5. Definition
An electronic integrated circuit which
transforms a signal from analog(continues) to
digital(discrete) form
Analog signals are directly measurable
quantities
Digital signals only have two states for digital
computer we refer to binary states, 0 and 1
5
5
6. Continue
The heart of computer-based data acquisition
is usually the analog to digital converter
Basically this device is digital volt meter
Digital Systems require discrète digital data
Analog
?
Digital
6
Digital System
6
7. Continue
Digital computers require signals to be in
digital form whereas most instrumentation
transducers have an output signal in analogue
form.
ADC conversion is therefore required at the
interface between analogue transducers and the
digital computer
7
7
8. Examples of use
• Voltmeter
7.77 V
ΔV
• Cell phone (microphone)
Wave
Voice
8
8
9. Why we need ADC
Microprocessors can only perform complex
processing on digitized signals
When signals are in digital form they are less
susceptible to the deleterious effects of
additive noise
ADC Provides a link between the analog
world of transducers and the digital world of
signal processing and data handling.
9
9
10. Types of analog to digital converter
There are many different types of analog to
digital converters
Each offers something in the way of
Speed
Cost
Power dissipation
complexity
10
10
11. Types of analog to digital converter
Counter type
Successive approximation
There are many types such as flash
type and sigma-delta but we will
cover these two types
11
11
12. Counter type
One of the simplest types of analog to digital
converter is counter type ADC
The input signal of ADC is connected to the
signal input of its internal comparator
The ADC then systematically increases the
voltage of the reference input of the
comparator until the reference becomes larger
than the signal
12
12
13. Continue
And the comparator output goes to 0
Ex: consider an input signal is 4.78 volts. The
initial comparator’s input would be 2.5 volts
The comparator compares the two value then
the result this is less than 4.78 then the next
higher voltage (5.00 volts) is applied
The comparator compares the two value and
says this is greater than 4.78 and switches 0
13
13
14. Continue
The digital output of the ADC is the number of
times the ADC increase the voltage after
starting at the initial 2.5 volts
This scheme is relatively simple , but as the
number of ADC increases the time it takes to
scan through all possible values lower than
input will grow quickly
14
14
15. Components of counter type
This type of converter uses some type of
counter as part of its operation
Counter type contains the following elements:
Digital to analog converter
Some type of counting mechanism
Comparator
clock
15
15
16. Features of counter type
Use a clock to index the counter
Use DAC to generate analog signal to compare
against input
Comparator is used to compare VIN and VDAC
where VIN is the signal to be digitized
The input to the DAC is from the counter
16
16
17. Operation of counter type
START
Comparator
Vin
Control Logic
clock
Counter
DA C
Digital Output
17
17
18. Operation of counter type
START
Comparator
Vin
Control Logic
clock
Counter
DA C
Digital Output
18
18
19. Successive approximation
A Successive Approximation Register (SAR)
is added to the circuit
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
19
20. Continue
The SAR architecture mainly uses the binary
search algorithm
The SAR ADC consists of fewer blocks such
as one comparator, one DAC (Digital to
Analog Converter) and one control logic.
The algorithm is very similar to like searching
a number from telephone book
20
20
21. How Successive Approximation Works
• Example : analog input = 6.428v, reference =
10.000v
MSB
5.000V
2SB
2.500V
3SB
1.250V
LSB
0.625V
VIN > 5.000V
VIN > 7.500V
VIN > 6.250V
VIN > 6.875V
YES
NO
YES
NO
0
1
0
1
21
21
22. Applications
Scanner : when you scan a picture with a
scanner , what scanner is doing is an analog to
digital conversion : it is taking the analog
information provided by the picture(light) and
converting into digital
Recording a voice : when u=you record your
voice or use a VoIP solution on your computer
you r using analog to digital converter to
convert you voice , which is analog into
digital information
22
22
28. continue
ADC is function that converts digital data(usually
binary) into analog signal(current , voltage, or
electric charge)
digital-to-analog converter, a device (usually a
single chip) that converts digital data into analog
signals.
Modems require a DAC to convert data to analog
signals that can be carried by telephone wires.
Video adapters also require DACs, called
RAMDACs, to convert digital data to analog
signals that the monitor can process.
28
28
29. Types of DAC
There are two types of ADC
Weighted Resistor or Resistive Divider type
And there is an other type of R -2R ladder
N bit
digital data
0
1
2
Digital to analog
converter
Analog data
n-2
29
29
30. Weighted Resistors
• In this type of DAC components used is
– Operational amplifier
– Switches
– Resistors
R
– Voltage source
MSB
– Ground
Rf = R
Ii
2R
4R
8R
LSB
-VREF
30
30
31. Definition of weighted resistors
Binary Weighted resistors are used to
distinguish each bit from the most significant
to the least significant
Binary weighted resistors Reduces current by a
factor of 2 for each bit
31
31
32. Continue
Binary Weighted resistors is reliable, and
simple to do
The circuit shown is a digital to analog
converter 4-bits weighted binary resistance
network circuit types.
Resistor values can be calculated using
the weight of the binary number.
32
32
35. Continue
For example
Referring to the circuit as shown, the highest
value resistor (150KΩ) is a digital input
resistor. The smallest bit (least significant bit),
and the values of other resistor is
35
35
36. Circuit analysis to find Vout
If binary input is 0001
R1 = 150KΩ, RF = 20KΩ, Vref = 3V
Voltage Gain (AV) = RF = 20KΩ = 0.133
R1 150KΩ
Vout = Vref X AV
= 3V X 0.1333
= 0.4V
36
36
37. Continue
If binary input is 0110
R2 = 75KΩ,
R3 = 37.5KΩ, RF = 20KΩ, Vref = 3V
RT = R2//R3 = 25KΩ
Voltage Gain (AV) = RF
RT
Vout
= 20KΩ = 0.8
25KΩ
= Vref X AV
= 3V X 0.8
= 2.4V
37
37
38. Calculate
If binary input is 1100
R3 = 37.5KΩ, R4=18.75 RF = 20KΩ, Vref = 3V
RT = R3//R4 = 12.5KΩ
Voltage Gain (AV) = RF
RT
Vout
= 20KΩ = 1.6
12.5KΩ
= Vref X AV
= 3V X 1.6
= 4.8
38
38
39. Simply that we can see the resulting output is shown in the table below
Decimal
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
Digital input
D
0
0
0
0
0
0
0
0
1
1
1
1
1
1
1
C
0
0
0
0
1
1
1
1
0
0
0
0
1
1
1
B
0
0
1
1
0
0
1
1
0
0
1
1
0
0
1
A
0
1
0
1
0
1
0
1
0
1
0
1
0
1
0
Vout (V)
0
0.4
0.8
1.2
1.6
2.0
2.4
2.8
3.2
3.6
4.0
4.4
4.8
5.2
5.6
39
39
40. Example
Find output voltage and current for a binary
weighted resistor DAC of 4 bits where :
R = 10 k Ohms, Rf = 5 k Ohms and VR = 10
Volts. Applied binary word is 1001.
40
40
44. Binary Weighted Resistor
Advantages
Simple Construction/Analysis
Fast Conversion
Disadvantages
Requires large range of resistors (2000:1 for 12bit DAC) with necessary high precision for low
resistors
Requires low switch resistances in transistors
Can be expensive. Therefore, usually limited to
8-bit resolution.
44
44
45. Limitations of binary weighted
Has problems if bit length is longer than 8 bits
For example, if R = 10 k Ohms
R8 = 28-1(10 k Ohms) = 1280 k Ohms
If VR = 10 Volts,
I8 = 10V/1280 k Ohms = 7.8 A
Op-amps to handle those currents are expensive
because this is usually below the current noise
threshold.
45
45
46. Limitations Cont’d
If R = 10 Ohms and Vref = 10 V
I = VR/R = 10V/10 Ohms = 1 A
This current is more than a typical op-amp
can handle.
Large resistors more error
46
46
48. Resolution
Resolution: is the amount of variance in
output voltage for every change of the LSB in
the digital input.
How closely can we approximate the desired
output signal(Higher Res. = finer
detail=smaller Voltage divisions)
A common DAC has a 8 - 12 bit Resolution
VRef
N = Number of bits
Resolution VLSB
N
2
48
48
49. Resolution continue
Better Resolution(3 bit)
Poor Resolution(1 bit)
Vout
Vout
Desired Analog
signal
Desired Analog signal
111
110
8 Volt. Levels
2 Volt. Levels
1
101
110
101
100
100
011
011
010
010
001
0
Approximate
output
0
001
000
000
Digital Input
Approximate
output
49
Digital Input
49
50. Reference voltage
Reference Voltage: A specified voltage used to
determine how each digital input will be
assigned to each voltage division.
Types:
Non-multiplier: internal, fixed, and defined by
manufacturer
Multiplier: external, variable, user specified
50
50
51. Reference voltage types
Multiplier: (Vref = Asin(wt))
Non-Multiplier: (Vref = C)
Voltage
Voltage
11
11
10
10
10
01
01
10
01
01
0
0
00
00
Digital Input
51
00
00
Digital Input
51
52. Settle time
Settling Time: The time required for the input
signal voltage to settle to the expected output
voltage(within +/- VLSB).
Any change in the input state will not be
reflected in the output state immediately. There
is a time lag, between the two events.
52
52
54. Linearity
Linearity: is the difference between the
desired analog output and the actual output
over the full range of expected values.
Ideally, a DAC should produce a linear
relationship between a digital input and the
analog output, this is not always the case.
54
54
56. Speed
Speed: Rate of conversion of a single digital
input to its analog equivalent
Conversion Rate
Depends on clock speed of input signal
Depends on settling time of converter
56
56
58. Non linearity: differential
Analog Output Voltage
Differential Non-Linearity: Difference in
voltage step size from the previous DAC
output (Ideally All DLN’s = 1 VLSB)
Ideal Output
2VLSB
Diff. Non-Linearity = 2VLSB
VLSB
Digital Input
58
58
59. Non linearity: integral
Integral Non-Linearity: Deviation of the
actual DAC output from the ideal (Ideally all
INL’s = 0)
Analog Output Voltage
Ideal Output
Int. Non-Linearity = 1VLSB
1VLSB
Digital Input
59
59
60. Gain error
Gain Error: Difference in slope of the ideal
curve and the actual DAC output
High Gain
High Gain Error: Actual
slope greater than ideal
Low Gain Error: Actual
slope less than ideal
Analog Output Voltage
Desired/Ideal Output
Low Gain
Digital Input
60
60
61. Offset
Offset Error: A constant voltage difference
between the ideal DAC output and the actual.
– The voltage axis intercept of the DAC output curve is different than the
ideal.
Output Voltage
Desired/Ideal Output
Positive Offset
Digital Input
Negative Offset
61
61
62. Applications of DAC
Digital Motor Control
Computer Printers
Sound Equipment (e.g. CD/MP3 Players, etc.)
Function Generators/Oscilloscopes
Digital Audio
62
62