Transcript DAC

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What is a DAC?
Weighted Resistor DAC
R-2R DAC
Capacitive DAC
DAC associated error
Delta-Sigma DAC
Conclusion
Digital-to-Analog Converter: An electronic
device, often an integrated circuit, that
converts a digital number into a
corresponding analog voltage or current.
A/D Converter
• In order to change an analog signal to
digital, the input analog signal is
sampled at a high rate of speed.
• The amplitude at each of those
sampled moments is converted into a
number equivalent – this is called
quantization.
• These numbers are simply the
combinations of the 0s and 1s used
in computer language – this called
encoding.
Digital to Analog (D/A, DAC, or
D-to-A) Conversion
• Converting discrete signals into discrete
analog values that represent the magnitude of
the input signal compared to a standard or
reference voltage
– The output of the DAC is discrete analog steps.
– By increasing the resolution (number of bits), the
step size is reduced, and the output approximates
a continuous analog signal.
DAC
In an electronic circuit, a combination of high voltage (+5V) and low voltage (0V) is
usually used to represent a binary number. For example, a binary number 1010 is
represented by
Weighting
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22
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Binary Digit
1
0
1
0
State
+5V
0V
+5V
0V
DACs are electronic circuits that convert digital, (usually binary) signals (for example,
1000100) to analog electrical quantities (usually voltage) directly related to the digitally
encoded input number.
DACs are used in many other applications, such as voice synthesizers, automatic test
system, and process control actuator. In addition, they allow computers to
communicate with the real (analog) world.
Input Binary
Number
Register
Analog Voltage
Output
Voltage
Switch
Resistive
Summing
Network
Amplifier
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The two most popular types of resistive
summing networks are:
Resistive divider circuit/Weighted binary
resistance type
-Utilizies summing amplifier to form the
weighted sum of all non-zero bits in the
input
b)Ladder resistance (R-2R) type
-Maintains constant current through all
branches (No voltage transients)
Uses a summing amplifier
circuit to generate
output.
Analysis uses KCL and OpAmp properties to
determine Vout.
Rf = R
I
R
2R
4R
i
Vo
8R
Most
Significant Bit
Least
Significant Bit
-VREF
Disadvantages
 Large number of resistors required for high
bit DACs
 High resistor values required for high bit
DACs (32,768R for common 16-bit DACs)
 Larger resistors cause more error in analog
output
Only Input 4 is HIGH
Only Input 3 is HIGH
Used for Digital-to-analog converter!
Binary Weighted
R-2R
Pros
Easily understood
Only 2 resistor values
Easier implementation
Easier to manufacture
Faster response time
Cons
Limited to ~ 8 bits
Large # of resistors
Susceptible to noise
Expensive
Greater Error
More confusing analysis
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CD Players
MP3 Players
Digital Telephone/Answering Machines
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1. http://electronics.howstuffworks.com/cd.htm
2. http://accessories.us.dell.com/sna/sna.aspx?c=us&cs=19&l=en&s=dhs&~topic=odg_dj
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3. http://www.toshiba.com/taistsd/pages/prd_dtc_digphones.html
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Digital Oscilloscopes
◦ Digital Input
◦ Analog Ouput
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Signal Generators
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Sine wave generation
Square wave generation
Triangle wave generation
Random noise generation
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1. http://www.electrorent.com/products/search/General_Purpose_Oscilloscopes.html
2. http://www.bkprecision.com/power_supplies_supply_generators.htm
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Cruise Control
Valve Control
Motor Control
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1. http://auto.howstuffworks.com/cruise-control.htm
2. http://www.emersonprocess.com/fisher/products/fieldvue/dvc/
3. http://www.thermionics.com/smc.htm
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For the selection of an IC DAC, there are several parameters that can determine
the suitability of a particular device.
Resolution
The number of bits making up the input data word that will ultimately determine the
output step voltage as a percentage of full-scale output voltage.
Example: Calculate the resolution of an 8-bit DAC.
Solution: Resolution = 8 bits
Percentage resolution =
1
1
 100 % 
 100 %  0.391 %
8
2
256
What is ADC(Analog to
Digital Converter)
Why ADC is needed
Application of ADC
A/D conversion process
What is ADC
An electronic integrated circuit which
transforms a signal from analog (continuous)
to digital (discrete) form.
Analog signals are directly measurable
quantities.
Digital signals only have two states. For
digital computer, we refer to binary states, 0
and 1.
• A simple hypothetical A/D converter circuit with one analog input signal and
three digital output lines with eight possible binary combinations: 000 to 111
– Shows the graph of digital output for FS V analog input
• The following points can be summarized in the above process:
– Maximum value this quantization process reaches is 7/8 V for a 1 V analog signal;
includes 1/8 V an inherent error
– 1/8 V (an inherent error) is also equal to the value of the Least Significant Bit
(LSB) = 001.
– Resolution of a converter is defined in terms of the number of discrete values it
can produce; also expressed in the number of bits used for conversion or as 1/2n
where n =number of bits
– The value of the most significant bit (MSB) -100- is equal to ½ the voltage of the
full-scale value of 1 V.
– The value of the largest digital number 111 is equal to full-scale value minus the
value of the LSB.
– The quantization error can be reduced or the resolution can be improved by
increasing the number of bits used for the conversion
Why ADC is needed
Microprocessors can only perform complex
processing on digitized signals.
When signals are in digital form they are less
susceptible to the deleterious effects of
additive noise.
ADC Provides a link between the analog
world of transducers and the digital world of
signal processing and data handling.
Application of ADC
ADC are used virtually everywhere where an
analog signal has to be processed, stored, or
transported in digital form.
Some examples of ADC usage are digital volt
meters, cell phone, thermocouples, and
digital oscilloscope.
Microcontrollers commonly use 8, 10, 12, or
16 bit ADCs, our micro controller uses an 8 or
10 bit ADC.
Numerous methods are used for converting analog signals to digital form. Five most
commonly used methods are listed below:
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Staircase ramp
Successive approximation
Dual slope
Voltage to frequency
Parallel (or flash)
Conversion from analog to digital form
inherently involves comparator action
where the value of the analog voltage at
some point in time is compared with some
standard.
A common way to do that is to apply the
analog voltage to one terminal of a
comparator and trigger a binary counter
which drives a DAC.
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The clear pulse resets the counter to the zero. The
counter then records in the binary form the number
of pulses from clock line. The clock is a source of
pulses equally spaced in time. Since the number of
pulses counted increases as the input linearly with
time, the binary word representing this count is used
as the input of a DAC (Digital to Analog Converter)
whose output is the stair case waveform
As long as the analog input VA is greater than VD, the
comparator has an output which is high and the AND
gate is open for transmission of the clock pulse to
the counter. When VD exceeds VA the comparator
output changes to low value and the AND gate is
disabled. This stops the counting at the time
when VA ≈ VD and the counter can be read out as the
digital word representing the analog input voltage.
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Advantages of Counter ADC
Very simple design
Cheap due to its simple design
Disadvantage of Counter ADC
Variable conversion time.
Slow operation
Illustration of 4-bit SAC with 1 volt step size
• Much faster than
the digital ramp
ADC because it
uses digital logic to
converge on the
value closest to the
input voltage.
• A comparator and
a DAC are used in
the process.