4. What is a DAC?
Digital Analog
Each binary number sampled by the DAC
corresponds to a different output level.
10111001 10100111 10000110010101000011001000010000
Digital Input Signal
AnalogOutputSignal
10111001 10100111 10000110010101000011001000010000
AnalogOutputSignal
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 Vref and ground (bit high or
low)
7. Binary Weighted Resistor
Assume Ideal Op-
amp
No current into op-
amp
Virtual ground at
inverting input
Vout= -IRf
-
+
R
2R
4R
2nR
Rf
Vout
I
Vref
8. Binary Weighted Resistor
R
V
R
V
R
V
R
V
RIRV 1-n
n321
ffout
242
MSB
LSB
Voltages V1 through Vn are either
Vref if corresponding bit is high or
ground if corresponding bit is low
V1 is most significant bit
Vn is least significant bit
I
-
+
R
2R
4R
2n-1R
Rf
Vout
Vref
V1
V2
V3
Vn
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.
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
0 2
n
f i
n i
i
R B
V V
R
B2
B1
B0
Rf
Compare to binary weighted circuit:
1
out ref ( 1)
0 2
n
f i
n i
i
R B
V V
R
15. Reference Voltage
Internal vs. External Vref?
Internal External
•Non-Multiplier DAC
•Vref fixed by manufacturer
•Qualified for specified
temperature range
•Multiplying DAC
•Vary Vref
•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. 5Vref DAC
1LSB=5/28 =0.0195V resolution (8-bit)
1LSB=5/23 =0.625V resolution (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
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
AnalogOutputSignal
010101000011001000010000 010101000011001000010000
Digital Input Signal
AnalogOutputSignal
010101000011001000010000
Digital Input Signal
AnalogOutputSignal
010101000011001000010000 010101000011001000010000
Digital Input Signal
AnalogOutputSignal
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
nth 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 18th, 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.