11 data acquisition and manipulation

7/29/2019 11 Data Acquisition and Manipulation

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Data acquisition and

manipulation

Chapter Eleven

11.1 - 11.3

Dr. Gheith Abandah 1


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Outline

Introduction

The main features of a data acquisition system

The characteristics of an analog-to-digitalconverter

The characteristics of the 16F873A analog-to-

digital converter Summary



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Analog vs. Digital

Property Analog Digital

RepresentationContinuous voltage

or currentBinary Number

PrecisionInfinite range of

values

Limited by the

numbers length

Resistance to

Degradation

WeakTolerant to signal

degradationProcessing Limited Powerful

Storage Impossible Possible



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Analog to Digital Conversion

Most physical signals are analog.

Analog signals are captured by sensors or

transducers.

Examples: temperature, sound, pressure,

Need to convert to digital signals to facilitate

processing by the microcontroller. The device that does this is analog-to-digital

converter (ADC).



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ADC Types

1. Dual ramp (slow with very high accuracy, for

precision measurements)

2. Flash converter (fast, lesser accuracy, for

video or radar)

3. Successive approximation (medium speed

and accuracy, for general-purpose industrial

applications, commonly found in embedded

systems)



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Conversion

characteristic

Voltage Range

Vr = Vmax 0

Resolution =

Vr / 2n

Quantization error =

Resolution / 2= Vr / 2

n+1



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Conversion Steps

1. Get Sample

2. Start conversion

3. Wait4. Read digital value (in parallel or serially)

Usually need a voltage reference



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Elements of a data acquisition system


1. Transducers: physical to electrical

2. Amplify and offset circuits

The input voltage should traverse as much of its input

range as possible

Voltage level shifting may also be required3. Filter: get rid of unwanted signal components

4. Multiplexer: select one of multiple inputs

5. Sampler: the conversion rate must be at least twicethe highest signal frequency (Nyquist sampling

criterion)

6. ADC


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Elements of a data acquisition

system



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Sample and hold, and acquisition

time



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Sample and hold, and acquisition

time



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Example What should be the acquisition time for a 10-bit

ADC?

The voltage should rise to

Vs quantization error = Vs - Vs / 2n+1

Vs(2047/2048) = 0.9995 Vs

VC = Vs {1 exp(t/RC)}

0.9995Vs = Vs {1 exp(t/RC)}

exp(t/RC) = 1 0.9995

t = RC ln(0.0005)

t = 7.6RC Dr. Gheith Abandah 12


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Typical timing

requirement of

one A-to-Dconversion



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Data acquisition in the

microcontroller environment

To operate to a good level of accuracy, an ADC

needs a clean power supply and ground and

no electromagnetic interference.

When an ADC is integrated inside a

microcontroller, it will be affected by the noisy

internal power sources.

So, integrated ADCs are not very accurate,

typically 8- or 10-bit.



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The PIC 16 Series

Device Pins Features

16F873A

16F876A

28 3 parallel ports,

3 counter/timers,

2 capture/compare/PWM,

2 serial,5 10-bit ADC,

2 comparators

16F874A

16F877A

40 5 parallel ports,

3 counter/timers,

2 capture/compare/PWM,

2 serial,

8 10-bit ADC,

2 comparators



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The PIC 16F87XA ADC module



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Controlling the

ADC

The ADC is controlledby two SFRs:

ADCON0 (1Fh)

ADCON1 (9Fh)

The result of the

conversion is placed

in:

ADRESH (1Eh)

ADRESL (9Eh)



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ADCON0: A/D Control Register 0

(address 1Fh) ADCS1:0: conversion clock select

CHS2:0: analog channel select

GO/DONE: conversion status

The ADC interrupt flag ADIF and interrupt

enable ADIE bits can also be used

U: unimplemented

ADON: A/D On



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A/D Conversion Clock Select bits


A full 10-bit conversion takes around 12 TAD cycles

TADshould be equal to or just greater than 1.6 s

Minimum 2TAD between two successive conversions

Maximum conversion rate is 30 kHz, higher rate by switching to higher

TAD after starting the conversion


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ADCON1: A/D Control Register 1

(address 9Fh) ADFM: result format select

1 = Right justified

0 = Left justified

ADCS2: conversion clock select

U: unimplemented

PCFG3:0: port configuration control



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A/D result format



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A/D Port Configuration Control



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The analog input model



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Calculating acquisition time

tac = Amplifier settling time

+ Hold capacitor charging time

+ Temperature coefficient

tac = 2 s

+ 7.6RC for 10-bit accuracy+ (Temperature 25C)(0.05 s/C)



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Calculating acquisition time -

Example

RSS = 7k, RIC = 1k (VDD = 5V), RS = 0,

Temp = 35C, TAD = 1.6 s

tac = 2 s

+ 7.6(7k + 1k + 0)(120pF)

+ (35 25)(0.05 s/C)= 2 + 7.3 + 0.5 = 9.8 s

Total time = tac + 12TAD = 9.8 + 19.2 s = 29 s



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A/D Example Page 1

...

bsf status,rp0

movlw B'00001011' ;set port A bits,

movwf trisa ;ADC set as inputsmovlw B'10000100' ;bits 0,1,3 analog input

movwf adcon1 ;right justify result

...



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A/D Example Page 2

...

bcf status,rp0

movlw B'01000001' ;set up ADC: clock Fosc/8,

;switch ADC on but not converting,;channel selection now is irrelevant

movwf adcon0



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A/D Example Page 3

main_loop

movlw B'01000001' ;select channel 0

movwf adcon0

call delay20u ;acquisition time

bsf adcon0,go ;start conversionbtfsc adcon0,go_done ;conversion ended?

goto $-1

movf adresh,0 ;read ADC output data high

movwf ldr_left_hibsf status,rp0

movf adresl,0 ;read ADC output data low

bcf status,rp0

movwf ldr_left_lo



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Summary - 1 Most signals produced by transducers are analog in

nature, while all processing done by a microcontroller

is digital.

Analog signals can be converted to digital form using

an analog-to-digital converter (ADC). The ADCgenerally forms just one part of a larger data

acquisition system.

Considerable care needs to be taken in applying ADCs

and data acquisition systems, using knowledge among

other things of timing requirements, signal

conditioning, grounding and the use of voltage

references.Dr. Gheith Abandah 29


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Summary - 2 The 16F873A has a 10-bit ADC module that contains

the features of a data acquisition system.

Data values, once acquired, are likely to need further

processing, including offsetting, scaling and code

conversion. Standard algorithms exist for all of these,and Assembler libraries are published.

A simple interface between the analog and digital

world is the comparator, which is commonly used to

classify an analog signal into one of two states.


11 data acquisition and manipulation

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