Bypass Caps

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1

Some weeks ago Karl Dalen (KD) pointed out that bypass capacitors are missing in most modules. Using these caps for each IC device is a general design rule, they improve the power integrity. Although you possibly never noticed problems without these caps, there is no reason why they shouldn’t be added in the future.

Therefore I’ve updated following schematics. There are also snapshots which help to find the right places at the bottom of the PCBs - don’t try to place them exactly like me, just take care that one end of the cap has to be soldered as close to the +5V power input of the device as possible, and the other end to ground.

Core Module: two additional 100 nF ceramic caps:

http://www.ucapps.de/mbhp/mbhp_core.pdf

AINX4 Module: four additional 100 nF ceramic caps for the PIC:

(I’ve used a so called “shrink hose” (german: “Schrumpfschlauch”) to isolate the cap leg which goes to ground)

http://www.ucapps.de/mbhp/mbhp_ainx4.pdf

DINX4 Module: four additional 100 nF ceramic caps for the 74HC165:

http://www.ucapps.de/mbhp/mbhp_dinx4.pdf

DOUTX4 Module: four additional 100 nF ceramic caps for the 74HC595, one additional 100 uF elcap at the power input of the module:

http://www.ucapps.de/mbhp/mbhp_doutx4.pdf

LTC Module: two additional 100 nF ceramic caps for the 74HC00:

http://www.ucapps.de/mbhp/mbhp_ltc.pdf

SID Module: two additional 100 nF ceramic caps for the 74HC595:

http://www.ucapps.de/mbhp/mbhp_sid.pdf

SHX8 Module: nine additional 100 nF ceramic caps for the 74HC595 and NJU7304:

http://www.ucapps.de/mbhp/mbhp_shx8.pdf

(no picture)

Best Regards, Thorsten.

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2

Small tip: For all IC’s that have power on lower-left / upper-right pins (all 74HC… series) you can also buy sockets with cap inside. See for example GS-KO 14p on reichelt.

http://www.reichelt.de/inhalt.html?ACTION=3;LASTACTION=4;SORT=artikel.artnr;GRUPPE=C131;WG=0;SUCHE=GS%2014p;ARTIKEL=GS-KO%252014P;START=0;END=16;STATIC=0;FC=667;PROVID=0;TITEL=0;CASE=ignore;CONNECT=AND;WHOLE_WORD_SEARCH=0;SORT_SEARCH=standard

3

Yeah! i saw them some time ago but forgot in the meantime…

btw: there’s absolutely nothing to say against SMD caps where u think they might come handy. Mostly SMDs are more solid in physical terms when it comes to placing parts on the trace side of a pcb without soldermask.

(just to not forget as an option…)

Kind regards

Martin

4

Hi Martin,

I cannot recomment SMD caps for newbies - I tried the suggested 0805 size caps and noticed that they are difficult to solder, there is a high propability for unwanted shorts if somebody uses a common soldering iron without a special tip. Therefore I suggested common caps in the guide to avoid additional support effort. However, experts should know which ones are better :wink:

Best Regards, Thorsten.

5

Although you possibly never noticed problems without these caps, there is no reason why they shouldn’t be added in the future.

How necessary is the bypass caps?

What types of problems do they prevent (what would I notice, if anything, if I didn’t use them).

For example, the SID module I have from SmashTV  didn’t have these bypass caps. 

Would you recommend that I go to the trouble of ordering caps?

they improve the power integrity

what does this mean?  what exactly does “integrity” mean here…  smooth out the power to a better DC supply?

thanks…

6

Hey subatomc,

How necessary is the bypass caps?

What types of problems do they prevent (what would I notice, if anything, if I didn’t use them).

For example, the SID module I have from SmashTV  didn’t have these bypass caps. 

Would you recommend that I go to the trouble of ordering caps?

Read the hole story:

http://www.midibox.org/forum/index.php?topic=4074.0

(There is also an explanation of why to use them and what “power inegrity” means (ripple voltage and so on…)).

7

sorry but im still having trouble locating where the caps should go on the DINX & DOUT modules, and the shrink hose one on the AINX (well i just dont understand what that is, why is it so long & where is it going?

I’ve looked on the various pages and in the pdf & gifs but i still cant make it out.

8

Yeah, I found the pics are too small, AFAIK they’re not at all mentioned in the gifs, but in the PDFs you should see them quite clearly  :o

Take a look again into the PDF plans:

e.g.

Core: look at Pins “OSC1” and “OSC2”

DOUT: look for C1..C5

…

:wink:

9

Core: look at Pins “OSC1” and “OSC2”

Hold up a minute there…

The caps on OSC1 and OSC2 are loading caps, not bypass caps.  They help the crystal start up and continue to resonate as expected.  Your choice of cap types for this is probably not too crucial at this clock speed, but I use NPO in the kits for a predictably linear temperature coefficient since all other grades of ceramic caps have a rather unpredictable reaction to temperature/voltage variance.  For all I know some of you guys like to play with your Core boards in the snow…  ;D

Keep in mind that your MIDIbox will work fine if built without any bypass caps, assuming you use the right parts (linear supply, “HC” family chips).

Adding caps to a box that has jitter issues only serves to mask a symptom of whatever the real problem is causing the jitter.  Thousands of MIDIboxes were built and still running fine without them, before the day that someone showed up here screaming about how their trade school instructor said everything needs a bypass cap…  :wink:

I have included them in my kits since the idiotic “I have designed xx for xx years” pissing match started, and all of my boards have spaces for them, just to end the debate.  That’s really sad too, considering any mixed signal design done right will have a lower noise floor without them…But that’s something from the real world not the uni or trade school, so why should I expect anyone “educated” to believe such heresey?  ;D

Best!

Smash

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Getem Smash! Getem!! YAAHHHH!!  >:( >:(

;D

Sorry, that’s my gun-ho streak getting out of control again  :-\

11

hi

i am using one of those hotguns spitting warm plastic that quickly cools down and becomes  something like silicon, those ones you load with plastic sticks … (well you got it - hope so) .. i fix the bypass caps with this plastic so they stays where it s supposed to be, than i center the legs with the proper tracks and finally (and comfortably) sold them..

take care

 

12

Thousands of MIDIboxes were built and still running fine without them, before the day that someone showed up here screaming about how their trade school instructor said everything needs a bypass cap… 

Just two weeks ago I had the very first case where a bypass cap was really required. I supplied a SpeakJet chip from the core module of my MBSID, and noticed random resets of the SpeakJet - “ReReReaReaadyReadyRRRRe” was all I heard on the audio out, serial communication wasn’t possible. I don’t know how I came to the idea to add a 100 nF bypass cap between +5V and ground, but it was the solution! I think that I will never forget this design rule anymore :wink:

Best Regards, Thorsten.

13

so should we or shouldnt we?! sounds like the general rule is that its probably better to but then others are saying its not needed. sorry for dragging this out but i dont really even know what these caps are doing, you’re advice is gospel on these matters.

so if it is best to add them can some one please post another pic or explain when exactly they are going on  the modules i mentioned cos i really cant work it out from the pics, pdfs & gifs, i can see where one end goes on the pdf but i dont know where its going, maybe im reading it incorrectly.

indebted thanks & apologies for the spoon feeding needed!  :-*

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If somebody shoots new pictures or writes better documentation -> please add this to wiki

Best Regards, Thorsten.

15

That is a quote i like :slight_smile:

I just got myself a bag of caps to do this to my old boards

If i can work it out and do it right i’ll take some pics :wink:

I can’t help with more documentation though because i don’t really understand enough :frowning:

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I’ll do the written documentation. Give me a yell when the pics are done :slight_smile:

17

That’s really sad too, considering any mixed signal design done right will have a lower noise floor without them…

Hi, I was just wondering if you could please provide a reference for this material? I’ve studied noise, and the role of capacitors to some extent at uni, but would like to look into it further, as I haven’t understood the basis for what you’ve written. Thanks a lot!!

I know this has been covered before, but for people concerned about bypass capacitors, I will try to briefly summarise my understanding of the issues here:

No DC supply is perfect, and random noise and changes in loading on the power supply cause the power supply output to vary from what is desired. With good power supplies, good circuit layout, good grounding, minimal EMF etc, this variation will be small, and not cause any manjor issues. If a voltage is changing too much, then it can cause problems, such as irregular behaviour of digital chips, or excessive audible noise from a synth (this is not the limit of problems).

One precaution for this problem is to use to put a capacitor between the power input of a component, and the component’s ground. Good capacitors provide very high impedance for low frequencies (DC), and very much lower impedance for high frequencies (noise). So, by using a capacitor, noise voltages are conducted to ground, and the component ‘sees’ a cleaner supply level. It’s not really quite that simple, but that’s the basic idea.

So, if you are wondering where to put a bypass capacitor, you can generally just connect it between a power supply of a chip (close to the pin), and the nearest ground connection.

It seems that some people were confused by the heat shrink tubing that Thorsten used. This is just a plastic tube which covers a metal leg, or uninsulated wire. When you heat the tube, it shrinks around the metal, providing DIY insulation. Think of it like high quality electricians tape.

If you want to know if you need to use them: try without, and see if there are any noise/supply voltage related problems. It seems that most people can get away without them.

Bypass caps are more important at high frequencies, when lead inductance comes into play, or when one is using noisy devices like motors etc.  From my brief look over the MBHP circuits, it doesn’t seem likely that this will be an issue.  High frequency stuff is not so well suited for DIY anyway, and the standard rule of thumb about 10uF/100nF can really fall short for those applications.

18

Hey madox,

thanks for this interesting posting :slight_smile:

Maybe you want to copy (or link it) to the Wiki? (maybe to the FAQ-Section: http://www.midibox.org/dokuwiki/doku.php?id=questions_and_answers#building_your_midibox )?

This would surely help to understand what’s it for and where to put the caps!

Cheers,

Michael

19

Hey madox,

thanks for this interesting posting :slight_smile:

Maybe you want to copy (or link it) to the Wiki? (maybe to the FAQ-Section: http://www.midibox.org/dokuwiki/doku.php?id=questions_and_answers#building_your_midibox )?

This would surely help to understand what’s it for and where to put the caps!

Cheers,

Michael

I just wrote that pretty quickly. Actually, if people want information about this topic, there is a lot of it available online. Wiki has a brief description already. Just do a search for bypass or decoupling capacitor. It’s really just a low pass filter.

If I’m going to put the information somewhere more permanent, I will give it a bit more effort, and try to clarify the generalisations. Making simplifications and generalisations are a bit tricky on this topic, as in reality the situation is not as simple as often represented.

I really appreciate your feedback, and I will give your suggestion more thought when I have some time (I’m actually meant to be writing my honours thesis at the moment).

Cheers audiocommander!

P.S. When I do my own designs, I pretty much always use bypass caps.  I’m not an experienced engineer, and generally do this as a matter of caution.  However, I tend not to rely on rules of thumb for capacitor values.  Choosing appropriate capacitor types and values probably does take a bit more background in electronics though.  If it is for a low frequency application, then the rule of thumb is usually fine.  Basicly, you just don’t want to have the filter resonating.

20

On the Core-construction page, in the partslist, small mistake: http://ucapps.de/mbhp_core.html

Should have C7,C8 instead of C5,C6 listed again.

Preparing to  build my first core as we speak :smiley: