Electronic – Mixed Signal PCB, 2 or 4 Layer

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I'm currently designing a mixed-signal PCB for a client and I've been reading a lot about signal integrity and most of the books recommend a 4 layer board (or more) because of the noise resistance from having a solid ground plane and reduced routing.

The board in question has two 16-bit ADCs and two 16-bit DACs with OP amps in the analog section, a microcontroller with some level shifters and mosfets in the digital section, and two DC/DC converters and an LDO regulator in the power section. Space is not much of a constraint, but having high resolution and low noise in the analog section is important. There's an I2C and an SPI bus running between the digital section and the edge of the analog section, operating at less than 10 MHz.

Routing wise I can totally get this board done in 2 layers. Will I really notice a huge difference in signal integrity with a 4 layer board and dedicated ground plane? Is it worth the extra cost? I'm leaning towards 4 but I'd like to hear your opinions.

Thanks in advance guys.

The Board

Best Answer

first allow me to clearly state my answer. A four layer board will give better performance. The difference is not stark for all boards, but the four layer board will allow you to route signal, power, and ground, directly over each other in a larger variety of ways. Keeping ground directly under the power plane will reduce crosstalk for close proximity lines and reduce noise by allowing overall better routing choices. With four layers, single point ground is an achievable feat without also cutting the ground plane in two. Essentially, if the two layer solution requires cutting across the ground plane, then you are creating two ground planes connected by two points. This allows a current loop, causing noise, and this can be exacerbated by temperature differentials, nearby electronics, etc. I am not sure as to the use case of this device, but the temperature, nearby EMI, and operating frequencies will all play in. If this is a semi-high speed design, then a four layer will definitely save the integrity of those signal lines. A lot of this comes down to how much resolution you need from those ADCs. If you truly need 16 bits, then I would say a 5% performance increase from a four layer board is worth it.

A great reference which I pull my design guidelines from is... http://www.ti.com/lit/an/szza009/szza009.pdf

Cheers