SIC1203 MEASUREMENTS & INSTRUMENTATION UNIT - V DATA ACQUISITION
PREPARED BY : Ms.D.Jamuna Rani , Ms.S.Celin
UNIT :5
Introduction to ADC / DAC - Specifications, ADC Quantization Error, Types of
ADC - Flash, Counter, Successive Approximation, Dual-Slope types and
Introduction to Delta-Sigma, Types of DAC - Weighted-Resistor, 2R ladder and
PWM type, ADC and DAC Problems - Smart sensors.
ANALOG TO DIGITAL CONVERTOR
Definition:
An electronic integrated circuit which converts a signal from analog (continuous and can
take an inifinity of values) to digital (discrete digital data) form. Provides a link between
the analog world of transducers and the digital world of signal processing and data
handling.
Examples of A/D Applications
1. Microphones - Take your voice varying pressure waves in the air and convert
them into varying electrical signals
2. Strain Gages - Determines the amount of strain (change in dimensions) when a
stress is applied
3. Thermocouple – Temperature measuring device converts thermal energy to
electric energy
4. Voltmeters
,SIC1203 MEASUREMENTS & INSTRUMENTATION UNIT - V DATA ACQUISITION
PREPARED BY : Ms.D.Jamuna Rani , Ms.S.Celin
5. Digital Multimeters
Basic circuit:
Process of Analog to Digital Conversion:
1. Sampling and Holding
2. Quantizing - Breaking down analog value in to a set of finite states
3. Encoding - Assigning a digital word or number to each state and matching it to
the input signal
Sampling and Holding
The circuit is an analog device that samples the voltage of a continuously varying
analog signal and holds (freezes) its value at a constant level for a specified minimum
period of time.
Measuring analog signals at uniform time intervals
Ideally twice as fast as what we are sampling
Digital system works with discrete states
Taking samples from each location
The signal is only defined at determined times
The sampling times are proportional to the sampling period (Ts)
, SIC1203 MEASUREMENTS & INSTRUMENTATION UNIT - V DATA ACQUISITION
PREPARED BY : Ms.D.Jamuna Rani , Ms.S.Celin
Respose of sample and Hold Circuit
Switch ON – sampling of signal (time to charge capacitor w/ Vin)
Switch OFF - voltage stored in capacitor (hold operation)
Must hold sampled value constant for digital conversion
Aliasing
High and low frequency samples are indistinguishable
Occurs when the input signal is changing much faster than the sample rate.
Results in improper conversion of the input signal
For example, a 2 kHz sine wave being sampled at 1.5 kHz would be
reconstructed as a 500 Hz (the aliased signal) sine wave.
Usually exists when Nyquist Criterion is violated
Prevented through the use of Low-Pass (Anti-aliasing) Filters
Nyquist Rule:
Use a sampling frequency at least twice as high as the maximum frequency in
the signal to avoid aliasing.
fs>2*fmax
PREPARED BY : Ms.D.Jamuna Rani , Ms.S.Celin
UNIT :5
Introduction to ADC / DAC - Specifications, ADC Quantization Error, Types of
ADC - Flash, Counter, Successive Approximation, Dual-Slope types and
Introduction to Delta-Sigma, Types of DAC - Weighted-Resistor, 2R ladder and
PWM type, ADC and DAC Problems - Smart sensors.
ANALOG TO DIGITAL CONVERTOR
Definition:
An electronic integrated circuit which converts a signal from analog (continuous and can
take an inifinity of values) to digital (discrete digital data) form. Provides a link between
the analog world of transducers and the digital world of signal processing and data
handling.
Examples of A/D Applications
1. Microphones - Take your voice varying pressure waves in the air and convert
them into varying electrical signals
2. Strain Gages - Determines the amount of strain (change in dimensions) when a
stress is applied
3. Thermocouple – Temperature measuring device converts thermal energy to
electric energy
4. Voltmeters
,SIC1203 MEASUREMENTS & INSTRUMENTATION UNIT - V DATA ACQUISITION
PREPARED BY : Ms.D.Jamuna Rani , Ms.S.Celin
5. Digital Multimeters
Basic circuit:
Process of Analog to Digital Conversion:
1. Sampling and Holding
2. Quantizing - Breaking down analog value in to a set of finite states
3. Encoding - Assigning a digital word or number to each state and matching it to
the input signal
Sampling and Holding
The circuit is an analog device that samples the voltage of a continuously varying
analog signal and holds (freezes) its value at a constant level for a specified minimum
period of time.
Measuring analog signals at uniform time intervals
Ideally twice as fast as what we are sampling
Digital system works with discrete states
Taking samples from each location
The signal is only defined at determined times
The sampling times are proportional to the sampling period (Ts)
, SIC1203 MEASUREMENTS & INSTRUMENTATION UNIT - V DATA ACQUISITION
PREPARED BY : Ms.D.Jamuna Rani , Ms.S.Celin
Respose of sample and Hold Circuit
Switch ON – sampling of signal (time to charge capacitor w/ Vin)
Switch OFF - voltage stored in capacitor (hold operation)
Must hold sampled value constant for digital conversion
Aliasing
High and low frequency samples are indistinguishable
Occurs when the input signal is changing much faster than the sample rate.
Results in improper conversion of the input signal
For example, a 2 kHz sine wave being sampled at 1.5 kHz would be
reconstructed as a 500 Hz (the aliased signal) sine wave.
Usually exists when Nyquist Criterion is violated
Prevented through the use of Low-Pass (Anti-aliasing) Filters
Nyquist Rule:
Use a sampling frequency at least twice as high as the maximum frequency in
the signal to avoid aliasing.
fs>2*fmax
