Re: Thanks so far guys
Scarr said:
Sorry for the late reply but I've been trying to figure it out for myself.
First an update to a few questions you asked. It’s a battery design then fed to a linear regulator (see pic)
I have just increased the caps from 0.1uf to 33uf Tants
I think the ripple is being generated by my board, looking at the various points across the PCB it changes from being OK at reg output to rather nasty ripple at Attiny13 input. Also the nasty at VCC of Attiny13 only appear when the PGS is on, so it looks like it's the GPS generating this.
I am doing a re-design of the PCB adding a few more caps and ensuring no tracks go under the GPS module, also adding two caps to the RX+TX lines (470pf) as per GPS 9805-III pdf recommendations and ensuring the tracks from decoupling caps to pics are short and wide (<0.5mm length , 1mm wide)
One a few new question before I go, what is a good cap for decoupling I get the feeling that 0.1uf is a good general value after reading quite a lot of the internet, but what type XR7 Z5U COG? also does size matter e.g. 0805?
ok Good for putting down some bulk capacitance (33uF) bad for removing the high-freq decoupling (100nF)
Put the 100nF capacitor back but still keep the 33uF capacitor.
Since you say you are powing from a battery the ripple MUST be coming from your cct (as you say)
Make sure all IC's have a 100nF capacitor at their supply pins to reduce the ripple it sees.
IF there is a higher-power device (relativily speaking) on the board think about putting some 10uF bulk storage capacitance near it
As far as the dialectric (X7R, Z5U,COG) its just stability.
COG has execlent stability (and COG capacitors should always be used for timing ccts, ie 555), but it is limited in capacitance.
the others have better capacitance, but at the expense of stability.
as for package size (0805 et al) it is more to do with ripple-current. majority of cases 0805 is fine, only rarely (or if you deal with switching ccts) do you need bigger, unless for conviniece, 0805 can be fidly if you are not use to them