Analog output

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Digital to Anaolg Conversion (D/A or DAC)
Purpose To take a digital signal and convert to
an Analog signal The digital signal is in
the form of a binary code. The analog signal will
either be a voltage or current which varies over
a predetermined range.
Example of a DAC (D/A converter) System 4-bit
D/A Converter 4 bits ? 24 16 different values
Digital inputs
Analog output
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If we assume the output voltage has a value equal
to that of the equivalent binary number
The binary number 1011 (1110) would
produce 11mV The binary number 0110 (610) would
produce 6 mV
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There is no need for such a direct relationship
as in last example e.g. There can be some
proportionality factor..say 7 in this
case 1011 would give 77mV at VOUT
and 0110 would give 42mV at VOUT
Example Question (DAC1) A 5-bit D/A Converter
has a voltage output. For a binary input of
10100, an output voltage of 12mV is
produced. Determine the output voltage when the
binary input is 11100. Solution given in
Lecture! Calculate proportionality
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D/A Converter Resolution
The smallest step that can occur in the analog
output due toa change in the digital input. The
resolution in the first example was 6mV. The
resolution is derived from a change in the A
input (or the LSB input).
The equivalent of the LSB is known as the step
size, being the amount the output changes as the
input rises by one step.
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Example A 4-bit D/A Converter driven by a
4-bit Binary Counter
4-bit counter
D/A Converter
VOUT
Clock input
As the counter continues to increase, it advances
from 00002 to 11112 - assuming the carry forward
on the counter can be ignored, after 11112 the
counters next count reverts to 00002
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The corresponding D/A output increases
step-by-step, but eventually reverts to zero.
The resolution is expressed as a percentage of
the maximum output.
For above e.g. Step size 1 and maximum output
is 15 ? Resolution (Step size / Max. output)
x 100 (1/15) x 100
6.67
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Example DAC4 An 8-bit D/A converter has a step
size of 5 mV. Determine the maximum output and
hence the percentage resolution.
Solution DAC4 An 8-bit system ? 28 1 255
steps (we subtract 1to take into account the
zero condition not being a step). Hence
Maximum Voltage output 255 x 5mV 1275mV
Resolution (Step Size / Maximum output) x 100
(5 / 1275) x 100
0.392 NOTE The max. o/p value is known as the
Full Scale Output!
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Example DAC5 An 10-bit D/A converter has a
step size of 3 mV. Determine the maximum output
voltage, and hence the percentage resolution.
Example DAC6 An 15-bit D/A converter has a
step size of 7 mV. Determine the maximum output
voltage, and hence the percentage resolution.
NOTE The greater the number of input bits, the
greater the number of steps required to reach the
full scale output. The greater the number of
bits, the smaller the percentage resolution.