Data - converter

circuits A/D and

D/A
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 A/D and D/A Converters
2

Analog to Digital Digital to

Analog

What parts of your iPhone operation Your internet access: Analog ?

are Analog ? / Digital Digital ?

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 3 ADC Conversion Process
Two main steps of process
1. Sampling and Holding
2. Quantization and Encoding

Analog-to-Digital Converter

Quantizing
and
Encoding
Sampling and
Hold
t
Input: Analog t
Signal
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4

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 D/A Conversion
5
Normal Output from digital domain is staircase
Filtered to produce smooth Analog output

The analog samples at the output of a D/A converter are usually fed to a sample-
and-hold circuit to obtain the staircase waveform shown. This waveform can then be
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filtered to obtain the smooth waveform, shown in color. The time delay usually
introduced by the filter is not shown.
6 A/D Converter Types

 Flash ADC

 Delta-Sigma ADC

 Dual Slope (integrating) ADC

 Successive Approximation ADC

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7 Flash ADC

 series of comparators, each one compares

input to a unique reference voltage.

 comparator outputs connect to a priority

encoder circuit produces binary output

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 Flash Analog to Digital Converter
Fast
8 – but more expensive :
Single cycle - Uses many Comparators in
parallel with different reference voltages
Digital
Analog

• 2N-1 comparators for N-bits

• Each reference voltage
equivalent to a quantization
level
• Encoding logic produces word

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9 Flash ADC Circuit

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10 How Flash Works

 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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11 Flash ADC Output

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 Flash
12

Advantages Disadvantages
 Simplest in terms of
operational theory  Lower resolution

 Expensive

 Most efficient in terms of  For each additional output

speed, very fast bit, the number of
comparators is doubled
 limited only in terms
of comparator and  i.e. for 8 bits, 256
gate propagation comparators needed
delays

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Successive Approximation ADC
13

 A Successive Approximation Register (SAR) is added to the

circuit. It reduces the conversion time from milliseconds to
microseconds.

 Instead of counting up in binary sequence, this register counts

by trying all values of bits starting with the MSB and finishing at
the LSB.

 The register monitors the comparators output to see if the

binary count is greater or less than the analog signal input and
adjusts the bits accordingly.

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 Successive Approximation ADC
14
Circuit

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15 Output

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 Successive Approximation
16

Advantages Disadvantages

 Capable of high speed and reliable  Higher resolution successive

approximation ADC’s will be slower
 Medium accuracy compared to other
ADC types  Speed limited to ~5Msps

 Good tradeoff between speed and cost

 Capable of outputting the binary

number in serial (one bit at a time)
format.

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17 ADC Applications

 ADC are used virtually everywhere where an analog signal

has to be processed, stored, or transported in digital form

➢ Microphones

➢ Strain Gages

➢ Thermocouple

➢ Digital Multimeters

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 What is a DAC
18

 A digital to analog converter (DAC) is a

device that converts digital numbers (binary)
into an analog voltage or current output.

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Principal components of DAC
19

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 Typical Output
20

DAC

Output typical of a real,

practical DAC due to sample
& hold
Ideally Sampled Signal
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21 Types of DAC
implementations
❖ Binary Weighted Resistor

❖ R-2R Ladder

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 Binary Weighted Resistor
22

•Start with
summing op-amp
circuit
•Input voltage
either high or
ground
•Adjust resistor
weighting to
achieve desired
Vout
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 Binary Weighted Resistor
23

• Details
– Use transistors to switch
between high and ground
– Use resistors scaled by
two to divide voltage on
each branch by a power
of two
– V1 is MSB, V4 LSB in this
circuit
• Assumptions:
– Ideal Op-Amp
– No Current into Op-Amp
– Virtual Ground at Inverting
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– Vout = -IRf
 Binary Weighted Resistor
24

Assume
binary
B5
inputs B0
B4 (LSB) to Bn-
B3 1 (MSB)
Each Bi = 1
B2
or 0 and is
B1 multiplied
B0
by Vref to
get input
 Bn −1 Bn − 2 B1 Bvoltage

Vout = − IRf = − Rf Vref  + + ... n − 2 + n-10 
 R 2R
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25 Binary Weighted Resistor

 B3 take
 Example: B2 a 4-bit B0  Rf = aR
B1 converter,
Vout = −aVref  + + + 
 1 2 4 8 

 Input parameters:
 Input voltage Vref = -2V
 Binary input = 1011
 Coefficient a = ½
1  1 0 1 1  11
Vout = − ( −2 )  + + +  = = 1.375V
2 1 2 4 8  8

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26 Binary Weighted Resistor

 Resolution: find minimum nonzero output

Rf Vref
Vmin =
R 2n-1
Vref
 If Rf = R/2 then resolution 2
isn

 1 
and maxVVout is=
max Vref 1 − n 
 2 

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27 Binary Weighted Resistor

Advantages
❖ Simple
❖ Fast

Disadvantages
❖ Need large range of resistor values (2048:1 for 12-bit) with
high precision in low resistor values
❖ Need very small switch resistances
❖ Op-amp may have trouble producing low currents at the
low range of a high precision DAC

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28 R-2R Ladder
 Circuit may be analyzed
using Thevenin’s theorem
(replace network with B2
equivalent voltage source Rf
B1
and resistance)
B0
 Final result is:

Rf n −1
Bi
Vout = −Vref 
R i =0 2 n −i

Compare to binary weighted circuit:

Rf n −1
Bi
Vout = −Vref 
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R 2 i =0
( n −1) −i
29 R-2R Ladder

 Rf Vref
Resolution
Vmin =
R 2n
Vref
n
 If Rf = R then resolution2is

 1 
and maxVVout is=
max Vref 1 − n 
 2 

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 R-2R Ladder
30

 Advantages:
❖ Only 2 resistor values
❖ Lower precision resistors acceptable

 Disadvantages
❖ Slower conversion rate

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 Common Applications:
31
Function Generators
 Digital Oscilloscopes  Signal Generators

 Sine wave generation

Digital Input
 Square wave generation
Analog Output
 Triangle wave generation
 Random noise generation

1 2

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