3.3. ADC CONVERSION TECHNOLOGIES 55
3.3.1 SUCCESSIVE APPROXIMATION
The successive approximation technique uses a DAC, a controller, and a comparator to perform the
ADC process. Starting from the MSB down to the LSB, the controller turns on each bit at a time
and generates an analog signal, with the help of the DAC, to be compared with the original input
analog signal. Based on the result of the comparison, the controller changes or leaves the current bit
and turns on the next MSB. The process continues until decisions are made for all available bits.
One can consider the process similar to a game children play of ten. One child thinks of a number
in the range of 0 to 10 and asks another child to guess the number within n turns. The first child
will tell the second child whether a guessed number is higher or lower than the answer at the end
of each turn. The optimal strategy in such a situation is to guess the middle number in the range,
say 5. If the answer is higher than 5, the second guess should be 8. If the answer is lower than 5, the
second guess should be 3.The strategy is to narrow down to the answer by partitioning the available
range into two equal parts at ev ery turn. The successive approximation method works similarly in
that the MSB is used to partition the original input range of an ADC into hal ves, the second MSB
divides the remaining half into two quarters of the input range, and so forth. Figure 3.12(a) shows
the architecture of this type of converter. The advantage of this technique is that the conversion time
is uniform for any input, but the disadvantage of the technology is the use of complex hardware for
implementation.
3.3.2 INTEGRATION
The integration technique uses an integrator, a comparator, and a controller to convert analog signals
to digital signals. A sampled analog signal is integrated over a fixed period, say n clock cycles of the
digital system. Another fixed reference signal is integrated over time and compared with the input
analog signal integrated. Although the value of the reference signal integrated is smaller than the
input analog signal integrated, the reference signal is continuously integrated, and the time for the
integration is measured. When the two integrated values equal, the measured time is converted
to a digital encoded value. Figure 3.12(b) shows the system components of the converter. One
disadvantage of this technique is the varying time for the conversion process. A small analog value
will take less time to convert compared with a large value.
3.3.3 COUNTER-BASED CONVERSION
The third technique to convert an analog signal to a digital signal is the counter-based conversion.
This conversion is performed with the help of a counter, a DAC, and a comparator. The counter
starts at 0 and counts up. As the counter counts up, the corresponding value is converted to an
analog value and compared with an input analog signal. As long as the input analog signal is greater
than the signal generated by the DAC, the counter counts up and the process continues. When the
comparator detects that the signal from the DAC is greater than the input analog signal, the counter
value is then converted to a digital value representing the sampled analog signal. Figure 3.12(c)

Get Atmel AVR Microcontroller Primer, 2nd Edition now with O’Reilly online learning.

O’Reilly members experience live online training, plus books, videos, and digital content from 200+ publishers.