DAC ppt,....................................

DAC

1. Module6
ADC andDAC

2. Data conversionSystems
•Bothdataaboutthephysicalworldandcontrol
signalssenttointeractwiththephysicalworldare
typically"analog"orcontinuouslyvaryingquantities.
•Inordertousethepowerofdigitalelectronics,one
mustconvertfromanalogtodigitalformonthe
experimentalmeasurementendandconvertfrom
digitaltoanalogformonthecontroloroutputendof
alaboratorysystem.

3. Data Collection andControl

4. What is aDAC?
•A digital to analog converter (DAC) converts adigital
signal to an analog voltage or currentoutput.
DAC
100101…

5. 3 bit DAC outputwaveform

6. Digital-to-AnalogConversion
•Dataincleanbinarydigitalformcanbe
convertedtoananalogformbyusinga
summingamplifier.
•Forexample,asimple4-bitD/Aconvertercan
bemadewithafour-inputsummingamplifier.

7. SummingAmplifier
Inverting summer circuit is an
operational amplifier using
negative feedback forcontrolled
gain, with several voltage inputs
and one voltageoutput.
Theoutputvoltageistheinverted
(oppositepolarity)sumofallinput
voltages:
Applyingkclatthejunction)321(
321
VVVVout
R
Vout
R
V
R
V
R
V




8. Digital-to-AnalogConversion
•2 BasicApproaches
–Weighted SummingAmplifier
–R-2R NetworkApproach

9. Weighted ResistorDAC
•For a simple inverting summer circuit, all resistors mustbe
of equalvalue.
•If any of the input resistors were different, the input
voltages would have different degrees of effect on the
output,andtheoutputvoltagewouldnotbeatruesum.
•Suppose, input resistor values at multiple powers oftwo:
R, 2R, and 4R, instead of all the same valueR:

10. Binary Weighted ResistorDAC
MSB
LSB
Voltages V
1 through V
nare
V
ref
or
if corresponding bit ishigh
groundif corresponding bit islow
V
1 is most significantbit
V
n is least significantbit
I
-
+
R
2R
4R
2
n-1R
Rf
V
out
V
ref
V
1
V
2
V
3
V
n)
24
3
2
2
1
1
(
1


nf
f
VnVVV
RVout
IRVout

11. 3 bit binary-weightedDAC

12. Binary WeightedResistor
For example, a 3-Bitconverter
V
out=-V
ref(b
3/2+ b
2/4+b
1/8) )
28
3
4
2
2
1
(
2
n
f
f
VnVVV
Vout
IRVout
R
IfR




13. No B3 B2 B1 Analog Output
0 0 0 0 0
1 0 0 1 -V/8
2 0 1 0 -V/4 or -2V/8
3 0 1 1 -3V/8
4 1 0 0 -V/2 or -4V/8
5 1 0 1 -5V/8
6 1 1 0 -3V/4 or-6V/8
7 1 1 1 -7V/8

14. 3 bit DAC outputwaveform
Input
States
Discrete
output
Voltage(V)
000 0.0
001 1.25
010 2.50
011 3.75
100 5.00
101 6.25
110 7.50
111 8.75V
out=-V
ref(b
3/2+ b
2/4+b
1/8)
With Vref negative (-10V) and R
f=R/2

15. •Uses a parallel network ofbinary-weighted
resistors to feed theop-amp.
•Seldom used since a wide range of resistor
values is required for a large number ofbits.
•Difficult to achieve accuracy for a highnumber
ofbits.
Weighted Resistor D/AConverter