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
