Decoupling capacitor for ic

for all subjects/topics not covered by the other forum categories
duikkie
Posts: 2782
Joined: Fri Feb 07, 2014 3:28 pm
Contact:

Decoupling capacitor for ic

Postby duikkie » Fri Jan 12, 2018 9:44 am

i am still playing with pcb exp board for pi zero w to connect with beeb b

i am not a electronic men from my school days , i am a lab rat :)

but now my question:

i did put 104 (10 with 4 zero's =100nf) on my buffer board for the pi , after the problems with the sd card it works

looking at a old board beeb all the blue ones are 22ns by ic ??

so what is the value of a decoupling cap for ic . i wonder ?? ( i don't have anymore 104 ones left :) )

and is it really needed if you connect it to the tube connector ? is it not decoupled by all other decoupling caps in beeb ?

johnkenyon
Posts: 130
Joined: Wed Jul 20, 2011 2:21 pm
Location: Coventry
Contact:

Re: Decoupling capacitor for ic

Postby johnkenyon » Fri Jan 12, 2018 11:13 am

duikkie wrote:i am still playing with pcb exp board for pi zero w to connect with beeb b

i am not a electronic men from my school days , i am a lab rat :)

but now my question:

i did put 104 (10 with 4 zero's =100nf) on my buffer board for the pi , after the problems with the sd card it works

looking at a old board beeb all the blue ones are 22ns by ic ??

so what is the value of a decoupling cap for ic . i wonder ?? ( i don't have anymore 104 ones left :) )

and is it really needed if you connect it to the tube connector ? is it not decoupled by all other decoupling caps in beeb ?


Your decoupling capacitor is there for the benefit of the buffer IC on the board - when the output(s) of the buffer change state they cause brief high frequency transients on the power supply lines which the decoupling capacitor effectively smooths out. (For a Pi-copro buffer board there should be one cap per buffer IC located close to the IC, because they are decoupling transients originating from the buffer, not the beeb itself).

Each decoupling capacitor on the Beeb board is there to decouple the supply for the IC they are located next to. They can't decouple the supply to chips further away because the copper track between the capacitor and that chip several inches away has a parasitic inductance which negates the effect of the capacitor.

Lower value capacitors are more effective at higher frequencies (lower parasitic inductance), so you find that you need to have to have lots of small capacitors to filter power supply transients on digital circuits compared to power supply capacitors for an audio circuit.

If you don't have any 100nF capacitors, throw in anything between 10nF and 220nF. Note that for me, 100nF polyester capacitors are an essential spares box basic component, and tend to get ordered 50 at a time (because you know they'll get used eventually).

duikkie
Posts: 2782
Joined: Fri Feb 07, 2014 3:28 pm
Contact:

Re: Decoupling capacitor for ic

Postby duikkie » Fri Jan 12, 2018 12:56 pm

yeah all the 100nf caps are used :) , so 100nf is standaard no calculation for it.

if i read it good it is for the dip on the power line of the ic ? or peak ?

but the range of the ic power line is that not big enough to work ? (74xxx from 4.5 till 6 volt or so)

it is not for the other lines ( under 2.5 v = low , above=high) ?

User avatar
1024MAK
Posts: 7018
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Decoupling capacitor for ic

Postby 1024MAK » Fri Jan 12, 2018 2:11 pm

There is lots of information on decoupling capacitors in this thread and further on, here.

In general, a 100nF capacitor is used. But the value is not critical in most cases. So a 47nF or 220nF capacitor may well be okay.

Manufacturers can sometimes use smaller values, as they can either use wider PCB tracks so that smaller capacitors can be used (saving money), or do testing and determine that using a smaller value capacitor still meets the criteria (again, saving money).

For us, buying low quantities, there is no significant cost saving, so it is easier to "play safe" and use 100nF capacitors. This also avoids the hassle of the strange intermittent faults that poor (or nonexistent) power line decoding can cause.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

DigitalDunc
Posts: 51
Joined: Fri Sep 02, 2016 6:27 pm
Location: Oadby, Leicster, UK.
Contact:

Re: Decoupling capacitor for ic

Postby DigitalDunc » Fri Jan 12, 2018 6:56 pm

Stick 'em in and stick 'em close. Now may I suggest that the EEVBlog has episodes thst give people a scratch and sniff start to the why and how of decoupling.

User avatar
myelin
Posts: 284
Joined: Tue Apr 26, 2016 9:17 pm
Location: San Francisco, CA, USA
Contact:

Re: Decoupling capacitor for ic

Postby myelin » Fri Jan 12, 2018 9:09 pm

At 2MHz you don't have to be super precise with your capacitors. You may even find that your hardware works fine without them... I've seen microcontrollers running off battery power with no decoupling. There's a tiny amount of capacitance on the chip itself, plus you get some for free from the traces on your PCB. My intuition here is that the decoupling requirements depend very much on how much power the chip draws, and especially how much of the power requirement is dynamic.

FPGAs, which run in the 100-1000MHz range, recommend to have dozens of capacitors with a mix of sizes (typically 10nF, 100nF, 1uF, 10uF), to deal with all the different power spikes. 10nF for very short pules, 10uF for longer term smoothing, etc...

Big capacitors (100-1000uF) keep the power supply stable and protect you against power supply sag in the case of sudden changes in current, like a power-hungry component switching on. Tiny capacitors (10-100nF) keep the power supply stable during power spikes measured in nanoseconds.
SW/EE from New Zealand, now in San Francisco, making BBC/Electron hardware projects for fun.
Most popular: fast serial port, FX2+PiTubeDirect Tube/Cartridge adapter, USB cart interface.

DigitalDunc
Posts: 51
Joined: Fri Sep 02, 2016 6:27 pm
Location: Oadby, Leicster, UK.
Contact:

Re: Decoupling capacitor for ic

Postby DigitalDunc » Sat Jan 13, 2018 8:42 pm

The transistors on chip outputs are massive compared to the ones for internal logic.

A lot about decoupling depends on how many lines the chip is driving as it has to charge and discharge them, one or two lines being easier than say 30 odd. It's also not all about the chip itself struggling. A standard bipolar 555 timer is known for upsetting some of it's neighbours unless decoupled.

User avatar
daveejhitchins
Posts: 3862
Joined: Wed Jun 13, 2012 5:23 pm
Location: Newton Aycliffe, County Durham
Contact:

Re: Decoupling capacitor for ic

Postby daveejhitchins » Sun Jan 14, 2018 8:58 am

Just to put-in-perspective about operating at 2MHz . . . I remember when we developed the AQR (Advanced Quarter-Meg RAM) - Assisted greatly by a good friend of mine, Pete Wallis. Pete insisted that the development was carried out 'without' any decoupling capacitors! The design consisted of 8 x DRAM, 1 x delay line and 2 x PLDs - Pete's reckoning was: if it works without decoupling capacitors then when there're added the design should be stable for production. Well, the prototype did work and nearly went into production without decoupling capacitors - due to not having much room! That was until I found the type of capacitor designed just for this situation: they're flat and fit under the IC with legs that match the supply pins of the IC. I've just been trying to find an image of them, via Google, but failed. Anyway, I think you get the picture.

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

crj
Posts: 539
Joined: Thu May 02, 2013 4:58 pm
Contact:

Re: Decoupling capacitor for ic

Postby crj » Sun Jan 14, 2018 9:02 am

daveejhitchins wrote:Pete's reckoning was: if it works without decoupling capacitors then when there're added the design should be stable for production.

That's just tempting fate into letting yours be one of those rare designs that's broken by additional caps. :-p

User avatar
daveejhitchins
Posts: 3862
Joined: Wed Jun 13, 2012 5:23 pm
Location: Newton Aycliffe, County Durham
Contact:

Re: Decoupling capacitor for ic

Postby daveejhitchins » Sun Jan 14, 2018 9:06 am

crj wrote:
daveejhitchins wrote:Pete's reckoning was: if it works without decoupling capacitors then when there're added the design should be stable for production.

That's just tempting fate into letting yours be one of those rare designs that's broken by additional caps. :-p
There was that thought, at the time . . .

Dave H :D
Parts: UM6502CE, GAL22V10D, GAL16V8D, AS6C62256A, TC514400AZ, WD1772, R6522, TMS27C512, AT28C256
Products: ARA II, ABR, ATI, AP6, MGC, AP5 . . .
For a price list, contact me at: Retro Hardware AT dave ej hitchins DOT plus DOT com

DigitalDunc
Posts: 51
Joined: Fri Sep 02, 2016 6:27 pm
Location: Oadby, Leicster, UK.
Contact:

Re: Decoupling capacitor for ic

Postby DigitalDunc » Tue Jan 16, 2018 8:17 pm

Y'know, I've recently seen on eBay a set of dip sockets with I integrated capacitors. Can't remember much about 'em but I thought they looked quite novel.

User avatar
1024MAK
Posts: 7018
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: Decoupling capacitor for ic

Postby 1024MAK » Tue Jan 16, 2018 11:42 pm

DigitalDunc wrote:Y'know, I've recently seen on eBay a set of dip sockets with I integrated capacitors. Can't remember much about 'em but I thought they looked quite novel.

They've been around for many years. But in the past, unless I was very tight on space, I've found it's much cheaper to use a normal DIP/DIL socket and separate capacitor.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...


Who is online

Users browsing this forum: No registered users and 2 guests