Microcomputer Circuits and Processes

16 rnA
R
8 rnA
2R
2R
2R
2R
R
S,
R
current-to-voltage
converter
Figure 4.21
The R-2R ladder D-to-A converter. Switches So to S3 switch currents either into the
current-to-voltage converter, or else down to 0 V.These could be high-speed transistor
switches. If R= 1kn, then the I-to-V converter produces 1V output per rnA of input
current.
and so produces an output voltage of -15 V. If only So and S3 had led
their currents to the I-to- V converter, and S1 and S2 had been
connected to 0 V, then 8+ 1= 9 rnA would have arrived at the I-to- V
converter and been converted to - 9 V. So you see that any 4-bit binary
number from 0000 to 1111is converted into its equivalent voltage, from
o to -15 V. Figure 4.22 gives a table of all outputs.
Switch positions Output voltage
S3 S2 S, So
(V)
0 0 0 0 0
0 0 0 1 -1
0 0 1 0 -2
0 0 1 1 -3
0 1 0 0 -4
0 1 0 1 -5
0 1 1 0 -6
0 1 1 1 -7
1 0 0 0 -8
1 0 0 1 -9
1 0 1 0 -10
1 0 1 1 -11
1 i 0 0 -12
1 1 0 1 -13
1 1 1 0 -14
1 1 1 1 -15
Figure 4.22
Table of output voltages obtainable from the converter shown in figure 4.21.
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