In The Trenches
by Gerard Fonte
The Business of Electronics Through Practical Design and Lessons Learned
In The Trenches
Analog to Digital Conversion Considerations
Using an Analog to Digital
Converter (A/D) is really
a fairly easy task for
most non-critical applications.
Unfortunately, there seems to be a
lot of confusion about the steps that
are necessary for a good conversion
to take place.
This month, we'll examine the
error sources and problems that
occur when the necessary attention
to detail is ignored.
The Sample and
Hold
We’ll limit this discussion to the
common successive-approximation
class of A/D converters. Perhaps
because the manufacturers have
made them so "easy" to use, hobbyists and engineers don't always stop
to think about what actually happens
when they press the "convert" switch.
Without a doubt, carelessness is
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NUTS & VOLTS
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Circle #147 on the Reader Service Card.
the major cause of improper A/D
function.
There was a time when you
had to buy a separate sample/hold
(or track/hold) IC and attach it to
your A/D.
Nowadays, everything is integrated
into one chip and, in many cases —
as with microcontrollers (µCs) — the
A/D is only another feature. Just
because you don't see the
sample/hold doesn't mean that it isn't
important or that it can be ignored.
It is a critical piece of the conversion
process. Understanding how it operates is important in understanding
the limitations of the A/D.
Fundamentally, the sample/hold
is just a means of stopping the input
signal from changing during A/D
conversion. The successive approximation procedure requires that the
input signal be fixed; otherwise, the
conversion can be corrupted. This is
because there are N sequential comparisons for an N-bit converter. An
eight-bit A/D needs eight comparison
steps and a 16-bit A/D needs 16
steps, etc.
Obviously, these steps take time.
If the input signal changes halfway
through the sequence, the second
half will represent the changed value.
You can see that even a small
input change can result in a really
fouled-up binary number.
A practical example will show
the importance of the sample/hold.
Let's say that you have a 12-bit A/D
— without a sample/hold — that
takes 1 µS for each of the 12 steps
for a conversion speed of 12 µS.
What is the fastest sine wave it can
JANUARY 2005
