Microcomputer Circuits and Processes

except that it is sent out bit after bit. This is an example of a serial
format of data which you will meet in the next section.
It took just four comparisons to convert the input signal into a 4-bit
binary number. For a standard 8-bit conversion, only 8 comparisons
are needed. That is much faster than the maximum of 255 comparisons
for the linear ramp technique. The AD571 SAR-type converter comes
with the D-to-A converter, SAR, comparator, and buffer on an 18-pin
chip taking just 25 JlS for a conversion. For several thousand US dollars,
you can even buy a SAR-type converter which will manage 10 million
conversions per second.
I!~r~ ~_~ __~~~ _!Q.~~R converter handles the transient sig~al.
Gl
Cl
III
~ o
>
desired
-'------ actual result
measurement
Time
start
conversion
conversion
complete
Figure 4.31
Successive approximation conversion technique. Note how each D-to-A output
increment is half of the previous increment. Here an 8-bit converter makes its eight
comparisons, but only the first four are able to home in on the input voltage.
It gets much closer to the desired value, but there is an error of
another sort. Unlike the ramp converter which stops when it has
reached the correct code, the 8-bit SAR must always execute its 8
comparisons. In this example, the converter fails to work properly after
the fourth approximation. Its final result is a little too low. This
problem is not major - in the example here, the converter was not given
a fair chance. The engineer needs only to remember that this may
happen, and to avoid it. In any case, the SAR technique is the most
popular today, and for most laboratory uses represents the best tradeoff
in terms of price versus speed.
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