[sdiy] Ferrite beads or 10ohms resistors in series with +/-15vdc ??

[Tony Clark]

On Fri, Jan 22, 2010 at 11:48 AM, Jean-Pierre Desrochers
<jpdesroc at oricom.ca> wrote:
>
> Hi list,
>
> Browsing a lot of DIY module schematics all around the net
> I noticed some builders use ferrite beads in series with their bipolar supply lines
> coming to the PCboard. Some other use 1/4w 10ohms resistors the same way.
> That makes me wonder..
> I'm on the making of a MIDI2DAC 6 voices polyphonic module using +/-15vdc and a separate 5vdc
> supplies for digital stuff. I started using 10ohms resistors in series with my 15vdc lines
> but rapidly found that these dropped my actual 15vdc voltage too much..
> I'm using 12 X 12bits DAC's here, a douzen of opamps  and don't want to disturb my 1v/oct outputs
> because of not stable supply voltages..
> So I changed the 2 resistors for 2 ferrite beads instead
> and now my 15vdc supply lines are precises but do I get
> a good noise barrier still compared to 10ohms resistors ?

If you want to continue using resistors on the power lines, then I
would suggest changing your strategy and put resistors on each chip as
opposed to just a set of resistors for the entire circuit.  The
current draw for a single chip is generally low enough to keep the
voltage drop across the resistors to a minimum.

It will add a lot more parts to your circuit, but if you are looking
for serious decoupling and noise elimination, this is the way to go.
Ferrite beads are okay for high frequency noise hash (IE, if you were
using a switching power supply), but I find that most power supply
noise is low-frequency and the beads aren't as effective there. if you
really want to go overboard, add caps to each chip as well so that you
have a true rc filter.

Best Regards,

Tony

-- 

Tony Clark
Great Lakes Modular
www.greatlakesmodular.com
Design, Engineering, and Manufacturing Services