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5.9 D/A AND A/D CONVERTERS AND THEIR
COMPUTER INTERFACE

A D/A converter converts a digital number to an analog signal, that is generally a voltage
level. An A/D converter does the opposite. It converts an analog signal to a digital number.
D/A and A/D converters are essential components in interfacing the digital world to the
analog world.
The process of converting signals from analog form to digital form involves two
operations:

1. sample the signal,
2. quantize the signal to the resolution the A/D can represent with n-bits.

Sampling is the process where a finite number samples of a continuous signal is
taken and converted into a discrete number sequence. The samples are the only information
available regarding the signal in the computer.
Let us consider how a D/A converter converts a digital number to an analog signal
(Figure 5.52). The voltage potential across the resistor bank at the output of the D flip-flop
memory device is V and held constant, that is 10 VDC. The resistor value R is selectable
h f
to usually one of four different values. This is used to change the output range of the D/A
converter. If a bit is ON (D0-D7, which is same as Q0-Q7), there will be a current flowing
through the corresponding resistor. If the bit is OFF, there will not be any current flowing
through that resistor. Therefore, since the sum of current at point A of the op-amp should
be zero, the output voltage for a given digital number N sent to the D/A converter can be
calculated as follows:

7
V ∕R = V (b ∕R + b ∕(R∕2) +…+ b ∕(R∕2 ))
h
0
f
0
7
1
V = V (R ∕R)N
h
f
0
where, b is zero (0) or one (1) representing the value of bit i, N is the corresponding number
i
sent to the D/A converter. The range it can cover is zero to V range , where
n
V range = V (R ∕R) ∗ (2 − 1) (5.315)
f
h
R
R f
Data bus A V O
D Flip-flop
N R
7
D7 Q7 R/2
CPU D6 Q6 R/2 6
D5
D4
D3
D2
D1 Q1 R/2 R
f N
O
D0 Q0 R V = V R R
C
Resolution
OUT
SELECT
D/A converter
FIGURE 5.52: Operating principles of a D/A converter.