Mixed Signal Engineering
Mixed Signal circuits consist of at least one analog and one digital part. Each
on it's own can be a rather tricky task, but together they bury caveats for
the unweary. Both parts interact not only at their defined interface points
but at many others, too. Engineers try to tackle that problem by extensive
simulation, but simulation can be only as good as the models and methods are.
Detailed models and thorough methods are preferable, but they drive up the time
for a simulation run. So most of the time simpler models and methods are
used to get the job done in time, bringing in the risk of missing important
The best way is to design a mixed signal device in such a way that it is doing
what it is supposed to and only this - the approach of "guaranteed by design".
Although this makes the task much more demanding it is the only way to be
sure that the product will perform as intended and without any quirks. Simulation
can then be used to verify the correctness of the approach and can be run with
the best methodes and exactest models, because it is run only once (if the design
is really flawless) or at least only a few times. Here are some ground rules to
give you a starting point:
- Keep the digital and the analog part(s) as far apart as possible on your
printed circuit board.
- If possible, create just one analog and one digital section on the PCB.
Else cluster functional sections together and form islands of them on the PCB.
- Keep the border lines between the analog and the digital section/islands
as straight and short as possible. Avoid "peninsulas" of one type reaching
into the area of the other. Imagine that you would have to put a wall between
the "hostile" islands/sections - and that you have to keep the cost of the
wall to the absolute minimum.
- Traces are "partisans" when they - even in a middle layer of a multilayer -
cross the border from one domain to the other: They will try to do as many
harm as possible! Only the "official" traces are allowed to cross the border
and only at one and only one point. This includes ground and all power lines!
- Use separate ground planes for the analog and the digital part and connect
these planes at only one point: where the power comes onto the PCB. Sometimes
this means to locate the power connection at an inconvenient or impossible
point, e.g. at the front panel for a board that gets it's power from a
backplane. In such a case first route the power/gound to the front and
distribute it from that point back into the circuit. The additional trace
length usually has a much lower impact on the board than disregarding this
- If your circuit is forced to have multiple ground connections, e.g. by
connectors at the front panel, and you have connectors for digital and
analog signals then locate the connection point of all the grounds as close
to the front panel as possible or use the front panel for it. (But only if
it is a very good conductor even after years in the field.) Anodized aluminium
is definetely a bad material for it because it tends to protect itself and
to "regrow" the insulating oxid layer. Consider a copper sheet behind the
aluminium front panel then.
- Keep the power lines as far apart as possible, too, and use plenty of
decoupling capacitors. More smaller ones distributed along a long run are
better than one big at only one location. Remember: each of these capacitors
has to go to that type of ground (analog or digital) that the power line
- If you have only one power supply voltage (e.g. 5V) still treat that for the
analog section(s) and that for the digital section(s) as two complete
separate ones that only happen to be connected at one point. A better solution
is to use a DC/DC converter to supply the analog part. Keep in mind: The
performance of your analog part is never better than the quality of it's
power supply - a noisy power line guarantees problems.
- Analog ICs are usually pretty sensitive to high frequency noise on the
power line(s) - much more than to low frequency noise. You can subtract at
least 20dB from the power supply rejection ratio when the noise goes into
the MHz-range. For some ICs even some tens of kilohertz are already too high.
- Never run digital traces parallel to analog ones for longer than absolutely
unavoidable. In such a case try to place a ground line (of either type) as
a spacer between them. But connect this ground line only at one end! Best
is to route the traces so that analog and digital ones are at least
that distance apart that they run in parallel and if they must cross they do
© Paul Elektronik, 1998-2002