1. Sensors Interfacing

2. 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

3. Interfacing components
OPAMP
Filters
Comparators
ADC
Voltage References

4. Op-amp Characteristics
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

5. OPAMP classification criteria
Precision opamp
Single/dual supply opamp
Single ended/differential opamp
High Bandwidth opamp
Rail to rail IO opamp

6. Open loop condition

7. Unity gain – Voltage follower
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

8. Instrumentation Amplifier
Three opamp IA configuration
A IA amplifies the difference between V+ and V-

9. Instrumentation Amplifiers
IA are available as monolithic IC’s
Fixed gain range
Easy to set desired gain using a single resistor
Very high CMRR of the order of 100db and more

10. Filters 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

11. Filter Response Characteristics

12. Categories of Filters Low Pass Filters:
pass all frequencies from dc up to the upper cutoff frequency.
High Pass Filters:
pass all frequencies that are above its lower cutoff frequency
Low-pass response
High-pass response

13. Categories of Filters Band Pass Filters:
pass only the frequencies that fall between its values of the lower and upper cutoff frequencies.
Band Stop (Notch) Filters:
eliminate all signals within the stop band while passing all frequencies outside this band.
Band Pass Response
Band Stop Response

14. Single-Pole Low/High-Pass Filter
Low Pass Filter
High Pass Filter

15. DAC
Is a circuit whose output depend on digital input and associated reference voltage
DAC can be implemented using PWM
for PWM
Vavg=(Ton/T) X Vlh
PWM output filtered using RC filter

16. ADC Essentials Basic I/O Relationship ADC is Rationing System
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
Basic I/O Relationship
ADC is Rationing System
x = Analog input /
Reference
Fraction: 0 ~ 1

17. Conversion PARAMETERS
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

18. Converting bipolar to unipolar
Input signal is scaled and an offset is added
Using unipolar converter when input signal is bipolar
Scaling down the input
Adding an offset
Bipolar Converter
If polarity information in output is desired
Bipolar input range
Typically, 0 ~ 5V
Bipolar Output
2’s Complement
Offset Binary
Sign Magnitude …
Add
offset
scaled

19. Outputs and Analog Reference Signal
I/O of typical ADC
ADC output
Number of bits
8 and 12 bits are typical
10, 14, 16 bits also available
Errors in reference signal
From
Initial Adjustment
Drift with time and temperature
Cause
Gain error in Transfer characteristics
To realize full accuracy of ADC
Precise and stable reference is crucial
Typically, precision IC voltage reference is used
5ppm/C ~ 100ppm/C

20. Control Signals HBE / LBE Start BUSY / EOC From CPU
To read Output word after EOC
HBE
High Byte Enable
LBE
Low Byte Enable
Start
From CPU
Initiate the conversion process
BUSY / EOC
To CPU
Conversion is in progress
0=Busy: In progress
1=EOC: End of Conversion

21. A/D Conversion Techniques
Counter or Tracking ADC
Successive Approximation ADC
Most Commonly Used
Dual Slop Integrating ADC
Voltage to Frequency ADC
Parallel or Flash ADC
Fast Conversion

22. Counter Type ADC Operation Reset and Start Counter Block diagram
DAC convert Digital output of Counter to Analog signal
Compare Analog input and Output of DAC
Vi < VDAC
Continue counting
Vi = VDAC
Stop counting
Digital Output = Output of Counter
Disadvantage
Conversion time is varied
2n Clock Period for Full Scale input
Block diagram
Suitable for low frequency high resolution conversion

23. Tracking Type ADC Tracking or Servo Type
Using Up/Down Counter to track input signal continuously
For slow varying input
Advantage There output is continuously available

24. Successive Approximation ADC
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

25. Successive Approximation ADC
Circuit waveform
Logic Flow
Conversion Time
n clock for n-bit ADC
Fixed conversion time
Serial Output is easily generated
Bit decision are made in serial order

26. Parallel or Flash ADC 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

27. Type of ADC’s Type Speed (relative) Cost (relative) Dual Slope Slow
Med
Flash
Very Fast
High
Successive Appox
Medium – Fast
Low
Sigma-Delta