Is it necessary to use four-layer board for SigmaDSP?
We strongly recommend using four layers in order to maintain signal integrity, clock precision, and EMI/noise reduction. SigmaDSPs operate with clock signals in the range of tens of MHz, and as a general best practice for PCB design, at least 4 layers should be used for mixed signal designs with clock frequencies of more than a few MHz.
Some SigmaDSP users have tried to design 2-layer PCBs and encountered issues related to clock stability, communication failures, and poor analog audio performance.
Here are some SigmaDSP PCB Layout Best Practices:
I've just made a 2-layer board using a 1446, once it's assembled I'll post my findings! I've run no analog signals on it though, has serial breakouts for testing purposes.
Our board with a -1701 and a PIC is also two-layer. It works fine, although it may produce more EMI than we would like (see my post, http://ez.analog.com/message/57200#57200). The engineer who works with me prefers two-sided boards because they're less expensive and have fewer manufacturing issues. He puts a lot of front-end work into eliminating unnecessary crossovers and tight spacings, which pays off later when the products are built.
Here is my fully assembled board (excuse the messy soldering, got a bit eager and couldn't wait for my hot air pencil to arrive)
It fires up fine but yet to test anything on it as I'm waiting for more components for my adc & dac boards.
This is a ADAU-1446 based board with 4 serial ins and 5 serial outs and that is literally it.
Below are top and bottom layers, I have also designed a four layer board but shall await the results of this one first.
2-layer designs will benefit from pouring a complete ground plane on the top and bottom layers, stitching the top and bottom layers together with as many vias as possible. As Bob and Gary have pointed out, parts placement, trace routing and spacing play very important roles; every trace will be accompanied by a current return path, and controlling this path is the challenge. We look forward to seeing the results of your testing. Do you plan to test the EMI differences between the 2- and 4-layer boards? This information would be a great help to the community!
I'm using an ADAU1701 on a 2-layer card, parked next to 4 channels of class D amplification, with no problems at all. Provided you keep the ground plane under the device as unbroken as possible, keep traces away from the crystal oscillator and do a good job of decoupling the power supplies, you should be OK.
Here's the design files for that card, if you want some design inspiration. Board layout was done in Cadsoft Eagle V6.
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