Microcomputer Circuits and Processes

II
CI
l!
~L--~---- desired measurement
trahsient
input signal. V in
.x---actual
result
Time
start
conversion
conversion
done
Figure 4.28
What happens when a ramp converter tries to capture a transient signal. By the time the
conversion is complete, the desired voltage has passed. Ramp converters are useful only
for slowly changing signals.
changed, and the output number is nowhere near the peak value you
wanted. It is a big problem: a faster conversion method is needed.
The second method of conversion is called successive approximation,
and does rather better. Here the input voltage, ~n, to be converted is
again compared with the output of a D-to-A converter. Instead of being
driven by a counter, the D-to-A converter is driven by a special logic
circuit called a 'successive approximation register' (SAR). A comparator
is used, and its output is fed back to the SAR, telling it what to do. The
circuit is shown in figure 4.29, and the process is best illustrated by an
example. Think of the SAR as a 4-bit register with an input coming
from the comparator:
Assume the analogue input voltage ~n has binary value 0010. The
following sequence occurs:
1 The SAR outputs 0111, bit 3 being set to zero, all the others to 1. The D-
to-A converts this and sends the equivalent voltage to the comparator,
which receives ~m 0010, onits other input. The comparator says 0111 is
too big, and so outputs a logical 0 to the SAR. The SAR latches this 0
into bit-3 position.
2 The SAR now makes bit 2 zero. This, with the previous latched zero,
makes the output 0011. The D-to-A converts this to a voltage which the
comparator says is too big, and so outputs a logical 0 to the SAR. The
SAR latches this into bit-2 position. The SAR now holds 0011.
3 The SAR now makes bit 1 zero, and so outputs 0001, bits 2 and 3
remaining latched at zero. The D-to-A converts, and the comparator
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