[sdiy] Walt Jung and Richard Marsh's Capacitors article

[patchell]

Grant Richter wrote:

> > It had a 16 bit ADC, and ahead of was a single
> > pole low pass filter.  The problem turned out to be the low pass filter, made
> > with a ceramic capacitor.  They were only getting 12 bit performance rather
> > than the 16 they were expecting.  Changing the cap to a polystyrene fixed the
> > problem.  It was difficult to explain to the engineer that a $0.05 ceramic
> > capacitor was not the same as a$2.00 polystyrene.
> >
> Could this be dielectric absorption?

    Yes, it was (I was trying to keep the story as short as posible).  The engineer
had made the time constant about 1/10 the the sample rate of the ADC (which is
about what you need to settle to 1 bit for a 16 bit system).  The input voltages
were jumping (descrete steps) in some known random pattern.  He could not figure
out why he was not getting what he was expecting.  The Burr Brown ADC's were
getting the rap, but of course, all the ADC's they had, worked the same.  Faces
were pretty red when I pointed out that the filter capacitor was the problem (I
think I bid a fixed 20 hours to solve the problem, took me about 1/2 hour, not a
bad return).

--
 -Jim
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