Digital to Analog Conversion (D/A)
About converting from digital to analog
Digital-to-analog conversion is much
simpler to achieve than analog-to-digital conversion and the cost of
building the necessary hardware circuit is considerably less. It is
required wherever the output of an intelligent instrument needs to be
presented on a display device which operates in an analog manner. A
common form of digital-to-analog converter is illustrated in Figure
10.6. This is shown with 8 bits for simplicity of explanation,
although in practice 10 and 12 bit D/A converters are used more
frequently.
This form of D/A converter consists of a resistor ladder network on
the input to an operational amplifier. The analog output voltage from
the amplifier is given by:
VA= |
V7 |
+ |
V6 |
+ |
V5 |
+ |
V4 |
+ |
V3 |
+ |
V2 |
+ |
V1 |
+ |
V0 |
|
|
|
2 |
|
4 |
|
8 |
|
16 |
|
32 |
|
64 |
|
128 |
Vo . . . V7 are set at either the reference
voltage level Vref or at 0 volts according to whether an
associated switch is open or closed. Each switch is controlled by the
logic level of one of the bits 0-7 of the 8 bit binary signal being
converted. A particular switch is open if the relevant binary bit has
a value of 0 and closed if the value is 1.
Consider for example a digital signal with a binary value of 11010100.
The values of V7 . . . V0 are therefore:
V7 = V6 = V4 = V2 = Vref
V5 = V3 = V1 = V0 = 0
The analog output from the converter is
then given by:
VA = |
Vref |
+ |
Vref |
+ |
Vref |
+ |
Vref |
|
|
|
2 |
|
8 |
|
32 |

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