DigitaltoAnalog Converters (DAC's)
A
digitaltoanalog converter,
or simply
DAC,
is a semiconductor device that is used to convert a digital code into an
analog signal. Digitaltoanalog conversion is the primary means by
which digital equipment such as computerbased systems are able to
translate
digital data into realworld signals that are more understandable to or
useable by humans, such as music, speech, pictures, video, and the like.
It also allows
digital
control
of machines, equipment, household appliances, and the like.
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A typical
digitaltoanalog converter outputs an analog signal, which is usually
voltage
or
current,
that is
proportional
to the value of the digital code provided to its inputs. Most DAC's have
several digital input pins to receive all the bits of its input digital
code in
parallel
(at the same time). Some DAC's, however, are designed to receive the
input digital data in
serial
form (one bit
at a time), so these only have a single digital input pin.
A simple DAC may be
implemented using an opamp
circuit known as a summer, so named because its output voltage is the sum
of its input voltages. Each of its inputs uses a resistor of different
binary weight, such that if R_{0}=R, then R_{1}=R/2, R_{2}=R/4,
R_{3}=R/8,.., R_{N1}=R/(2^{N1}). The
output of a summer circuit with N bits is:
Vo =
V_{R }(Rf / R) (S_{N1}2^{N1} + S_{N2}2^{N2}+...+S_{0}2^{0})
where V_{R}
is the voltage to which the bit is connected when the digital input is
'1'. A digital input is '0' if the bit is connected to 0V (ground).
A 4bit summer circuit is
shown in Figure 1.


Figure 1.
An Op Amp Summer Circuit Used as a DAC; where
R_{0} =
2 R_{1} = 4 R_{2} = 8 R_{3 }


One problem with this circuit
is the wide range of resistor values needed to build a DAC with a high
number of digital inputs. Putting thinfilm resistors that come in a wide
range of values (e.g., from a few kΩs
to several MΩs) on a single semiconductor chip can be very difficult,
especially if high accuracy and stability are required.
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A betterdesigned and more
commonlyused circuit for digitaltoanalog conversion is known as the
R2R ladder
DAC, a 4bit version of which is shown in Fig. 2. It consists of a
network of resistors with only two values, R and 2R. The
input S_{N} to bit N is '1' if it is connected to a voltage V_{R}
and '0' if it is grounded. Thevenin's Theorem may be applied to prove that
the output Vo of an R2R ladder DAC with N bits is:
Vo =
V_{R}/2^{N} (S_{N1}2^{N1} + S_{N2}2^{N2}+...+S_{0}2^{0}).
Thus, the output of the R2R
ladder in Figure 2 is Vo = V_{R}/2^{4}
(S_{3}2^{3}+S_{2}2^{2}+S_{1}2^{1}+S_{0}2^{0})
or Vo = V_{R}
(S_{3 }/ 2 + S_{2 }/ 4 + S_{1}
/ 8 + S_{0 }/ 16) .
In effect, contribution of each bit to the analog output is proportional to
its binary weight.


Figure 2.
A 4bit R2R Ladder DAC 

See also:
DAC Parameters;
ADC's;
Op Amps
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