Hi,

In my project I can see ripple on the 3.3v line to a GPS engine, I know these things are sensitive to ripple and so I want to keep it as low as possible but I don't want to be trying to do something I will never achieve, like 0mv!!

Currently it's about 40mv of ripple, I have 0.1uf ceramic caps very close to all IC's and appropriate caps on regs, and this is a battery device so supply is clean. is there something more I can do? Is there a little component / IC you can stick between the supply and VCC pin of a device to smooth it?

P.S. Can't find acceptable supply ripple in PFG for the leadtek 9805-III

Thx

 

One critical fact that you left out is the ripple frequency. We can't always assume that it's the mains frequency as it might be internally generated by the circuitry itself or by a SMPS running in the KHz or MHz range.

Dean

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Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

you might want to place an inductor in the power rail as well


i.e.

BATTERY -> INductor -> Cap2GND -> 3.3V rail feeding cct.

That will form a better low-pass filter and smooth out the voltage rail

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details

 

It sounds to me like there might be a switching regulator between the battery and the GPS circuitry. An inductor can help reduce ripple, but it can introduce more ripple if not judiciously chosen.

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Ron

 

What's powering this thing? Is it a battery and a linear 3.3V reg or just a battery? Linear regs do not "ripple", they can oscillate (serious design problem) or can have poor transient response.
40mV sounds pretty darn small to me.

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I thought what I'd do was I'd pretend I was one of those deaf-mutes.

 

Ripple is relative. 40mV ripple on a 33v supply is small at 0.12%. On a 3.3v supply, it's 1.2%, still not a lot, but depending on the application, could be a problem in some analog circuitry. I don't know that I'd worry about it in digital applications. Your bottom line is this: are you really seeing any effects of the ripple in the end result? If not, there's no sense in trying to fix what ain't broke.

Dean

__________________
Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

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?

Attached Images

reg_210.jpg (18.7 KB, 248 views)

 

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

__________________
Nothing is impossible.
Once a problem is realised, the rest is just details

 

Hi Styx,

OK I understand why you recomend putting a 100nf back in but only on the output side, am I right in saying this?

Also all IC's have 100nf close to the VCC pin so got that right

Thanks for your help.