PCB's and Capacitors

I need to check a thought I've had...

When you're transferring schematic to layout (for those of you who make PCB's) to you have a "generic" footprint for a capacitor you like to use?

I don't want to specify surface mount, because surface mount caps and resistors are a lesson in pain. Surface mount IC's are okay though :green:

I was thinking of setting the package for all my non-electrolytics as 5mm MKT one's. I should be able to get them from WIMA or elsewhere.

yes/no???

Rochey
I'm interested as well...
Just because some of my 5mm Wimas now are coming in double width sizes (a bit of a pain when some of the pcbs expect them to be the old thin variety)
ie the fit the footprint - but you can;t put them next to each other - so you have to stick them on the underside of the board

When doing layout, I build a footprint for every different size of cap that I am using. It is very rare to layout a board and have every single cap be the same size anyway. With a good datasheet in front of me, it takes about 1 minute to draw up a footprint with pads and silkscreen info. It is definately worth taking the time so everything fits.

Ian

When doing layout, I build a footprint for every different size of cap that I am using. It is very rare to layout a board and have every single cap be the same size anyway. With a good datasheet in front of me, it takes about 1 minute to draw up a footprint with pads and silkscreen info. It is definately worth taking the time so everything fits.

Ian

Well, having looked at the libraries in EAGLE, there are WIMA type caps (MKT) in a pacakge that has 3 pins on it - so that you can either use a 5mm pitch or a 7.5mm pitch cap.

That seems pretty useful, as WIMA covers most sizes in 5mm, and for obscure values, Siemens makes 7.5mm pitch caps.

Thoughts?

[quote author="Rochey"]Thoughts?[/quote]

Some passives, especially caps, really need to be surface mount. Think power supply decoupling caps: you want those traces as short as possible, as every millimeter means extra inductance.

For filter/coupling stuff, a standard WIMA footprint should be fine (as long as you're sure you'll never want to upgrade to >0.22uF polyprop ;-).

HTH,

JDB.
[working on the coupling/decoupling for a PGA2500/PCM4202-board. Schematics will be posted once I'm done]

jdb- don't forget to send pictures when you're done

[quote author="Rochey"]
Thoughts?[/quote]

If you're using Eagle and prefer WIMA caps: there is a pretty complete WIMA library available at Cadsoft...

;Matthias

[quote author="Rochey"]jdb- don't forget to send pictures when you're done [/quote]

No worries. It looks like the PCB will end up around A4-sized, and my scanner has no problems imaging 3D objects (unlike a newer HP unit I tried which has an extremely narrow focus range).

JDB
[just finished the front-end up to and including the ADC. 30 caps per channel, ranging from 100pF NPO 0603s to 1uF polyprop. Still not sure if the PGA2500-PCM4202 interface should be AC- or DC-coupled; I ended up laying out the board so I could try both & pick which sounds best. Will post samples if & when I get it up & running]

the datasheets are normally pretty good for the Pro Audio parts from TI (I should know... :grin: )

Page 13: http://focus.ti.com/lit/ds/symlink/pga2500.pdf

I would suggest that as you need a VCOM of 2.5V for the PCM420x device then AC coupling might not be such a good idea.

PM me if you have any Q's.

cheers

R

conclusion: Use WIMA where possible (easy to buy) and specify other caps directly. :wink:

That'll do me Rochey - but as said before Wimas have now changed
Pin spacing is the same but the widths have changed between old and new - some old ones (blue/green/whatever were quite thin - the red/yellow equivs are now double thickness)
Or even triple thickness (which is a bloody pain)