Chapter : Data Acquisition and Digital Signal Processing, PPT, Semester, Engineering - Computer Science Engineering (CSE) PDF Download

Embedded Computer Systems
Lecture Notes
Data Acquisition
and
Digital Signal Processing

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Data Acquisition

Data acquisition is the sampling of the real world to generate data that can be manipulated by a computer.
Sometimes abbreviated DAQ or DAS, data acquisition typically involves acquisition of signals and waveforms and processing the signals to obtain desired information.
The components of data acquisition systems include appropriate sensors that convert any measurement parameter to an electrical signal, which is acquired by data acquisition hardware.

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Data Acquisition

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Data Acquisition and Control

􀁺Physical systems (real-world phenomena)
􀁺Transducers and Actuators
􀁺Signal Conditioning equipment
􀁺Data Acquisition & Control Hardware
􀁺Software

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Analog-to-Digital Conversion

Analog-to-digital conversion (ADC) and digital-toanalog conversion (DAC) are the processes that allow digital computers to interact with everyday signals: voltage, current, distance, velocity, temperature, altitude, force, acceleration, pressure etc.

Digital information is different from its analog counterpart in two respects:
- it is sampled
- it is quantized

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ADC Principles: S/H and Quantization

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Quantization Error

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Analog-to-Digital Conversion

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Sampling Theory

Nyquist theorem:
Analog signal that has been digitized can be perfectly reconstructed
if the sampling rate was 1/(2W) seconds,
where W is the highest frequency in the original signal.

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Sampling Theory

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Using Analog Filters

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Basic A/D Architecture

􀁺 Flash ADC
􀁺 Succesive-approximation ADC
􀁺 Sigma-delta ADC

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Flash ADC

• Requires 4095 parallel comparators for 12-bit.
• Very fast, conversion is done in one step.
• Complexity doubles with each added bit of resolution.

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Succesive-approximation ADC

􀁺 Uses an internal n-bit DAC
􀁺 Conversion logic is a simple n-bit counter
􀁺 N-bit ADC requires 2^n cycles to perform a conversion in worst case.

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Delta-sigma ADC

• Requires 1-bit DAC: less susceptible to noise
• Requires high sampling rate, suitable for audio applications
• It has digital filter, so no need for expensive low-pass filters at input
• Resolution can be very high.

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Digital-to-Analog Converter (DAC)

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Transducers

• Converts physical properties such as temperature, pressure, force etc. to electrical signals.

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Signal Conditioning

􀁺 Amplifying
􀁺 Scaling
􀁺 Offsetting
􀁺 Filtering
􀁺 Linearization
􀁺 Isolation

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Digital Signal Processing

1) Specialized microprocessors (DSP)
2) Digital Signal Controllers (DSC)
3) FPGA, ASIC
4) Powerful general purpose microprocessors

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Example: Digital Filters

• Reproducable response
• Temperature insensitive
• Programmable

Example: FIR filter with 4 coefficients
• Unable to pass power
• Requires a power supply
• Frequency range limitations

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Case Study

Getting the most from an ADC means scaling the maximum signal from each sensor to match the maximum input of the ADC. Therefore, we have to use an amplifier along with a scaling circuit. We may left the noise filtering on the acquired temperature to the program

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Table for Thermocouple

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Linear Interpolation

f(3)=0+(3-0)(0.261-0)/(10-0)=0.0783
(In the table, f(3) is given as 0.078)
Calculate the output of N-type thermocouple at 3 °C.
We will use two points: (0, 0) and (10, 0.261).

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Programmable Gain Amplifier

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Digital Filtering

􀁺 A very simple FIR filter (moving average filter) attenuating the high frequencies:

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A Solution
 

􀁺 Amplify the signal such that the output of the signal is approximately 10 volts at 120 C.
􀁺 Shift the signal 5 volts downwards.
􀁺 Implement a look-up table in the microcontroller
􀁺 Implement a digital Low Pass Filter to reduce EMI disturbance.
􀁺 Assume that ADC take the following samples
 

(Sampling rate is 10 ms):
− (2.81v, 2.94v, 2.68v, 2.56v, 2.78v, 2.12v, 2.48v, ...)
􀁺 What would be the current temperature according to your design?