[Soekris] DC-DC converter on net4801

Poul-Henning Kamp phk at phk.freebsd.dk
Mon Jan 31 20:04:33 UTC 2005

In message <Pine.LNX.4.58.0501311914320.13678 at pi1061.physik.uni-erlangen.de>, Kay Graf writes:
>Hi again,
>thanks for helping on that topic. I already included some capacitors to 
>get rid of the noise, which infact couples into the sensors themselves 
>(not the read-out).
>The problem I have is that the noise on the sensors may only be around 5mV 
>(RMS) and I can not use a different powersupply for them. The noise I 
>observe is around 40mV and I can not know where it actually comes from, my 
>first guess was the DC-DC-converter.
>So what I thought of was, that if anyone knows how the DC-DC-converter 
>reacts I can later on calibrate for the noise in the data. 

The behaviour is very hard to predict as it depends on the power-drain
from the batteries.

If the noise his high frequency (> 10 kHz) you should use ferrite
cores or even coils in the power-wire to the soekris to stop it,
capacitors will not help very much.

If it is low frequency, (< 10 kHz) then it is likely the changing
load of the soekris coupled with the batteries internal resistance
that makes the voltage change.

In that case your best bet is to use regulated power for your
sensors, if need be you will have to increase the battery voltage
for that.

A regulator like the LT1083 from www.linear.com gives very _very_
good regulation and will shave only 1V off your voltage, leaving
you with 9.8 V for your sensors.

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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