Martin Nilsson wrote:
>
> Matt Bonner <mbonner@SUNADA.COM> wrote:
>
> > Craig Webb wrote:
> >
> > > How
> > > many bytes would you use to keep good resolution at 13 bits? The person
 I'm
> > > working with who has already written some LPF code said we need four byte
 to
> > > keep precision. This seems a bit high to me.
> > >
> ...
> > First, you have to know the "noise-free" counts of your system.
[snip..]

> True, but there is another problem as well. The numerical computation
> in a digital filter is often ill-conditioned, and intermediate results
> need to be stored with higher precision. How much depends on the
> particular filter formula used, but according to my experience, a
> doubling is not uncommon. 2 * 13 bits = 26, which is 3 bytes, plus
> half a nibble.  I would say Craig's colleague's recommendation is very
> reasonable.
>
You're right - I was trying to point out that you have to be careful
using "rule-of-thumb" computations (if, in fact, it was rule of thumb).
(Craig wasn't sure how his colleague had come up with the number "4".)
One of most common mistakes I seen in data acquisition design is the
conception that you can double your accuracy by doubling your resolution
through oversampling and filtering.  In one design I analysed, it would
have taken about 50 oversamples (before filtering) to increase the
accuracy of a 14b system to 16b (and the A/D was a 20b delta-sigma).
Maybe somebody can point me to a ultra low noise military
instrumentation amp?

--Matt