Re: Low noise DC/DC converter

From: Fritz Schlunder (me_at_privacy.net)
Date: 12/27/04 

Date: Mon, 27 Dec 2004 13:17:33 -0700

"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.

What are the relevant frequencies in your circuit?

Are you familiar with and have addressed the other considerations when using
high resolution AD converters?

http://ww1.microchip.com/downloads/en/AppNotes/00688b.pdf

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