Design Guide: Basic principles in PCB design

Submitted by Antonb on February 5, 2010 - 9:14am.

Design Guide:

Basic principles in PCB design  

 

 

I thought this could come handy. EMI and power surge problems are common in digital circuit design. The following list is a compilation of suggestions I have been putting together as a reference for my projects, that can be employed in the PCB design stage and reduce EMI emission / reception as well as improve power distribution:

  • Clock signals are designed first placing the oscillator as close to the IC as the design allows. Clock traces much like any high-frequency signal are a great source of noise and therefore have to be designed as short as possible avoiding 90° bends.
  • Decoupling capacitors (100nF) are used to connect power and ground pins on every IC. These act as power reservoirs for the fast switching logic and must be placed as close as possible and on the same side as the IC. 
  • Analog signals must never run in parallel to clock and/or communications signals.
  • Power planes are utilised to reduce local potential differentials and also tend to cancel-out noise. A ground trace around your board will partially shield it against incoming EMI. In 4+ layer boards is a good practice to sandwich all signals between two ground planes.
  • It is preferable to use separate power and ground delivery paths for the analog part of your PCB and for each analog IC. The analog return path must be linked to the source via a unique point, as close to the source as possible. Care must be taken to avoid overlapping between digital and analog planes.
  • Inverted Schmitt-triggers can be used on all digital sensor inputs in order to address issues like switch bounce and temporarily fluctuating/floating inputs.
  • Tantalum capacitors are used since they come in a smaller form factor than their electrolytic counterparts, still able of accommodating appreciable ripple current amounts. As a rule of thumb these should be rated at least twice the operating voltage. Increased overall capacitance is of no harm to digital systems; on the contrary it also offers a temporary energy bank, addressing transients in power demand, often present in fast switching logic.

 

This is a good point to mention other measures that can be taken as an effort to avoid EMI problems in other parts of your system:

 

  • For motor suppression a 10-100nF capacitor can be connected across the motor’s leads. Higher capacitor values will cause the capacitor to appear as a short across the leads. An additional pair of capacitors is often used to link each of the motor’s leads to the motor’s casing.
  • Separate cables into 3 groups: analog, digital and power, in order to avoid cross-talk between them.
  • All analog cables must be shielded. The shield must be grounded.
  • Motor drivers and microcontrollers should use different power sources.
  • Analog and digital signals must never use the same connector. If this is unavoidable due to space constraints, the analog signals must be separated from the digital ones by a series of ground pins.

 

Some notes on topology:

 

  • Avoid placing components with different orientations on the PCB; preferably orient all IC’s in a similar fashion and all polarised caps with the +lead in the same location. This can help in avoiding mistakes and is particularly important in mass-production automated assembly where the robotic part-placement arm has to adapt to each part’s angular orientations. In other words having all components with the same orientation can save on production times.

  • Where applicable, try and group the connectors according to their function and place them near the edge of the PCB. Separate inputs from outputs and use LED status-indicator lights.
  • Avoid using male power-in connectors in order to minimise the possibility of accidental high-current shorts due to stray tools etc

 

For a PDF version of this document visit:   http://www.01mech.com/open-engineering

 

 

 

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