Specifications

CONSTRUCTION
R-2R D/A CONVERTER-BASED
FUNCTION GENERATOR USING
PIC16C84 MICROCONTROLLER
PRASANNA WAICHAL
S.C. DWIVEDI
D
igital to analogue conversion is a
process wherein the analogue
output voltage or current is a
function of the digital input word (binary
code). D to A converters (DACs) find ex-
tensive application in analogue input-out-
put (I/O) systems, waveform generators,
signal processors, motor-
speed-controllers, voice
synthesisers, attenuators,
etc.
DACs are characteri-
sed by the following two
main performance criteria:
1. Resolution. It is de-
fined as the smallest in-
cremental change in the
output voltage
that can be re-
solved by a linear
DAC and is equal
to 1/2
n
(or 2
-n
) of
the full-scale
span of the DAC.
Here, ‘n’ repre-
sents the number
of bits the DAC
can process.
Resolution can
also be expressed
in percentage of
full-scale or
in bits.
Higher the
number of
bits that a
DAC can
process,
the better
will be its
resolution.
2. Set-
tling time.
It is the
time re-
quired for the output to stabilise, or
change from its previous value to the new
value corresponding to fresh digital input
word. For a given converter, the output
does not change instantaneously when a
change in the input occurs.
For an ideal linear DAC, the transfer
curve is a linear function of input code
which produces a single analogue discrete
value and has a zero settling time
Basic classifications
A DAC can be classified into one of the
following three types:
1. Current output. Here the output
is a current proportional to the input digi-
tal word.
2. Voltage output. Here the output
is a voltage proportional to the input digi-
tal word.
3. Multiplying output. In this type
of DAC the output voltage or current is a
function of input digital word multiplied
by the reference input (i.e. the voltage
applied or current fed into its reference
terminal).
Fig. 2: 8-bit R-2R DAC
Fig. 3: Input code versus output voltage transfer function of R-2R DAC
Fig. 1: Binary weighted resistance DAC
Fig. 4: R-2R network used in conjunction with an op-amp for 3-bit application
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