ADC (Analog to Digital Converter)
& DAC (Digital to Analog Converter)
Analog to Digital Converters (ADCs) Digital to Analog Converters (DACs)
111
Digital Output Dout
110 VFSR
101
100 ΔV, VLSB
011
010
001
000
Vref 4 7
Vref Vref
8 8 8
Analog Input Vin
• Any analog quantity can be represented by a binary number.
• Longer binary numbers provide higher resolution, which gives a more
accurate representation of the analog quantity.
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Analog to Digital Converters
Objective: Representing an analog varying physical quantity by a
sequence of discrete numerical values.
01 07 10 15 09 03 00 05
Analog Digital
Sample
& Hold
Quantization
fsample
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Digitization Process
3
If you can exactly
reconstruct the signal from
the samples, then you
have done a proper
sampling and captured the
key signal information
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Types of ADCs :
1. Flash or Parallel ADC.
2. Successive Approximation (SAR) ADC.
3. Delta-sigma (ΔΣ) ADC.
4. Dual Slope ADC.
5. Pipelined ADC.
Flash or Parallel type ADC
• Vin connected with 2N comparators in parallel
• Comparators connected to resistor string
• Flash converters are extremely fast compared to
many other types of ADCs
• It is formed of a series of comparators, each
one comparing the input signal to a unique
reference voltage. The comparator outputs
connect to the inputs of
priority encoder circuit, which then produces a
binary output. The following illustration shows
a 3-bit flash ADC circuit.
• 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 active input,
ignoring all other active inputs.
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Successive Approximation type ADC
• Generate internal analog signal VD/A
• Compare VD/A with input signal Vin
• Modify VD/A by D0D1D2…DN-1 until closest possible value to Vin
is reached
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• The basic principle of this type of A/D converter is that the
unknown analog input voltage is approximated against an n-bit
digital value by trying one bit at a time, beginning with the MSB.
• The principle of successive approximation process for a 4-bit
conversion is explained here.
1. The MSB is initially set to 1 with the remaining three bits
set as 000. The digital equivalent voltage is compared with the
unknown analog input voltage.
2. If the analog input voltage is higher than the digital
equivalent voltage, the MSB is retained as 1 and the second MSB is
set to 1. Otherwise, the MSB is set to 0 and the second MSB is set
to 1. Comparison is made as given in step (1) to decide whether to
retain or reset the second MSB.
Selection of ADC
The parameters used in selecting an ADC are very similar to those
considered for a DAC selection:
• Error/Accuracy: Quantizing error represents the difference
between an actual analog value and its digital representation.
Ideally, the quantizing error should not be greater than ± ½
LSB.
• Resolution: V to cause 1 bit change in output
• Output Voltage Range Input Voltage Range
• Output Settling Time Conversion Time
• Output Coding (usually binary)
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Types of DACs
1. Weighted Resistor DAC.
2. R-2R Ladder DAC.
3. Inverted R-2R Ladder DAC.
Weighted Resistor type DAC
Voltages V1 through Vn are Vref
either Vref if corresponding V1 R
bit is high or ground if Rf
V2 2R I
corresponding bit is low
V3 4R
V1 is most significant bit - Vout
n-1
+
Vn 2 R
Vn is least significant bit
MSB
V1 V2 V3 Vn LSB
Vout IRf Rf
R 2R 4R 2 R
n-1
4-bit Weighted Resistor DAC
Sum of the currents from the input resistors; Consider binary weighting factor.
Advantages: Simple Construction/Analysis; Fast Conversion
Disadvantages: Requires large range of resistors (2000:1 for 12-bit DAC)
with necessary high precision for low resistors; Requires low switch
resistances in transistors
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Weighted Resistor (4 bit DAC)
B3 B2 B1 B0
REF R 2R 4R 8R
I V
B B2 B1 B0
VOUT I R f VREF 3
2 4 8 Rf = R
Bi
VOUT VREF
2ni1 I
i
VREF Digital Value Resolution
R 2R 4R 8R Vo
MSB
LSB
-VREF
R-2R Ladder type DAC
• R-2R ladder DAC circuit using op-amp. Here only two values of
resistors are required i.e. R and 2R
• The electronic switch positions decides the binary word.
• The typical value of feedback resistor is Rf = 2R. The resistance R
is normally selected any value between 2.5 kΩ to 10 kΩ.
Let us find the value of
analog output voltage of
3-bit R-2R Ladder DAC
for a binary input
b2b1b0 =100.
Therefore, the output
voltage of 3-bit R-2R
Ladder DAC is VR/2 V
for a binary input of 100.
Digital to Analog Converters
Selection Criteria of DAC
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.
Resolution = 8 bits
1 100% 1 100% 0.391%
Percentage resolution = 8
2 256
Output Voltage Range
This is the difference between the maximum and minimum output voltages
expressed in volts.
Example: Calculate the output voltage range of a 4-bit DAC if the output
voltage is +4.5V for an input of 0000 and +7.5V for an input of 1111.
Output voltage range = 7.5 – 4.5 = 3.0V
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