62 ATMEL AVR MICROCONTROLLER PRIMER: PROGRAMMING AND INTERFACING
3.4.4 Digital-to-Analog Conversion
Once a s i gnal is acquired to a digital system with the help of the ADC process and has been
processed, frequently, the processed signal is converted back to another analog signal. A simple
example of such a conversion occurs in digital a udio processing. Human voice is converted to a
digital signal, modified, processed, and converted back to an analog signal for people to hear. The
process to convert digital signals to analog signals is completed by a DAC. T he most commonly
used technique to convert digital signals to analog signals i s the summation method shown in
Figure 3.11.
With the summation method of DAC, a digital signal, re prese nted by a s e t of 1’s and 0’s,
enters the DAC from the M SB to the LSB. For e ach bit, a comparator checks its logic state , high
or low, to produce a clean digital bit, represented by a voltage level. Typically, in a microcontroller
MSB
LSB
Comparators
2.5 v
1/2
1/4
1/8
1/2
n
Adder
Quantized
Analog
Signal
Scalar
Multipliers
FIGURE 3.11: A summation method to convert a digital signal into a quantized analog signal.
Comparators are used to clean up incoming signals, and the resulting values are multiplied by a scalar
multiplier, and the results are added to generate the output signal. For the final analog signal, the
quantized analog signal should be connected to a low pass filter followed by a transducer interface circuit.

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