bypass capacitors

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!

Ross

Reply to
Ross Marchant
Loading thread data ...

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 batteries'.

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?

Rob

Reply to
Rob Turk

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. :)

Ross.

Reply to
Ross Marchant

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
 Click to see the full signature
Reply to
Tauno Voipio

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'

Cheers

PeteS

Reply to
PeteS

.....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.

--
********************************************************************
Paul E. Bennett ....................
 Click to see the full signature
Reply to
Paul E. Bennett

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.