bypass capacitors

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Just solved a nasty problem where my LCD display was being corrupted when
another part of the circuit was switched on. I needed a 10uF bypass
capactitor in addition to the 0.1uF already there.

What basic rules do people use for bypass capacitors, and capacitors in
general, when designing a new embedded circuit? My general rule is 0.1uF
bypass cap for each IC to remove high frequency AC and a few 10uF scattered
around, particularly in areas of high current usage.

Considering the importance of such capacitors, its amazing I wasn't taught
about them in embedded design at university!


Re: bypass capacitors
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If you need such large capacitors then I think you fixed the symptom, not
the cause. The 10uF caps you installed are not bypass caps but 'temporary

This actual cause is most likely a case of mixing signal ground with power
ground, using power traces that are too thin, or an insufficient power
supply. I'm sure you were taught to separate high-power sections of the
design from low-level signal sections?


Re: bypass capacitors
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Hehe, you're right. I posted way to soon. Damn optimism. Problem was
incorrect port setup on the MCU turning on a latch for a brief moment when
it shouldn't have been. The latch enables various circuits, and the current
rush must have been too much for the lcd. Hence the 'temporary battery' that
fixed the problem.

At uni we were taught nothing about circuit layout in digital systems.
Everything was just 1's and 0's, no mention of the analogue side, race
conditions, ground bounce, etc. I've stumbled upon all the problems by
chance, and thanks to this newsgroup solved all of them. :)


Re: bypass capacitors
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One of Don Vonada's engineering maxims:

   Digital circuits are made of analog parts.

Lesson 1.

Just my EUR .02.


Tauno Voipio
tauno voipio (at) iki fi

Re: bypass capacitors
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There is no such thing as a purely digital circuit. Electromagnetic
Compliance testing proves this daily.

On the bypassing question, there was a long thread not so long ago.
Although rules of thumb are useful, they remain that, not solid rules
that may always be used.

I have parts that require both bulk and high frequency bypass. The
short answer is 'check the datasheet'



Re: bypass capacitors

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.....and more importantly these days they operate as though they were
little RF transmitters and receivers. Therefore, some RF theory would go a
long way in helping you to lay out PCB's so that they stood a chance of
behaving properly.

Look at the di/dt rates for all your circuits and ensure that the
transmission line (which is what the PCB tracks tend to be) impedance is
configured to be low enough at the appropriate frequencies.

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