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SENSORS INTERFACING
Sensors to ADC

Sensors Output span rarely fit input span of ADC
 Offset
(a) – require level shifting
 Unequal span (b) – require amplification
 Both (c) –Require both level shift and amplification
 An OpAmp Level shift and amplify simultaneously
Interfacing components
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OPAMP
Filters
Comparators
ADC
Voltage References
Op-amp Characteristics
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High Input resistance
Low Output resistance
Ability to drive capacitive load
Low input offset voltage
Low input bias current
Very high open loop gain
High common mode rejection ratio
OPAMP classification criteria
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Precision opamp
Single/dual supply opamp
Single ended/differential opamp
High Bandwidth opamp
Rail to rail IO opamp
Open loop condition
Unity gain – Voltage follower
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Provide impedance conversion from high level to
low level
A follower design should have following
characteristics
 For
current generating sensors – input bias current of
opamp should be at least hundred time smaller than
sensors current
 Input offset voltage should be smaller
than required LSB
Instrumentation Amplifier
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Three opamp IA configuration
A IA amplifies the difference between V+ and V-
Instrumentation Amplifiers
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IA are available as monolithic IC’s
Fixed gain range
 Easy
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to set desired gain using a single resistor
Very high CMRR of the order of 100db and more
Filters
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To remove unwanted signal components in the input
signal
Analog Filters
 Passive
filters
 Designed
using passive R,L,C components
 Simple to design 1st order filters
 Active
filters
 Based
on active component like transistors or opamp
 Possible to amplify signal of interest

Digital Filters
Filter Response Characteristics
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Av
Butterworth
Bessel
Chebyshev
f
Categories of Filters
12
Low Pass Filters:
High Pass Filters:
pass all frequencies from dc
up to the upper cutoff
frequency.
pass all frequencies that are
above its lower cutoff
frequency
Av(dB)
Av(dB)
-3dB
{
-3dB
f2
Low-pass response
f
{
f1
High-pass response
f
Categories of Filters
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Band Pass Filters:
Band Stop (Notch) Filters:
pass only the frequencies
that fall between its values
of the lower and upper
cutoff
frequencies.
A
eliminate all signals within
the stop band while passing
all frequencies outside this
A
band.
v(dB)
v(dB)
-3dB
{
-3dB
f1
f2
Band Pass Response
{
f
f1
f2
Band Stop Response
f
Single-Pole Low/High-Pass Filter
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+V
R1
+V
C1
+
vin
vin
C1
+
R1
vout
vout
-
Rf1
-
Rf1
-V
-V
Rf2
Rf2
Low Pass Filter
High Pass Filter
DAC
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Is a circuit whose output depend on digital input and
associated reference voltage
DAC can be implemented using PWM
for PWM
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Vavg=(Ton/T) X Vlh
PWM output filtered using RC filter
ADC Essentials
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
Basic I/O Relationship

ADC is Rationing System

x = Analog input /
Reference
•
Fraction: 0 ~ 1
n bits ADC
Number of discrete output level
: 2n
Quantum
LSB size
Q = LSB = FS / 2n
Quantization Error
1/2 LSB
Reduced by increasing n
Conversion PARAMETERS
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Conversion Time
 Required time (tc) before the converter can provide valid output data
 Input voltage change during the conversion process introduces an
undesirable uncertainty
 Full conversion accuracy is realized only if this uncertainty is kept low
below the converter’s resolution
Converter Resolution
 The smallest change required in the analog input of an ADC to
change its output code by one level
Converter Accuracy
 The difference between the actual input voltage and the full-scale
weighted equivalent of the binary output code
 Maximum sum of all converter errors including quantization error
Converting bipolar to unipolar
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
Using unipolar converter when
input signal is bipolar
 Scaling
down the input
 Adding an offset
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Bipolar Converter
 If
polarity information
in output is desired
 Bipolar input range

Typically, 0 ~ 5V
 Bipolar Output
 2’s Complement
 Offset Binary
 Sign Magnitude
 …
Input signal is scaled and an offset is
added
Add
offset
scaled
Outputs and Analog Reference Signal
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I/O of typical ADC
Errors in reference signal
From
Initial Adjustment
Drift with time and temperature
Cause
Gain error in Transfer
characteristics
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ADC output
 Number of bits
 8 and 12 bits are typical
 10, 14, 16 bits also available
To realize full accuracy of ADC
Precise and stable
reference is crucial
Typically, precision IC voltage
reference is used
5ppm/C ~ 100ppm/C
Control Signals
HBE / LBE
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
Start
From CPU
 Initiate the conversion
process

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BUSY / EOC
To CPU
 Conversion is in progress

0=Busy: In progress
 1=EOC: End of Conversion

From CPU
To read Output word after
EOC
HBE
High Byte Enable
LBE
Low Byte Enable
A/D Conversion Techniques
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Counter or Tracking ADC
Successive Approximation ADC
 Most
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Commonly Used
Dual Slop Integrating ADC
Voltage to Frequency ADC
Parallel or Flash ADC
 Fast
Conversion
Counter Type ADCOperation
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Block diagram
Reset and Start Counter
DAC convert Digital output of
Counter to Analog signal
Compare Analog input and
Output of DAC
Vi < VDAC
Continue counting
Vi = VDAC
Stop counting

Suitable for low frequency high
resolution conversion
Digital Output = Output of
Counter
Disadvantage
Conversion time is varied
2n Clock Period for Full Scale
input
Tracking Type ADC
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
Tracking or Servo Type

Using Up/Down Counter
to track input signal
continuously

For slow varying input
Advantage
There output is continuously
available
Successive Approximation ADC
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Most Commonly used in medium
to high speed Converters
Based on approximating the
input signal with binary code
and then successively revising this
approximation until best
approximation is achieved
SAR(Successive Approximation
Register) holds the current binary
value
Block Diagram
Successive Approximation ADC
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Circuit waveform
Conversion Time
n clock for n-bit ADC
Fixed conversion time
Serial Output is easily
generated
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Logic Flow
Bit decision are made in
serial order
Parallel or Flash ADC
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Very High speed conversion
 Up to 100MHz for 8 bit
resolution
 Video, Radar, Digital
Oscilloscope
Single Step Conversion
 2n –1 comparator
 Precision Resistive Network
 Encoder
Resolution is limited
 Large number of comparator
in IC
Type of ADC’s
ADC Resolution Comparison
Dual Slope
Flash
Successive Approx
Sigma-Delta
0
5
10
15
Resolution (Bits)
20
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Type
Speed (relative) Cost (relative)
Dual Slope
Slow
Med
Flash
Very Fast
High
Successive Appox
Medium – Fast
Low
Sigma-Delta
Slow
Low