I worked in audio in the past (radio station gear) I always used a star ground. With line level audio you don't need to worry too much about large ground currents changing your ground potential, but with the power amplifier sections you do. But I almost never used a ground plane on the audio side, but everything came back to a single point, and it was always near the lowest potential on the board. That's usually right at the point of entry for the power, or nearby the smoothing caps if they are on the same board.

author: jahonen
author_rank: Super Contributor
timestamp: June 14, 2012, 09:51:31 am
content: If it helps to make a decision, I used a multilayer board for a8 channel digital volume controllerwithout any issues. All traces which carry the audio signal, are placed directly upon the continuous ground plane, and dielectric thickness is in order of 0.2 mm which is much less than with ordinary 1.6 mm thick two-sided PCB. Trace widths were also 0.2 mm. Close proximity of the ground plane also helps to control the crosstalk between traces. There are no evident crosstalk problems even if the closest spacing between those audio traces between input buffers and PGA2320's is only 0.2 mm. At least up to 200 kHz.No stability problems whatsoever and noise immunity is absolutely wonderful. The result is usually much more robust than just wires hanging in the air (PCB without continuous ground plane under traces). The most important thing is to ensure that parts are placed correctly, so there are clearly own sections for analog and digital.There might be some issues with current feedback amplifiers (feedback nodes and output pins of those are sensitive to reactive loading, but I guess these special situations are easy to note and to take care). With normal audio opamps, like OPA2134, no problems.Regards,Janne

author: JuKu
author_rank: Frequent Contributor
timestamp: June 14, 2012, 11:59:19 am
content: There is no such thing as ground. There are only supply and return currents. But for convenience, let's call a common reference plane "ground". What many people don't realize is that currents don't flow arbitrary ways in ground planes. Instead, they automatically find the optimum return route. The high frequency noisy return currents don't pollute the sensitive audio return currents, provided that the supplies are correctly separated. When you route ALL signals correctly, an unbroken ground plane automatically gives you the optimal star ground.In practical terms: Have a solid ground plane. Do not split it; best case would be that the split does not hurt you. Most likely, there would be at least some penalty. Be very careful how to route digital and analog signals. Take any digital signal away from the audio side as short route you can. Consider any digital signal as a potential aggressor to audio. If you can't avoid crossing, make the crossings in 90 deg. angles, don't run wires in parallel. Consider system wide design, too. Make audio side control lines active only when necessary. For example, while it could technically be possible to use one i2c control bus for the whole system, it is much better idea to have two or more buses in a system to keep audio side quiet.In two layers, this would be hugely more difficult. But consider: A four layer board is usually only 30-40% more expensive than a two layer board, but also easily 30-40% smaller. Since I've realized that, I have done very few two layer designs.And the above is valid only for a single box. For real system (different products connected together) there is no good universal solution (read about "pin 1 problem).

author: obiwanjacobi
author_rank: Super Contributor
timestamp: June 14, 2012, 12:47:25 pm
content: Thanks for your remarks. Amazing to see so many different "opinions" on how stuff works and what is "right".The way I'm now going with this is to make two separate boards. This also allows me to plugin more Audio (Switch Matrix) boards with a single MCU controller board (I like modular design). And I think I bypass a good chunk of the problems with ground planes (or no) etc. All will be mounted into a 19" 1U or 2U rack case, so there no real space requirement.My current design uses a kind of extended SPI bus (SPI plus one extra line) for the audio board and the chip-select is handled right at the beginning. So no SPI signals (meant for other components) travel far into my board. The audio board has connectors for audio input and output, SPI control and Power Supply.

author: T4P
author_rank: Super Contributor
timestamp: June 14, 2012, 04:07:20 pm
content: Yeah, 4 layer boards are easier to route, no matter what the "audiophools" saySome might even say "1 layer boards have superior sound quality!"BullshitI just finally realized after using eagle for 1 year that i can do 16 layer on it