Interfacing to Analog World - WordPress.com · •Introduction to Analog to digital Conversion •...

• Introduction to Analog to digital Conversion

• Why Analog to Digital?

• Basics of A/D Conversion.

• A/D converter inside PIC16F887

• Related Problems

Interfacing to Analog World Sensor Interfacing

Prepared By-

Mohammed Abdul Kader

Assistant Professor, EEE, IIUC

Lecture Materials on "Interfacing to Analog World", By- Mohammed Abdul Kader, Assistant Professor, EEE, IIUC 2

Introduction to Analog to digital Conversion

Signals in the real world are analog: light, sound, temperature, pressure, acceleration or

other phenomenon. So, real-world signals must be converted into digital, using a circuit

called ADC (Analog-to-Digital Converter), before they can be manipulated by digital

equipment.

When you scan a picture with a scanner what the scanner is doing is an analog-to-digital

conversion: it is taking the analog information provided by the picture (light) and

converting into digital.

When you record your voice on your computer, you are using an analog-to-digital

converter to convert your voice, which is analog, into digital information.

When an audio CD is recorded at a studio, once again analog-to-digital is taking place,

converting sounds into digital numbers that will be stored on the disc.

Whenever we need the analog signal back, the opposite conversion – digital-to-analog,

which is done by a circuit called DAC, Digital-to-Analog Converter – is needed. When

you play an audio CD, what the CD player is doing is reading digital information stored

on the disc and converting it back to analog so you can hear the audio.


Why Analog to Digital? 1. Reducing Noise: Since analog signals can assume any value, noise is interpreted as

being part of the original signal. Digital systems, on the other hand, can only understand

two numbers, zero and one. Anything different from this is discarded.

Noise is added with

Signal

Information is

unchanged


Why Analog to Digital? (Continued)

2. Signal Processing: The most strong tool of processing or analyzing signal is

microprocessor. Microprocessor is a digital device. It can understand only digital signal. So,

we should convert the analog physical parameters into digital to process it by microprocessor.

3. Data compression capability: Another advantage of digital system against analog is the

data compression capability. Since the digital counterpart of an analog signal is just a bunch of

numbers, these numbers can be compressed, just like you would compress a Word file using

WinZip to shrink down the file size, for example. The compression can be done to save

storage space or bandwidth.


The main use of ADC in embedded system is to measure the voltage outputs of sensors.

Most electronic sensors produce a voltage that corresponds to temperature, pressure, acceleration

or other phenomenon.

Music, speech, or other signals can be converted to digital form by A/D converters for storage or

additional processing.

ADC (A/D Converter) in Embedded System

Sensor Physical

parameter A/D Converter

Converts physical parameter to

corresponding analog voltage

Converts analog voltage to

digital voltage

Microprocessor

Analyze or process the

physical parameter

Gives a decision (output)

according to user instruction


Related terms of A/D conversion

Resolution: The resolution of an A/D converter is specified by number of bits and determines how

many distinct output levels or codes (2^N) the converter is capable of producing. For example, an 8-bit

A/D converter produces 2^8, or 256, output codes or levels. Mathematically the resolution for an A/D

converter is

In other Words, Resolution is the smallest change or steps in the analog voltage for which output digital

voltage (codes) remain unchanged.

For an 3 bit A/D converter with

conversion range (0-5V):

Resolution= 5/2^3 = 5/ 8= 0.625 V


conversion range (0-5V):

Resolution= 5/2^8 = 5/ 256 = 0.0196 V


conversion range 0-5V:

Resolution = 5/2^10 = 5/1024 = 0.00488 V

Here, Reference means the conversion range

and N is the no of bit in digital output.


Related terms of A/D conversion (Cont.)

Resolution of an 2-bit A/D converter is 1.25V

Resolution of an 10-bit A/D converter is 0.00488V

Another definition ,The resolution of the A/D converter is the voltage change that will result in the

returned value (output digital value) changing by 1.

This means that with an 3 bit A/D the returned value will change by 1 when the voltage changes by

about 1.25V. With a 10 bit A/D, a change of only 4.88 mV will change the count by 1.


Clearly an A/D converter with more bits will give better accuracy.

But, for a higher bit A/D converter you need more storage memory to store the result and at the

same time A/D converter circuit become more complex and conversion time will increase.


Problem 1: A weight sensor changes its output voltage 0.1 mV for 1gm change in input and it’s

output varies from 0-5V. What should be the resolution and no of bits in the A/D converter if you

want to get the weight in gram.

The resolution should be 0.1mV = 0.0001 V



Quantization Error : If an AC signal is applied to an ideal A/D converter, noise present in the

digitized output due to quantization error.

If the input to the quantizer is moved through its full range and subtracted from the discrete output

levels the error signal will be a sawtooth waveform with a peak-to-peak value Q as shown in figure (b).

The quantization error is dependent on the number of quantization level (discrete levels). The

quantization levels are an index of the resolution. The output of a quantizer can be considered as a noise

signal with an rms value of

Which is the quantization error.

Fig. (a) Fig. (b)



Sampling:

An analog signal is continuous and it has infinite number of samples. It is not possible to take the

sample of analog signal at every instant of time. What the ADC circuit does is to take samples

from the analog signal in a regular time interval. Each sample will be converted into a number,

based on its voltage level.

The frequency on which the sampling will occur is called sampling rate. If a sampling rate of

22,050 Hz is used, for example, this means that in one second 22,050 points will be sampled.

Thus, the distance of each sampling point will be of 1 /22,050 second (45.35 µs, in this case). If a

sampling rate of 44,100 Hz is used, it means that 44,100 points will be captured per second. In

this case the distance of each point will be of 1 /44,100 second or 22.675 µs. And so on.


Basic of A/D Conversion

Sampling Rate= 10 samples/Sec

3-bit A/D converter

So, to store the signal of length 1.1 s we need

(10*1.1*3) = 33 bit memory. 3-b

it A

/D

co

nv

ert

er

wit

h s

amp

lin

g r

ate

of

10 s

amp

les/

sec


Sampling Rate= 20 samples/Sec

4-bit A/D converter

So, to store the signal of length 1.1 s we need

(20*1.1*4) = 88 bit memory. = 11 Byte 4-b

it A

/D

co

nv

ert

er

wit

h s

amp

lin

g r

ate

of

20 s

amp

les/

sec

Basic of A/D Conversion


Basic of A/D Conversion: Performance analysis of two A/D converters

4-b

it A

/D

co

nv

ert

er

wit

h s

amp

lin

g r

ate

of

20 s

amp

les/

sec

3-b

it A

/D

co

nv

ert

er

wit

h s

amp

lin

g

rate

of

10 s

amp

les/

sec

4-bit A/D converter with sampling rate 20 sample/sec is better than 3-bit A/D converter with

sampling rate 10 sample/sec because converter output of 2nd one is more closer to original signal.

But, 2nd one need 88-bit memory to store the signal where 1st one need only 33-bit memory.


Basic of A/D Conversion: Minimum Sampling Rate (Nyquist Rate)

So, we have this dilemma: if the sampling rate is too high, the output quality will be close to

perfection, but you will need a lot of storage space to hold the generated data (i.e., the generated file

will be very big); if the sampling rate is too low, the output quality will be bad.

How can you know the best sampling rate to be used during analog-to-digital conversions to have the

best storage/quality balance? The answer is the Nyquist Theorem.

“This theorem states that the sampling rate on analog-to-digital conversions must

be at least two times the value of the highest frequency you want to capture.”

In practice it is necessary to sample at least 5 times the value of the highest frequency in order to

reduce the effect of noise and non-sinusoidal filters.

Since the human ear listens to sounds up to the frequency of 20 kHz, for music we need to use a

sampling rate of at least 40,000 Hz. In fact, the CD uses a 44,100 Hz sampling rate, thus capturing

more than our ears can hear (this value was arbitrated by Phillips and Sony when they created the

CD). Some professional audio applications use an even higher sampling rate.

The phone system, on the other hand, was created to transmit only human voice, which has a lower

frequency range, up to 4 kHz. So on the digital part of the phone system, an 8,000 Hz sampling rate

is used. That’s why if you try to transmit music through the phone the quality is bad: the phone

circuitry cancels all frequencies above 4 kHz.


Problem 2: There is an embedded system which has an 16-bit A/D converter with 8KHz sampling

rate. To store a signal of 1 minute length, how much memory you need?

Basic of A/D Conversion: Explanation of Nyquist Rate


ANALOG MODULES of PIC16F887

Features of Analog Modules:

The A/D converter module has the following features:

The converter generates a 10-bit binary result using the method of successive approximation and

stores the conversion results into the ADC registers (ADRESL and ADRESH);

There are 14 separate analog inputs;

The A/D converter converts an analog input signal into a 10-bit binary number;

The minimum resolution or quality of conversion may be adjusted to various needs by selecting

voltage references Vref- and Vref+.


The operation of A/D converter is in control of the bits of four registers:

• ADRESH Contains high byte of conversion result;

• ADRESL Contains low byte of conversion result;

• ADCON0 Control register 0; and

• ADCON1 Control register 1.

Control of A/D Converter inside PIC 16F887


ADRESH and ADRESL Registers

The result obtained after converting an analog value into digital is a10-bit number that is to be

stored in the ADRESH and ADRESL registers.

There are two ways of handling it - left and right justification which simplifies its use to a great

extent. The format of conversion result depends on the ADFM bit of the ADCON1 register. In the

event that the A/D converter is not used, these registers may be used as general-purpose registers.

Control of A/D CONVERTER inside PIC 16F887 (Cont.)


ADCON0 Register

ADCS1, ADCS0 - A/D Conversion Clock Select bits select clock frequency used for internal

synchronization of A/D converter. It also affects duration of conversion.

ADCS1 ADCS2 CLOCK

0 0 Fosc/2

0 1 Fosc/8

1 0 Fosc/32

1 1 RC *

GO/DONE - A/D Conversion Status bit determines current status of conversion:

1 - A/D conversion is in progress.

0 - A/D conversion is complete. This bit is automatically cleared by hardware when the A/D

conversion is complete.


ADON - A/D On bit enables A/D converter.

1 - A/D converter is enabled.

0 - A/D converter is disabled.



CHS3-CHS0 - Analog Channel Select bits select a pin or an analog channel for A/D conversion,

i.e. voltage measurement:

ADCON0 Register (cont.)


ADFM - A/D Result Format Select bit

1 - Conversion result is right justified. Six most significant bits of the ADRESH are not used.

0 - Conversion result is left justified. Six least significant bits of the ADRESL are not used.

VCFG1 - Voltage Reference bit selects negative voltage reference source needed for the operation of

A/D converter.

1 - Negative voltage reference is applied to the Vref- pin.

0 - Power supply voltage Vss is used as negative voltage reference source.

VCFG0 - Voltage Reference bit selects positive voltage reference source needed for the operation of

A/D converter.

1 - Positive voltage reference is applied to the Vref+ pin.

0 - Power supply voltage Vdd is used as positive voltage reference source.


ADCON1 Register


A/D ACQUISITION REQUIREMENTS

In order to enable the ADC to meet its specified accuracy, it is necessary to provide a certain time

delay between selecting specific analog input and measurement itself. This time is called 'acquisition

time' and mainly depends on the source impedance. There is an equation used to calculate this time

accurately, which in the worst case amounts to approximately 20uS. So, if you want the conversion to be

accurate, don’t forget this important detail.

The time needed to complete a one-bit conversion is defined as TAD. It is required to be at least 1-6

uS depends on ADC clock frequency which is set by ADCS1 and ADCS0 bits. One full 10-bit A/D

conversion is slightly longer than expected and amounts to 11 TAD periods. S


In order to measure voltage on an input pin by the A/D converter, the following should be done:

Step 1 - Port configuration:

• Write a logic one (1) to a bit of the TRIS register, thus configuring the appropriate pin as an

input.

• Write a logic one (1) to a bit of the ANSEL register, thus configuring the appropriate pin as an

analog input.

Step 2 - ADC module configuration:

• Configure voltage reference in the ADCON1 register.

• Select ADC conversion clock in the ADCON0 register.

• Select one of input channels CH0-CH13 of the ADCON0 register.

• Select data format using the ADFM bit of the ADCON1 register.

• Enable A/D converter by setting the ADON bit of the ADCON0 register.

Step 3 - ADC interrupt configuration (optionally):

• Clear the ADIF bit.

• Set the ADIE, PEIE and GIE bits.

Steps of using A/D Module


Step 4 - Wait for the required acquisition time to pass (approximately 20uS).

Step 5 - Start conversion by setting the GO/DONE bit of the ADCON0 register.

Step 6 - Wait for ADC conversion to complete.

• It is necessary to check in the program loop whether the GO/DONE pin is cleared or wait for

an A/D interrupt (must be previously enabled).

Step 7 - Read ADC results:

• Read the ADRESH and ADRESL registers.

Steps of using A/D Module (Cont.)


Problem 3: Write codes to read analog value from channel 2 and displays it on PORTD and PORTB

as 10-bit binary number. Use A/D converter reference voltage from external pin.


unsigned int temp_res;

void main() {

ANSEL = 0x0C; // Pins AN2 and AN3 are configured as analog

TRISA = 0xFF; // All port A pins are configured as inputs

ANSELH = 0; // Rest of pins is configured as digital

TRISB = 0x3F; // Port B pins RB7 and RB6 are configured as outputs

TRISD = 0; // All port D pins are configured as outputs

ADCON1.F4 = 1 ; // Voltage reference is brought to the RA3 pin.

do {

temp_res = ADC_Read(2); // Result of A/D conversion is copied to temp_res

PORTD = temp_res; // 8 LSBs are moved to port D

PORTB = temp_res >> 2; // 2 MSBs are moved to bits RB6 and RB7

} while(1); // Endless loop

}

In order to make this example work properly, it is necessary to tick off the ADC library in

the Library Manager prior to compiling:

• ADC

Solution of Problem 3


Problem 4: Lamp on street are used to lighten the street at night or cloudy time when day light

becomes very dim. Design a circuit using PIC16F887 microcontroller for automatic control of street

light, such that, lamp on street will lighten only at night or cloudy time, when there is not enough

light. Use LDR to sense the light. Suppose, resistance of LDR is less than 1M when there is enough

light and greater than 1M when light is not enough. Use internal A/D converter of microcontroller to

measure the voltage across LDR.



unsigned int light;

void main() {

ANSEL = 0x04; // Pins AN2 is configured as analog



TRISB.F0 =0; // Port B pin PB0 is configured as output

ADCON1.F4 = 0 ; // Power supply voltage Vdd is used as positive voltage reference source.

ADCON.F5=0; // Power supply voltage Vss is used as negative voltage reference source.

while(1)

{

light= ADC_Read(2); // Result of A/D conversion is copied to light

if(light>512)

PORTB.F0=1;

else PORTB.F0=0;

}

}

Program (Problem-4)


Problem 5: Gas content in the air is measured by ppm (parts per million) unit. Suppose, when 500

ppm CH4 gas is present in the air, it is harmful for health and when it exceeds 700 ppm, the situation is

dangerous. Develop an embedded system for an industry to give- beef sound when CH4 content in the

air is greater than 500 ppm and continuous alarm when CH4 content exceeds 700 ppm. The

characteristics curve of the gas sensor and its pin diagram is given below. Use a buzzer to produce

sound.



Circuit Diagram

Out

RB0


unsigned int gas;

void main() {

ANSEL = 0x04; // Pins AN2 is configured as analog



TRISB.F0 =0; // Port B pin PB0 is configured as output

ADCON1.F4 = 0 ; // Power supply voltage Vdd is used as positive voltage reference source.

ADCON.F5=0; // Power supply voltage Vss is used as negative voltage reference source.

while(1)

{

gas= ADC_Read(2); // Result of A/D conversion is copied to gas

if(gas>307 && gas<409)

{ PORTB.F0=1;

delay_ms(300);

PORTB.F0=0;

delay_ms(300); }

else if (gas>409)

PORTB.F0=1;

else PORTB.F0=0;

} }

Program (problem-5)

Interfacing to Analog World - WordPress.com · •Introduction to Analog to digital Conversion •...

Documents

Transcript of Interfacing to Analog World - WordPress.com · •Introduction to Analog to digital Conversion •...