Re: gEDA-user: basic anti-EMI design q

[Bill Sloman <bill.sloman@xxxxxxxxx>]

At 04:05 24-3-2006, you wrote:
> DJ Delorie wrote:

<snipped lots of sensible stuff>

> Also, many regulators don't have stellar rejection of glitches at 
> their input so you might look specifically for that when picking 
> one.  If you can tolerate it, you might be able to improve things by 
> cascading 2 regulators.  Depends on spectral content of what you're 
> trying to reject.  I've done that in a case where I really wanted a 
> very quite supply.  I used an off the shelf regulator to produce 
> something larger than I needed and used a custom design to further improve it.

The high frequency ripple ejection you get from linear regulators 
isn't impressive - it starts rolling off between 1kHz and 10kHz and 
decreases in proportion to frequency above that. The obvious way of 
dealing with this is to add a passive filter on the input side of the 
regulator. Most people put a 100nF ceramic disk or metalised film 
capacitor on the input side of a regulator anyway; if you add 
something like 10uF of electrolytic capacitor in parallel with that 
you've got an impedance of 10R at 1.5kHz - and you can make it into a 
RC filter, rolling off the ripple from 1.5kHz with a 10R resistor, or 
you can get ambitious, and make an LC filter with a 1mH of inductor. 
A 1mH inductor is bulky - around half an inch/10mm in every dimension 
- and expensive at a dollar or two, but it gives you a two pole 
filter until the electrolytic capacitor starts looking like a 
resistor at 100kHz or so. Eventually the parallel capacitance of the 
inductor windings  by-pass the inductance - typical self-resonant 
frequencies for 1mH inductor fall around 2MHz - so you want to add a 
non-wound ferrite bead inductor in series with your wound 1mH inductor.

This gives you much better high frequency attenuation than a second regulator.

Bill Sloman, Nijmegen