1. Data acquisition and manipulation
Chapter Eleven
Dr. Gheith Abandah

2. Outline Introduction The main features of a data acquisition system
The characteristics of an analog-to-digital converter
The characteristics of the 16F873A analog-to-digital converter
Summary
Dr. Gheith Abandah

3. Analog vs. Digital Property Analog Digital Representation
Continuous voltage or current
Binary Number
Precision
Infinite range of values
Limited by the number’s length
Resistance to Degradation
Weak
Tolerant to signal degradation
Processing
Limited
Powerful
Storage
Impossible
Possible
Dr. Gheith Abandah

4. 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).
Dr. Gheith Abandah

5. ADC Types
Dual ramp (slow with very high accuracy, for precision measurements)
Flash converter (fast, lesser accuracy, for video or radar)
Successive approximation (medium speed and accuracy, for general-purpose industrial applications, commonly found in embedded systems)
Dr. Gheith Abandah

6. Conversion characteristic
Voltage Range
Vr = Vmax – 0
Resolution =
Vr / 2n
Quantization error =
± Resolution / 2
= Vr / 2n+1
Dr. Gheith Abandah

7. Conversion Steps Get Sample Start conversion Wait
Read digital value (in parallel or serially)
Usually need a voltage reference
Dr. Gheith Abandah

8. Elements of a data acquisition system
Transducers: physical to electrical
Amplify and offset circuits
The input voltage should traverse as much of its input range as possible
Voltage level shifting may also be required
Filter: get rid of unwanted signal components
Multiplexer: select one of multiple inputs
Sampler: the conversion rate must be at least twice the highest signal frequency (Nyquist sampling criterion)
ADC
Dr. Gheith Abandah

9. Elements of a data acquisition system
Dr. Gheith Abandah

10. Sample and hold, and acquisition time
Dr. Gheith Abandah

11. Sample and hold, and acquisition time
Dr. Gheith Abandah

12. 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) = Vs
VC = Vs {1 − exp(−t/RC)}
0.9995Vs = Vs {1 − exp(−t/RC)}
exp(−t/RC) = 1 −
−t = RC ln(0.0005)
t = 7.6RC
Dr. Gheith Abandah

13. Typical timing requirement of one A-to-D conversion
Dr. Gheith Abandah

14. 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.
Dr. Gheith Abandah

15. 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,
8 10-bit ADC,
Dr. Gheith Abandah

16. The PIC® 16F87XA ADC module
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17. Controlling the ADC The ADC is controlled by two SFRs: ADCON0 (1Fh)
The result of the conversion is placed in:
ADRESH (1Eh)
ADRESL (9Eh)
Dr. Gheith Abandah

18. 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
Dr. Gheith Abandah

19. A/D Conversion Clock Select bits
A full 10-bit conversion takes around 12 TAD cycles
TAD should be equal to or just greater than 1.6 μs
Minimum 2TAD between two successive conversions
Maximum conversion rate is 30 kHz, higher rate by switching to higher TAD after starting the conversion
Dr. Gheith Abandah

20. 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
Dr. Gheith Abandah

21. A/D result format
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22. A/D Port Configuration Control
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23. The analog input model
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24. Calculating acquisition time
tac = Amplifier settling time + Hold capacitor charging time + Temperature coefficient tac = 2 μs + 7.6RC for 10-bit accuracy + (Temperature − 25◦C)(0.05 μs/◦C)
Dr. Gheith Abandah

25. Calculating acquisition time - Example
RSS = 7kΩ, RIC = 1kΩ (VDD = 5V), RS = 0, Temp = 35 ◦C, TAD = 1.6 μs tac = 2 μs + 7.6(7kΩ + 1kΩ + 0)(120pF) + (35 − 25)(0.05 μs/◦C) = = 9.8 μs Total time = tac + 12TAD = μs = 29 μs
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26. A/D Example – Page 1
... bsf status,rp0 movlw B' ' ;set port A bits, movwf trisa ;ADC set as inputs movlw B' ' ;bits 0,1,3 analog input movwf adcon1 ;right justify result
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27. A/D Example – Page 2
... bcf status,rp0 movlw B' ' ;set up ADC: clock Fosc/8, ;switch ADC on but not converting, ;channel selection now is irrelevant movwf adcon0
Dr. Gheith Abandah

28. A/D Example – Page 3
main_loop movlw B' ' ;select channel 0 movwf adcon0 call delay20u ;acquisition time bsf adcon0,go ;start conversion btfsc adcon0,go_done ;conversion ended? goto $-1 movf adresh,0 ;read ADC output data high movwf ldr_left_hi bsf status,rp0 movf adresl,0 ;read ADC output data low bcf status,rp0 movwf ldr_left_lo
Dr. Gheith Abandah

29. 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 ADC generally 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

30. 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.
Dr. Gheith Abandah