Picking a Fabrication Facility

After designing our analog front end and picking our ICs, we began by creating a schematic for the board using the ExpressPCB design tools.  We elected to use ExpressPCB for two reasons: quick turn around and previous experience.  ExpressPCB advertises a 1 week turn around time on their boards, making it ideal for us on a tight time schedule.  Additionally, we also used them to fabricate our board for CSE462, meaning we were already familiar with their software saving us even more time. 

Board Layout

A major challenge in designing mixed-signal circuitry is maintaining signal integrity
throughout the board. There is usually an abundance of noise sources. High frequency switching in digital circuits can cause electromagnetic interference (EMI) in all surrounding circuits. Some environmental noise is always present, but the issue can be exacerbated if large loops are put in different parts of the circuit. Additionally, keeping digital and analog power and ground rails from interfering with each other can be difficult. Therefore, designing the board with as little noise as possible becomes a very tricky task.  In general we followed these design guidelines: 

Expand Guidelines

  • Do not route signal traces beneath parts on the top layer.

  • Try to make traces as short as possible to reduce the signal’s vulnerability to EMI.

  • Route signal traces on different layers perpendicular to each other (reduces crosstalk).

  • Maximize the spacing between traces (reduces EMI).

  • Place bypass capacitors as close as possible to IC pins.

  • Minimize the size of loops in the circuit.

  • Put the clock as close to clocked ICs as possible.

  • Minimize the number of vias in the signal pathway (to reduce capacitance).

  • Group together parts of similar function (reduces noise and complexity).

  • Use wide traces for power transmission (low impedance, low power consumption).

  • Use the smallest trace size possible for signal traces (for higher impedance).

When you combine all of these with the limitations imposed by the board house (maximum of 4 layers, no right angle traces, etc.), it became very tricky to get the components into their necessary sections and then route the traces accordingly.  Additionally,  the board house only puts silkscreen on one side of the board, which discouraged us from using the backside for any major ICs.  After a few weeks of board designs, design reviews, and revisions our design was done.  The final design from the ExpressPCB tools is shown below.