Types of ADC

• Flash A/D Converter

• Successive Approximation A/D Converter
• Single Slope A/D Converter
• Dual Slope A/D Converter
• Delta-Sigma A/D Converter
 Flash A/D Converter

• Uses the 2N resistors to form a ladder voltage divider, which

divides the reference voltage into 2N equal intervals.
• Consists of a series of comparators, each one comparing the
input signal to a unique reference voltage.
• The comparator outputs connect to the inputs of a priority
encoder circuit, which produces a binary output
3-bit Flash A/D Converter
Comparator

If Output
VIN
+ VOUT
VREF
VIN > VREF High
-

VIN < VREF Low

Truth Table of Priority Encoder
Flash ADC operation

• As the analog input voltage exceeds the reference voltage at each

comparator, the comparator outputs will sequentially saturate to a high
state.

• The priority encoder generates a binary number based on the highest-

order, ignoring all other active inputs.
Advantages and Disadvantages

Advantages:
• Very Fast .
• Very simple operational theory .
• Speed is only limited by gate and comparator propagation delay .

Disadvantages:
• Expensive.
• Each additional bit of resolution requires twice the comparators.
 Successive Approximation ADC
• It consists of a successive
approximation register (SAR), DAC
and comparator.
• The output of SAR is given to N-bit
DAC.
• The equivalent analog output
voltage of DAC, VDA is applied to
the inverting input of the
comparator.
• The second input to the
comparator is the unknown
analog input voltage VIN.
Figure: Block diagram of N-bit
• The output of the comparator is Successive Approximation ADC
used to activate the successive
approximation logic of SAR.
• A/D conversion begins by setting the MSB = 1 and keeping
rest of the bits = 0
• DAC converts the digital output of the SAR to analog signal,
VDA which is compared with VIN
• If VIN > VDA; then MSB of SAR is kept at 1 and next lower
significant bit is set to 1
• If VIN < VDA; then MSB of SAR is cleared to 0 and next lower
significant bit is set to 1
• Again, DAC converts the digital output of the SAR and
comparing VDA with VIN , the respective bit is either kept at 1
or it is cleared to 0
• This process is repeated till all the bits have been checked
• A/D conversion is then completed and the contents of SAR are
the Digital representation of VIN
 3-bit Successive Approximation ADC (Example)
Let, VREF = 10V

D2 D1 D0 VDA
0 0 0 0.00
0 0 1 1.25
0 1 0 2.50
0 1 1 3.75
1 0 0 5.00
1 0 1 6.25
1 1 0 7.50
1 1 1 8.75
Figure: Block diagram of 3-bit
Table 1: Input bit pattern and o/p
Successive Approximation ADC
analog voltage range of 3-bit DAC
Let, VIN = 3.2 V
A/D conversion takes place
according to the following 3
cycles:
• MSB (D2) = 1 , D1 =0 , D0 = 0.
Input to DAC is ‘1 0 0’
According to Table 1
VDA = 5 V
As, VIN < VDA ; D2 is cleared to
0
Resulting bit pattern at this
stage:
0 0 0
• Next, D1 is set to 1.
Input to DAC is ‘0 1 0’
According to Table 1
VDA = 2.5 V
As, VIN > VDA ; D1 is unchanged

Updated bit pattern at this

stage:
0 1 0
• Next, D0 is set to 1.
Input to DAC is ‘0 1 1’
According to Table 1
VDA = 3.75 V
As, VIN < VDA ; D0 is cleared to
0
Updated bit pattern at this
stage:
0 1 0

• A/D conversion is completed

and the digital representation
of VIN = 3.2V is ‘0 1 0’
Figure: Conversion sequence of 3-bit successive
approximation ADC
 Alternative Approach
In the previous example, VREF = 10V
So, voltage contribution from each bit of DAC is:
Table 2
Bit D2 D1 D0
Voltage Contributed 5V 2.5 V 1.25 V

The 3 cycles of A/D conversion:

• MSB (D2) = 1 , D1 =0 , D0 = 0.
Input to DAC is ‘1 0 0’
According to Table 2
VDA = (5 + 0 + 0) V
As, VIN < VDA ; D2 is cleared to 0

Resulting bit pattern at this stage: 0 0 0

• Next, D1 is set to 1. • Next, D0 is set to 1.
Input to DAC is ‘0 1 0’ Input to DAC is ‘0 1 1’
According to Table 2 According to Table 1
VDA = (0 + 2.5 + 0) V VDA = 3.75 V
As, VIN > VDA ; D1 is unchanged As, VIN < VDA ; D0 is cleared to
0
Updated bit pattern at this Updated bit pattern at this
stage: stage:
0 1 0 0 1 0

A/D conversion is completed and the digital

representation of VIN = 3.2V is ‘0 1 0’
 Advantages and Disadvantages SA ADC

Advantages :
• Capable of high speed and reliable .
• Medium accuracy compared to other ADC types.
• Good tradeoff between speed and cost.

Disadvantages :
• Higher resolution successive approximation ADC’s will be
slower
Single Slope ADC
Single Slope ADC
Dual Slope ADC
Dual Slope ADC
Dual Slope ADC
Dual Slope ADC
Single Slope ADC
Single Slope ADC
DIGITAL TO ANALOGUE CONVERSION
(DAC)
 Weighted-Resistor D/A Converter
• Can be designed using an operational amplifier
and appropriate combination of resistors
• Resistors connected to data bits are in binary
weighted proportion, and each is twice the value
of the previous one.
• Each input signal can be connected to the op amp
by turning on its switch to the reference voltage
that represents logic 1.
• If the switch is off, the input signal is logic 0.
 Weighted-Resistor D/A Converter

Total Current :

Output Voltage :
Disadvantages of Binary Weighted DAC
• One disadvantage of this DAC is that N inputs require N
binary-weighted resistor values.
• Another is that the circuit require very accurate resistors, as
the DAC would require that the error in each resistor be less
than the smallest resistor value. This type requires large range
of resistors with necessary high precision for low resistors.
• Requires low switch resistances in transistors.
• Can be expensive. Hence resolution is limited to 8-bit size.
 R-2R Ladder D/A Converter

• A resistive ladder network is a special type of series-parallel

circuit.
• One form of ladder network is commonly used to scale down
voltages to certain weighted values for digital-to-analog
conversion
– Called R/2R Ladder Network
• To find total resistance of a ladder network, start at the point
farthest from the source and reduce the resistance in steps.
 Figure: 4-bit R-2R Ladder D/A Converter

Figure: Thevenin equivalent to the left of point A

 Figure: 4-bit R-2R Ladder D/A Converter

Thevenin’s
equivalent
voltage can be
determined
using
superposition
theorem

Figure: Thevenin equivalent to the left of point B

 Figure: 4-bit R-2R Ladder D/A Converter

Figure: Thevenin equivalent to the left of point D

Advantages and Disadvantages

Advantages:
• Only two resistor values are used in R-2R ladder type.
• It does not need as precision resistors as Binary weighted DACs.
• It is cheap and easy to manufacture.

Disadvantages:
• It has slower conversion rate.