Digital to Analog Converter
Digital to Analog Converter
Jungchul Lee Kamran Jeelani Jonathan Beckwith
Topics of Discussion
? What is a DAC? ? Types of DAC Circuits
? Resistor-string DAC ? N-Bit Binary weighted DAC ? R-2R Ladder DAC
? Specifications of DAC ? Applications
What is a DAC?
? Digital-to-Analog Converter: An electronic device, often an integrated circuit, that converts a digital number into a corresponding analog voltage or current.
? Relation between analog signal and digital equivalent
n
Va = bi 2-i ? Vref i=1
? AD conversion ? Va -> bi (encoder) ex) Transducer interface ? DA conversion ? bi -> Va (decoder) ex) motor, heater control
DAC configurations
? Assume the analog signal is a voltage
N-bit digital word Analog voltage signal
... DN-1, DN-2 ...
D1, D0
VRef
=VDrVefAO*DCut /2N VOut
Types of DAC Circuits
1. Resistor-string 2. N-Bit Binary Weighted Resistor 3. R-2R Ladder
V Ref
R
1
R
0
R
1
R
0
R
1
R
0
R
1
R
0
B2
A Resistor-string DAC
1 0
1 0
B1
1 0
B0
Example of 3 bit resistor-string DAC Required component
- a resistor string - a set of switches ? select output to use - opamp ? buffer
Vo
V0
=
VRe f
b N-1 i 2i+1 i=0
Resistor String DAC Example
? How many resistors and switches would be required to implement an 8 bit resistor-string DAC?
Ans)
# of resistors = 2N=28=256
N -1
# of switches = 2i = 28 -1 = 255
i=0
Impractical for converters with more than a few bits of
resolution
N-Bit Binary Weighted Resistor
R
MSB 1
2
I0
2R
1
2
4R
1
2
8R
1
2
... ...
LSB
1
2^(N-1)R 2
V Ref
3
+ -
R/2
1 4
2
V out
I0
=
VR
N i=1
bi 2(i-1) R
5
V0 = -Rf I0
N-bit binary weighted Example
? Find output voltage, current, and resolution for a binary weighted resistor DAC of 4 bits - given condition R = 10 k, Rf = 5 k, VR = -10 V Applied binary word is 1001
Example Solutions
-10 V VR
1 2
3 3
2 1 3
2 1 1
2 3
Rf=R/2 5 k
3
R 1
4 2
? Vout
+ -
5
8R
4R
2R
R
10 k
0 Bit
1 Bit
2 Bit
3 Bit
1 0 0 1 ? binary input
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