Analog-to-Digital Converters (ADC's)
is a semiconductor device that is used to convert an analog signal into a
digital code. In the real
world, most of the signals sensed and processed by humans are analog
signals. Analog-to-digital conversion is the primary means by which analog
signals are converted into digital data that can be processed by computers
for various purposes.
signal is a signal that may assume any value within a continuous range.
Examples of analog signals commonly encountered every day are sound,
light, temperature, and pressure, all of which may be represented
electrically by an analog voltage or current. A device that is used to
convert an analog signal into an analog voltage or current is known as a
An analog-to-digital converter is used to further translate this analog
voltage or current into digital codes that consist of 1's and 0's.
A typical ADC, therefore, has
an analog input and a digital output, which may either be 'serial'
(consisting of just one output pin that delivers the output code one bit
at a time) or 'parallel' (consisting of several output pins that deliver
all the bits of the output code at the same time).
come in many forms. One example is the
parallel comparator-type ADC, which
basically consists of: 1) a set of comparators that compare the input
analog voltage to different values of fixed voltages; 2) a corresponding
set of D-type flip-flops that hold the digital outputs of the comparators;
and 3) an encoder that converts the outputs of the D-type flip-flops into
the final output digital code.
Another implementation of the
ADC is known as the
successive-approximation ADC. This circuit
consists of: 1) a sample and hold circuit to accept the analog input Va;
2) a successive approximation register (SAR) consisting of clocked
flip-flops and gates designed to systematically and progressively
approximate the digital code corresponding to the analog input Va; 3) an
internal reference DAC that gets its digital inputs from the SAR; and 4) a voltage
comparator that compares the analog output of the internal DAC to the
analog input Va.
In a successive approximation
ADC, the SAR generates a series of digital codes as it is clocked, which
are fed into the reference DAC one at a time. The digital codes are
binary search fashion, i.e., the bits are toggled to logic
'1' one at a time starting with the MSB. If the bit toggled to '1'
causes the DAC to output an analog voltage that exceeds Va, then it is
returned to '0', otherwise it is kept at logic '1'.
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Eventually all the bits would
have been exercised, and the resulting digital code is the one that causes
the DAC to produce an analog voltage that is as
close to Va
as possible without exceeding it. Thus, this will be the same
digital code released by the ADC to its outputs, since it was basically
the code that produced a voltage equal to Va using the internal reference
Another ADC design that
operates similarly to the successive approximation ADC is the
It also employs an internal reference DAC, except that in this case it is fed with
digital data that are generated by a
counter. As the counter is
clocked, the digital code fed to the DAC increases which causes the DAC to
increase its analog output proportionately. Eventually the DAC
output exceeds the analog input Va and the counter is stopped. The
digital code fed to the DAC at this point becomes the output of the
The ADC's discussed earlier
all employ what is referred to as
Pulse Code Modulation (PCM), wherein
an N-bit digital code is assigned to each sample taken from the analog
signal. Another major class of ADC's employs a process known as
Delta Modulation (DM) instead of PCM to digitize analog signals.
A basic linear DM ADC
has an internal
that generates an analog signal that approximates the analog signal
being digitized. It also has a
the processor's analog output to the actual input analog voltage.
If the comparator determines that the analog input is greater than the
processor output, then the processor increases its output by a step S0;
otherwise the processor output is decreased by
of linear DM is the ease by which the analog signal can be reconstructed
from the digitized signal. The drawback of linear DM is that its
output can only change in steps of just
This limits the slope of the digitized signal, which becomes a problem
when the input analog signal is changing rapidly.
Delta Modulation (ADM)
limitation of linear DM ADC's by allowing variations in the step sizes
at which the digitized signal changes. Under ADM, the step size by
which the digital output of the ADC changes increases whenever the
analog signal being digitized is changing rapidly.
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