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Data Acquisition SystemsData Acquisition Systems

Data acquisition is the process of sampling signals that measure real world physical conditions and converting the resulting samples into digital numeric values that can be manipulated by a computer. Data acquisition systems (abbreviated with the acronym DAS or DAQ) typically convert analog waveforms into digital values for processing.

Contd. Data Acquisition System A data acquisition system consists of

many components that are integrated to: Sense physical variables (use of

transducers) Condition the electrical signal to make it

readable by an A/D board. Convert the signal into a digital format

acceptable by a computer Process, analyze, store, and display the

acquired data with the help of software

Data Acquisition SystemData Acquisition System

Analog Signal

Signal Conditioner

ADC

Digital Processin

g

Communication

Data Acquisition SystemBlock Diagram

Sensor(Transducers)

Sensors that convert physical parameters to electrical signals

Sense physical phenomena and translate it into electric signal.

Displacement Level Electric

signals ON/OFF

switch

Temperature Pressure Light Force

The components of data acquisition systems include:

Signal ConditioningSignal Conditioning

Signal conditioning circuitry to convert sensor signals into a form that can be converted to digital values.

Functions: modify the analog signal to match the performance of the ADC

Pre-filtering: remove undesirable high frequency components

Amplification: amplify the signal to match the dynamic range of the ADC

Analog-to-Digital Conversion• Most signals are naturally analog (e.g. a voltage)

• Digital techniques are often more useful: data storage, processing, computing, error free signal transmission etc.

• We need ways to convert analog signals to digital

• Want A/D converters to be fast, accurate and cheap

Before a computer can process analog information, we must first use an analog-to-digital converter (ADC) to transform the analog values into digital binary values.

Data Acquisition Software It can be the most critical factor in

obtaining reliable, high performance operation.

Transforms the PC and DAQ hardware into a complete DAQ, analysis, and display system.

Different alternatives: Programmable software. Data acquisition software packages.

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Successive-approximation A/D Successive-approximation A/D converter converter The successive approximation ADC has been the mainstay of data acquisition systems for many years.A successive approximation ADC is a type of analog-to-digital converter that converts a continuous analog waveform into a discrete digital representation via a binary search through all possible quantization levels before finally converging upon a digital output for each conversion.

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Contd. Successive-approximation A/D Contd. Successive-approximation A/D converter converter The successive-approximation A/D converter consists of:•D/A converter•Comparator• Success-approximation register .•Conversion process starts at Maximum range

(MSB) and steps down through a defined sequence to the (LSB) until correct reading is found using a DAC and an analog comparator circuit.

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Contd. Successive-approximation A/D Contd. Successive-approximation A/D converter converter Successive Approximation ADC Block Diagram.

KeyDAC = Digital-to-Analog converterEOC = end of conversionSAR = successive approximation registerS/H = sample and hold circuitVin = input voltageVref = reference voltage

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Contd. Successive-approximation A/D Contd. Successive-approximation A/D converter converter The successive approximation Analog to digital converter circuit typically consists of four chief sub circuits:A sample and hold circuit to acquire the input voltage (Vin).An analog voltage comparator that compares Vin to the output of the internal DAC and outputs the result of the comparison to the successive approximation register (SAR).

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Contd. Successive-approximation A/D Contd. Successive-approximation A/D converter converter A successive approximation register sub circuit designed to supply an approximate digital code of Vin to the internal DAC.An internal reference DAC that, for comparison with VREF, supplies the comparator with an analog voltage equal to the digital code output of the SARin.

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Operation of the successive-Operation of the successive-approximation A/D converter approximation A/D converter

The successive approximation register is initialized so that the most significant bit (MSB) is equal to a digital 1. This code is fed into the DAC, which then supplies the analog equivalent of this digital code (Vref/2) into the comparator circuit for comparison with the sampled input voltage.If this analog voltage exceeds Vin the comparator causes the SAR to reset this bit; otherwise, the bit is left a 1.

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Operation of the successive-Operation of the successive-approximation A/D converter approximation A/D converter

Then the next bit is set to 1 and the same test is done, continuing this binary search until every bit in the SAR has been tested. The resulting code is the digital approximation of the sampled input voltage and is finally output by the SAR at the end of the conversion (EOC).

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Operation of the successive-Operation of the successive-approximation A/D converter approximation A/D converter

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Example:Operation of the successive-Example:Operation of the successive-approximation A/D converter approximation A/D converter

Successive Approximation ADC Example

Goal: Find digital value Vin

• 8-bit ADC• Vin = 7.65

• Vfull scale = 10

Successive Approximation ADC Example

• MSB LSB

• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 7• (Vfull scale +0)/2 = 5

• 7.65 > 5 Bit 7 = 1

Vfull scale = 10, Vin = 7.65

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Successive Approximation ADC Example

• MSB LSB• Average high/low limits• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 6• (Vfull scale +5)/2 = 7.5

• 7.65 > 7.5 Bit 6 = 1

Vfull scale = 10, Vin = 7.65

1  1           

Successive Approximation ADC Example

• MSB LSB• Average high/low limits• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 5• (Vfull scale +7.5)/2 = 8.75

• 7.65 < 8.75 Bit 5 = 0

Vfull scale = 10, Vin = 7.65

1  1  0         

Successive Approximation ADC Example

• MSB LSB• Average high/low limits• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 4• (8.75+7.5)/2 8.125• 7.65 < 8.125 Bit 4 = 0

Vin = 7.65

1  1  0  0       

Successive Approximation ADC Example

• MSB LSB• Average high/low limits• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 3• (8.125+7.5)/2 = 7.8125• 7.65 < 7.8125 Bit 3 = 0

Vin = 7.65

1  1  0  0 0      

Successive Approximation ADC Example

• MSB LSB• Average high/low limits• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 2• (7.8125+7.5)/2 = 7.65625• 7.65 < 7.65625 Bit 2 = 0

Vin = 7.65

1  1  0  0 0   0   

Successive Approximation ADC Example

• MSB LSB• Average high/low limits• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 1• (7.65625+7.5)/2 = 7.578125• 7.65 > 7.578125 Bit 1 = 1

Vin = 7.65

1  1  0  0 0   0 1  

Successive Approximation ADC Example

• MSB LSB• Average high/low limits• Compare to Vin

• Vin > Average MSB = 1

• Vin < Average MSB = 0

• Bit 0• (7.65625+7.578125)/2 = 7.6171875• 7.65 > 7.6171875 Bit 0 = 1

Vin = 7.65

1  1  0  0 0   0 1  1 

Successive Approximation ADC Example

• 110000112 = 19510

• 8-bits, 28 = 256• Digital Output

• 195/256 = 0.76171875• Analog Input

• 7.65/10 = 0.765

Exercise Given an analog input signal whose voltage

should range from 0 to 15 volts, and an 8-bit digital encoding, calculate the approximate encoding for 5 volts. Trace the successive-approximation approach to find the correct encoding.