dac-180418111805 (1).pdf

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1. DIGITAL TO ANALOG
CONVERTER
Name : Gauravsinh Parmar
Enrollment no : 170410117023

2. What is a DAC?
A digital to analog converter (DAC) is a device
that converts digitalnumbers (binary) intoan
analogvoltage or current output.

3. Principal components of DAC

4. What is a DAC?
Digital Analog
Each binary number sampled by the DAC
corresponds to a different output level.
101110011010011110000110010101000011001000010000
Digital Input Signal
Analog Output Signal
101110011010011110000110010101000011001000010000
Analog Output Signal
Digital Input Signal

5. Types of DACs
Many types of DACs available.
Usually switches, resistors, and op-
amps used to implement conversion
Two Types:
◦Binary Weighted Resistor
◦R-2R Ladder

6. Binary Weighted Resistor
Utilizes a summing op-amp circuit
Weighted resistors are used to
distinguish each bit from the most
significant to the least significant
Transistors are used to switch
between V
ref and ground (bit high or
low)

7. Binary Weighted Resistor
Assume Ideal Op-
amp
No current into op-
amp
Virtual ground at
inverting input
V
out= -IR
f
-
+
R
2R
4R
2
n
R
Rf
V
out
I
V
ref

8. Binary Weighted Resistor






R
V
R
V
R
V
R
V
RIRV
1-n
n321
ffout
242

MSB
LSB
Voltages V
1through V
nare either
V
refif corresponding bit is high or
ground if corresponding bit is low
V
1is most significant bit
V
nis least significant bit
I
-
+
R
2R
4R
2
n-1
R
Rf
V
out
V
ref
V
1
V
2
V
3
V
n

9. Binary Weighted Resistor
Advantages
◦Simple
◦Fast
Disadvantages
◦Need large range of resistor values
(2048:1 for 12-bit) with high precision in
low resistor values
◦Need very small switch resistances
◦Op-amp may have trouble producing low
currents at the low range of a high
precision DAC

10. R-2R Ladder
Each bit
corresponds to a
switch:
◦If the bit is high, the
corresponding
switch is connected
to the inverting input
of the op-amp.
◦If the bit is low, the
corresponding
switch is connected
to ground.

11. R-2R Ladder
B
2
B
1
B
0

12. R-2R Ladder
Circuit may be
analyzed using
Thevenin’s
theorem (replace
network with
equivalent
voltage source
and resistance)
Final result is:1
out ref
02
n
f i
ni
i
R B
VV
R



 
B
2
B
1
B
0
R
f
Compare to binary weighted circuit:1
out ref ( 1)
02
n
f i
ni
i
R B
VV
R



 

13. R-2R Ladder
Advantages:
◦Only 2 resistor values
◦Lower precision resistors acceptable
Disadvantages
◦Slower conversion rate

14. DAC Specifications:
Reference Voltages
Resolution
Speed
Settling Time
Linearity

15. Reference Voltage
Internal vs. External V
ref?
Internal External
•Non-Multiplier DAC
•V
reffixed by manufacturer
•Qualified for specified
temperature range
•Multiplying DAC
•Vary V
ref
•Consider current required
•Consider Voltage range
•Consider dynamic effects
of inner structure

16. Resolution
1 LSB (digital)=1 step size for DAC output (analog)
Increasing the number of bits results in a finer resolution
Most DAC -8 to 16-bits (256 to 65,536 steps)
e.g. 5V
refDAC
1LSB=5/2
8
=0.0195Vresolution (8-bit)
1LSB=5/2
3
=0.625Vresolution (3-bit)n
refV
2
Resolution 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
8-bit Resolution 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
3-bit Resolution
1 LSB

17. Speed(Max. Sampling Frequency)
The maximum rate at which DAC is reproducing usable
analog output from digital input register
Digital input signal that fluctuates at/ has high frequency
require high conversion speed
Speed is limited by the clock speed of the microcontroller
(input clock speed) and the settling time of the DAC
E.g. To reproduce audio signal up to 20kHz, standard CD
samples audio at 44.1kHz with DAC ≥40kHz
Typical computer sound cards 48kHz sampling freq
>1MHz for High Speed DACs

18. Settling Time
The interval between a command to update (change) its
output value and the instant it reaches its final value,
within a specified percentage (±½ LSB)
Ideal DAC output would be sequence of impulses 
Instantaneous update
Causes:
◦Slew rate of output amplifier
◦Amount of amplifier ringing and signal overshoot
Faster DACs have shorter settling time
Electronic switching fast
Amplifier settling time dominant effect

19. Settling Time
t
settle

20. DAC Linearity
The difference between the desired analog output and the actual
output over the full range of expected values
Does the DAC analog output vary linearly with digital input signal?
Can the DAC behavior follow a constant Transfer Function
relationship?
Ideally, proportionality constant –linear slope
Increase in input increase in output monotonic
Integral non-linearity (INL) & Differential non-linearity (DNL)010101000011001000010000
Digital Input Signal
Analog Output Signal
010101000011001000010000 010101000011001000010000
Digital Input Signal
Analog Output Signal 010101000011001000010000
Digital Input Signal
Analog Output Signal
010101000011001000010000 010101000011001000010000
Digital Input Signal
Analog Output Signal
Linear Non-Linear

21. Common Applications:
Function Generators
Digital
Oscilloscopes
◦Digital Input
◦Analog Ouput
Signal Generators
◦Triangle wave generation
◦Sine wave generation
◦Square wave generation
◦Random noise generation
1
2

22. Common Applications
Used when a continuous analog signal is
required.
Signal from DAC can be smoothed by a
Low pass filter
0 bit
n
th
bit
n bit DAC
011010010101010100101
101010101011111100101
000010101010111110011
010101010101010101010
111010101011110011000
100101010101010001111
Digital Input
Filter
Piece-wise
Continuous Output
Analog
Continuous Output

23. Applications –Video
Video signals from digital sources, such as a
computer or DVD must be converted to analog
signals before being displayed on an analog
monitor. Beginning on February 18
th
, 2009 all
television broadcasts in the United States will
be in a digital format, requiring ATSC tuners
(either internal or set-top box) to convert the
signal to analog.

24. Common Applications
Motor Controllers
Cruise Control
Valve Control
Motor Control
1 2 3