Re: Low noise DC/DC converter

From: Jeroen (jayjay.1974_at_xs4all.nl)
Date: 12/27/04 

Date: Mon, 27 Dec 2004 23:27:15 +0100

"Fritz Schlunder" <me@privacy.net> wrote in message
news:33b8saF3usut1U1@individual.net...
>
> "Jeroen" <jayjay.1974@xs4all.nl> wrote in message
> news:41cf366a$0$6219$e4fe514c@news.xs4all.nl...
>
> > I'm currently exploring the interesting details of building and
designing
> > transformers. I need an isolated 5V to -15V/+15V/3V3 convertor, that
> should
> > be very low noise, as it supplies a 12 bits ADC with additional signal
> > conditioning circuits. Currently I'm using two C&D Tech. 1W DC/DC bricks
> > followed by low noise LDOs. The noise (or better, the switching residue)
> can
> > generate errors up to 48 ADC counts. Actually these little bricks seem
to
> > generate more noise then three switchers in the digital part of the
board
> (a
> > LT1940 and a LT3431 to create a 1V5/1.4A, 3V3/1.4A and 5V/3A supply).
>
> Hi there.
>
> Have you tried hanging one or more extra LC low pass filter(s) on the
output
> of the DC/DC converter modules? In my experience playing with DC/DC
> converters this extra LC filter can sometimes greatly improve at least the
> aesthetic appearance of the output voltage as viewed on an oscilloscope.
On
> the other hand, I have been significantly disappointed by the noise
reducing
> capability of hanging a linear regulator on the output of the switching
> supply. A typical DC/DC converter runs at 100kHz+, so presumably most of
> the noise produced by it will be at that frequency and higher.
> Unfortunately the supply voltage rejection of normal linear regulators
seems
> to be fairly low at 100kHz and even lower in the MHz range which (at least
> for me) is the most aesthetically unpleasing noise to look at on the
> oscilloscope.
>

I did include LC filters on the inputs and outputs and I used low noise
linear post regulators. It's a six layer board with two ground planes. With
very close grounding measuring techniques I still have nice 100mVp-p spikes
on my scope reading, nicely coincident with the switching of the regulators
(there are a few test pins on them). Even with the 512x averaging mode on
the scope these spikes stand out in an otherwise rock stable supply.

These spikes contains a lot of higher harmonics that simply shoot through
every parasatic component of each part, including the filter inductors (the
winding-to-window capacitance). I think the operating frequency of the DC/DC
is of no point here; you either end up with 200.000 spikes per second or
2.000.000 spikes per second. The problem is even worsened by having two of
these DC/DC that are not synced, they both run at approx. 100KHz depending
on input voltage. The sampling rate of the ADC is around 37-40 ksamples/sec.
Digital noise is of no concern in this design; all digital parts are
non-operative while it's converting (it's a serial DAC).

I think no amount of mu shielding, bypassing, inductors, beads and ground
planes will kill these spikes; it's simply easier not to generate these
spikes to begin with.

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