RE: Quick note on Disk 18a

[Tom Droege <tdroege@veriomail.com>]

I agree.  The scheme of looking at random data is a very powerful technique 
to disclose the blemishes of an ADC.  That is not my point.  My point is 
that there is not much that you can do about it.  What you see as 
distribution defects change over time.  Power supply voltages, temperature, 
age, all affect the distribution that one sees.  I think we are fooling 
ourselves if we try to calibrate these things out.

OK, I have designed the system with a bunch of measurements that you might 
use to calibrate such errors.  In particular there are two reference 
voltages, the VCO temperature, and all the power supply voltages.  One 
could take data and try to find a relationship between the ADC bit errors 
and the other parameters that are measured.  Good Luck!

Tom Droege

At 10:16 AM 5/22/01 -0700, you wrote:

>I'm suggesting fixes really as a way to understand what's going on.
>OK I believe that the sky background in a given frame has more
>noise than the ADC chip introduces.  Now lets say we took an
>average of 100 frames.  What would you expect the major noise source
>to be?  The sky is down by a factor of 10 = sqrt(100) but I think
>non-random systematic errors get worse as you add them.
>
> > -----Original Message-----
> > From: Tom Droege [mailto:tdroege@veriomail.com]
> > Sent: Monday, May 21, 2001 9:44 PM
> > To: Albertson, Chris; tass@listserv.wwa.com
> > Subject: RE: Quick note on Disk 18a
> >
> >
> > Hey!  You guys are worrying about nits when it is raining boulders.
> >
> > Yes it is true that most successive approximation ADCs have a
> > momentary
> > fairly well defined differential linearity error.  But is is going to
> > change with the temperature and time of day and small
> > variations in the
> > power supply voltages.  It is not worth trying to calibrate.
> > I attempted
> > to match the ADC quality to the data.  It is way overkill.
> > This ADC is
> > always good to a few ADU.  The differential linearity is a 3 or 4
> > ADU.   The integral linearity is somewhat worse, but it is
> > normally better <SNIP>
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