A/D & D/A Converters
DAC Fundamentals
Resolution
• Resolution is defined as the ratio of change in
analog output voltage resulting from a change
of 1 LSB at the digital input.
Resolution =VFS/2n-1
Where n= number of digital inputs
• The full scale analog output voltage VFS is
defined as the output voltage corresponding
to the digital input with all digits 1.
D to A conversion
• The basic building blocks of a DAC are-
a resistive n/w
digitally controlled electronic switches
a voltage reference
current to voltage converter
Types of DAC
• Binary weighted resistor
• R-2R ladder DAC
• Inverted R-2R ladder DAC
Binary Weighted Resistor DAC
• Binary weighted resistor uses a network of binary
weighted resistors and a summing amplifier .There 2n
resistors and n number of SPDT(single pole double
throw) type switches.
• The position of the moving arm of every switch is
controlled by the binary input word.
• The n bit digital word decide the positions of all the
SPDT switches and connect their corresponding
binary weighted resistors to either ground or -VR
R-2R Ladder DAC
• The problem of using wide range of resistor
values can be solved by using R-2R ladder
type DAC.
• There are only two values of resistors are
required.
• R-2R provide results accurately.
• We can also increase the number of bits by
adding more sections of R/2R values.
Inverted R-2R Ladder DAC
• It is also known as current mode R-2R ladder
DAC because this circuit operates on the
current.
• It is called as inverted because the positions of
MSB and LSB in the original R-2R ladder DAC
have been interchanged.
Analog to Digital conversion
1. Sampling
2. Holding
3. Quantizing
4. Encoding
A to D Converter
• ADC performs a function which is exactly opposite
to that of a DAC . The input to an ADC is the analog
voltage VA and at the output is n bit digital word.
• In n bit digital output d1 represent MSB and dn is
the LSB.The analog input voltage VA produces an
output digital word having a function value D .
D=d12-1 +d22-2+……..+dn2-n
A to D(Contd.)
• ADC block has a reference voltage VR input
and control lines SOC and EOC .SOC is used to
start the A to D conversion whereas EOC
output goes high to indicate that the
conversion is complete.
Types of A to D converters
• Flash type
• Counter type
• Successive approximation
• Dual slope
Flash type or Parallel type ADC
• The parallel comparator uses a number OP-AMP
comparators.
• A 2 bit parallel comparator having
i. The non inverting(+) terminal that has been connected
to analog input
ii. The inverting(-) terminal that has been connected to a
different reference voltage. i.e. v1, v2 and v3 and
these voltage are derived from voltage source VR
.There are three comparators C1, C2 and C3 and a
priority encoder
2 bit flash type
Range Analog input Comparators o/p Encoder o/p
number voltage (VA ) C3 C2 C1 Y1 Y0
1. 0<VA<VR/4 0 0 0 0 0
2. VR/4<VA<VR/2 0 0 1 0 1
3. VR/2<VA<3VR/4 0 1 1 1 0
4. 3VR/4<VA<VR 1 1 1 1 1
Successive approximation ADC
• The successive approximation receives the
comparator output ,clock and SOC and
produces an n-bit digital output along with the
end of conversion .When SOC initiates then
SAR will set the MSB d1=1 with all bits 0.So
the trial code becomes 1000 0000 for an 8-bit
ADC . This trial code is then applied at the
input of a DAC.The corresponding output of
DAC VD is applied to comparator.
Dual slope integrator ADC
• In this ADC, an unknown analog voltage and a
known reference voltage are converted into
equivalent time periods using an integrator.
These time periods are measured by a
counter. This circuit is known as dual slope
ADC because the analog voltage and the
reference voltage are converted to ramp
signals of different slopes by the integrator.
