Unit-IV: Data Converter

• Objectives:
– Theory and operation DACs.
– DAC Specifications & Applications
– Analog-to-Digital Converter (ADC) Architectures.
– Sample-and-hold circuits in conjunction with
ADCs.
– Analog Multiplexing
 General Concept
• Most physical variables are analog, and can take
on any value within a continuous range of values.
– Normally a nonelectrical quantity.
• A transducer converts the physical variable to
an electrical variable.
– Thermistors, photo-cells, photodiodes, flow meters,
pressure transducers, tachometers, etc.
 General Concept
• The transducer’s electrical analog output is the
analog input to the analog-to-digital converter.
• The ADC converts analog input to a digital output
– Output consists of a number of bits that represent the
value of the analog input.
• The binary output from the ADC is proportional to the
analog input voltage.
 General Concept
• The digital representation of the process variable
is transmitted from the ADC to the digital computer
– The digital value is stored & processes according
to a program of instructions that it is executing.
• The program might perform calculations or other
operations to produce output that will eventually
be used to control a physical device.
 General Concept
• Digital output from the computer is connected to
a digital-to-analog converter (DAC).
– Converted to a proportional analog voltage/current.
• The analog signal is often connected to some
device or circuit that serves as an actuator to
control the physical variable.
– An electrically controlled valve or thermostat, etc.
Examples
 Digital to Analog Conversion
• A/D conversion involves converting a value
represented in digital input to a voltage or current
proportional to the digital value.
 Digital to Analog Conversion
• For each digital input, the D/A converter output
voltage is a unique

Where K is the proportionality factor signify the amount of voltage for each steps

• The quantity of possible output values can be

increased, and the difference between
successive values decreased—by increasing
the number of bits.
• More & more steps in the output looks like an
analog signal that varies continuously over a
range of values or a “Pseudo-Analog”
 Digital to Analog Conversion
• Each digital input contributes a different amount
to the analog output—weighted according to
their position in the binary number.

Weights are successively doubled

for each bit, beginning with the LSB.

VOUT can be treated to be the weighted

sum of the digital inputs.
 Digital to Analog Conversion
• The resolution of a D/A converter is defined
as the smallest change that can occur in analog output as a result of a
change in digital input.
• Always equal to the weight of the LSB, called the step size, it is the amount
VOUT will change as digital input value changes from one step to the next.
Binary Weighted Register DAC
Each digital input controls a semiconductor
switch such as a CMOS TG

When input is HIGH, the switch connects

a precision reference supply to the input.

When the input is LOW,

the switch is open.

The supply voltage is required to be stable

and precise for accurate analog output.
Binary Weighted Register DAC

It is a simple DAC using an op-amp

summing amplifier with
binary-weighted resistors.

Accuracy of VOUT depends primarily on

precision resistors and reference supply
voltage used.

Input resistor values are binarily

weighted. Starting with MSB,
resistor values increase by a factor
of 2, producing desired weighting
in the voltage output.
 R/2R Ladder Type DAC
• Circuits with binary weighted resistors cause a
problem due to the large difference in R values
between LSB and MSB.
– The R/2R ladder uses resistances that span only
a 2 to 1 range.
 An Integrated Circuit DAC
• Many DACs are available as ICs
• The AD7524, a CMOS IC is an eight-bit D/A
converter that uses an R/2R ladder network.
– This DAC has an eight-bit input that can be latched
internally by Chip Select and WRITE inputs.
• AD7524 8-bit DAC with latched inputs
• When control input goes HIGH, the digital input
data are latched, and the analog output remains
at the level corresponding to that
latched digital data.
 DAC Specifications
• Many DACs are available as ICs or self
contained packages, and their key specifications
are:
– Resolution: The number of possible output levels the DAC
is designed to reproduce.
– Accuracy: It is the deviation of DAC output from expected
value. Ex. ±0.01 F.S.
– Offset error: It is a measure of voltage when all inputs
are 0
– Settling time: It is the time required for o/p to settled within
±1/2 step size of its final value
– Monotonicity: The ability of a DAC's analog output to move
only in the direction that the digital input moves
Ref: 10th ed. of RJ Tocci ; Page 523-525
 GATE Exam
For the 4-bit DAC shown in Figure, the output voltage V0

GATE Questions: https://gateselfstudy.blogspot.com/2017/10/gate-questions-on-adc-and-dac-coverters.html

 DAC Applications
• Used when a digital circuit output must provide
an analog voltage or current.
– Control—use a digital computer output to adjust
motor speed or furnace temperature.
– Automatic testing—computer generated signals to
test analog circuitry.
– Signal reconstruction—restoring an analog signal
after it has been converted to digital.
– Digital amplitude control—used to reduce the
amplitude of an analog signal.
– Serial DACs—with a built-in serial in/parallel out shift
register—many have more than one DAC on the
same chip.
 Analog to digital Conversion
Analog Signals every where
• Microphones - take your voice varying pressure
waves in the air and convert them into varying electrical
signals

• Thermocouple – temperature measuring device

converts thermal energy to electric energy
• Digital Multimeters -
Analog to Digital Conversion

2-Step Process

• Quantizing - breaking down analog value is

a set of finite states.
• Encoding - assigning a digital word or
number to each state and after matching it to the
input signal.
 Quantizing
The number of possible states N that the converter can
acquire N=2n , n= no. of bits
Ex: For a 3 bit ADC, N=23=8.
Quantization size:
Q=(Vmax-Vmin)/N = (10V – 0V)/8 = 1.25V

Output State Encoding

Output Discrete Voltage Output Output Binary
States Ranges (V) States Equivalent
0 0.00-1.25 0 000
1 1.25-2.50 1 001
2 2.50-3.75 2 010
3 3.75-5.00 3 011
4 5.00-6.25 4 100
5 6.25-7.50 5 101
6 7.50-8.75 6 110
7 8.75-10.0 7 111
 A/D Converter Types

–Digital Ramp ADC

–Up/Down Digital Ramp ADC
–Successive Approximation ADC
–Flash ADC
–Dual Slope Integrated ADC
 Digital-ramp ADC
The comparator
compares VAX
with analog input
VA.
While VAX < VA
,comparator
output stays
HIGH.

When VAX
exceeds VA by at
least an amount
equal to VT
(threshold
voltage),
comparator out-
put goes LOW
and stops
modifying the
register number.

If counter used is Up/Down, then it is called Up/Down Digital

Digital-ramp ADC
 11-9 Digital Ramp ADC
• A/D resolution and accuracy
– Reducing the step size can reduce but not eliminate
potential error—called quantization error.
The waveforms illustrate how the computer acquires digital version of the analog signal (VA ).

• Conversion time,
tC(max) =(2N-1)
clock cycles

• Slow response

• Simpler Circuit
 Digital Ramp ADC
11-10 Data Acquisition

• The process by which the computer acquires

digitized analog data is called data acquisition.
• Acquiring a single data point’s value is
referred to as sampling the analog signal.
– That data point is often called a sample.

Microcomputer connected
to a digital-ramp ADC
for acquiring the data.
 Successive Approximation ADC
• It is one of the most widely used types of ADC. It has more complex circuitry
than the digital-ramp ADC but a much shorter conversion time. A fixed value
of conversion time not dependent on the value of the analog input.

• tC(max)
=N×1
The ADC0804 is a CMOS IC that performs Clock
A/D conversion using successive approximation. cycles
 Flash ADCs
• It is the highest-speed ADC.
– Requires much more circuitry than the other types.

IC flash converters are commonly

available in two- to eight-bit
units, and nine- and ten-bit
units.
• No. of resistors =2n
• No. of comparators=2n-1
 Flash ADCs
• Flash converters use no clock signal because no
timing or sequencing is required.
– The conversion takes place continuously.
• When analog input value changes, comparator
outputs change—causing encoder output change.
– Conversion time depends only on the propagation
delays of the comparators and encoder logic.
• Flash converters can be very expensive and tend
to have relatively low resolutions and high
power consumption.
 ADC Types Comparison
ADC Resolution Comparison
Dual Slope
Flash
Successive Approx
Sigma-Delta

0 5 10 15 20 25
Resolution (Bits)

Type Speed (relative) Cost (relative)

Dual Slope Slow Med
Flash Very Fast High
Successive Appox Medium – Fast Low
Sigma-Delta Slow Low
 Sample and Hold Circuits
• Analog voltage connected directly to an ADC input conversion can be
adversely affected if analog voltage is changing during the conversion
time.
• To hold the analog voltage constant while the A/D conversion is taking
place S/H circuit is used.
 Multiplexing
When analog inputs from several sources are to be converted, a
multiplexing technique can be used so that one ADC may be
time-shared.

Many integrated ADCs

contain the
multiplexing circuitry
on the
same chip as the
ADC.
The ADC0808 can
multiplex eight
different analog inputs
into one ADC.
GATE Exam
Concluding Remarks!
Explained the Importance of DACs
ADCs.

Specifications criteria were discussed.

A detail discussions on their working

were taken up