In The Trenches
With our 1,000 Hz signal, our new
sampling speed is 250,000 Hz (with a
12-bit A/D).
Think
The purpose of these exercises
is to show how carelessness can
create significant problems. It's
important to stop and think about
what actually happens during an
A/D conversion. It's important to be
able to determine these fundamental
sources of error.
Also, of course, it's critical to
understand how these errors can
affect your design.
Most of the time, you should
have a good idea of what signal you
expect your A/D to see. You should
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always know what is to be done with
the A/D data, even if that is not your
job. It makes no sense to design
eight-bit hardware when the software
requires 12 bits of resolution. Many
times, the A/D is only measuring a
slowly varying signal, which is
essentially DC (like room temperature).
In such cases, the error considerations are simple, but don't think all
situations are like that. It can't be
over-emphasized that you need to
know which signal parameters are
important (amplitude, frequency,
phase) and how to determine the
error budget for your application.
Error Is in the Eye
of the Tester
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92
Circle #100 on the Reader Service Card.
You may have noticed that there
are a number of ways to calculate the
above errors.
For example, you might argue
that the worst case frequency error
should be measured from peak to
peak rather than from zero crossing
to zero crossing. The zero crossing
procedure gives a smaller error.
Conversely, the phase error could be
reduced by measuring zero crossings
rather than peaks.
"Standard engineering practice"
dictates how these errors are measured. However, I am not aware of
any reference that details what
"standard engineering practice"
actually is. There are books on
"standard methods," but these are
chemical analytical techniques. I'm
also sure that there are probably
references in various text books and
manuals that provide occasional
examples, but I haven't found anything like a collection of standard
engineering practices. (If you know
of such a collection, let me know.)
These techniques are usually
learned from experience.
First, all errors should be defined
as worst case errors. "Average" errors
or "typical" errors should never be
used without first stating — very clearly — what the worst case error actually
is. Proper engineering must always
JANUARY 2005
