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11 ANALOG-TO-DIGITAL CONVERSION 357

ADC Adc;

int Quit = 0, key; unsigned char DACbyte;

clrscr();

Dac.SendData(0); // initialise to zero

while(!Quit)

{

gotoxy(10,10);

cprintf("The ADC output is: %3u",

(int) Adc.ADConvert());

if(bioskey(1)!=0)

{

DACbyte = Dac.GetLastOutput();

if(DACbyte>247) // limit max value DACbyte = 247;

Dac.SendData(DACbyte+8);

break;

/*Down Arrow*/ case 0x5000 :

if(DACbyte<8) // limit min value DACbyte = 8;

Dac.SendData(DACbyte-8);

}

}

}

}

We have made changes to just a few statements in the main function. The keyboard controls operate almost identically to the program in Listing 10-6 that used the VCO object. The difference being the addition of statements to limit the maximum and minimum value of the number written to the DAC (0 to 255).

358 11 ANALOG-TO-DIGITAL CONVERSION

Executable File Generation

The entire definition of the DAC class must be provided before the compiler can compile the main() function given in Listing 11-5. The VCO object is not being used. Therefore, its class definition and function definition can be eliminated, however, its inclusion will not affect the operation of our program. When compiled, linked and run, this program will display an integer value on the screen that corresponds to the voltage applied at the VIN input of the ADC.

The program can be modified slightly to display the analog voltage instead of an integer number. The ADC produces an output that is 8 bits wide. These 8 bits can represent any value in the range 0–255 both inclusive (which forms 256 numbers). The operation of the ADC requires the full-scale range to be quantised (segmented) into 256 equal quantum levels. Each quantum then represents the full-scale voltage (5V) divided by 256. The ADC’s integer output of 0 corresponds to 0 volts at VIN, and its integer output value of i corresponds to an applied voltage of:

volts

Note that from a C++ program’s point of view, the division operation 5/256 is considered as an integer division and the result will be 0. Therefore, when including the above expression in the program, it must be typed in as:

5.0/256.0*i

Now the compiler will treat the division and multiplication operations as floating point operations, and a non-zero result will be evaluated for the expression 5.0/256.0. The statement containing the cprintf() function in Listing 11-5 must now be modified to include the above factor as shown below:

cprintf("The ADC output is: %5.2f (V)", 5.0/256.0*Adc.ADConvert());

The program will now display the actual voltage applied at the analog input pin of the ADC (VIN). The connections that need to be made for this program to operate are given in Figure 11-19, Table 11-4 and Table 11-5.

11 ANALOG-TO-DIGITAL CONVERSION 359

NOTE

Ensure that the DAC is placed into unipolar mode (0 to +5V output) by fitting the

jumper across the header in the position marked as LINK1. Then connect the 9V battery before connecting the output of the DAC circuit to the ADC input.

Table 11-5 Partial connections for the ADC.

11.7 Measuring Temperature Using the

ADC

A program was developed in Chapter 10 to measure temperature using the interface board’s thermistor and VCO. That program (Listing 10-12) only requires minor changes to measure temperature using the ADC. This new modified program is shown in Listing 11-6. Note that in this case we can accurately characterise the thermistor circuit response over the full 0 to +5V range since the ADC has very good linearity over its entire input range. In comparison, the VCO has similar linearity between +2.2V and +2.8V.

Listing 11-6 Program measures temperature using ADC and thermistor – temp.cpp.

/***************************************************** This program uses the thermistor circuit on the interface board to generate the analog input voltage to the ADC. The

byte produced by the ADC will be proportional to the applied voltage (temperature of the thermistor).

The program also calibrates the thermistor circuit output using upper and lower temperature points. The calibration equation will then interpolate a straight line through these two points. Once calibrated, the program will be able to display actual temperatures.

*****************************************************/

#include <bios.h>

360 11 ANALOG-TO-DIGITAL CONVERSION

#include <conio.h> #include <iostream.h>

#include "adc.h"

void main()

{

ADC Adc;

int Quit=0, HiFlag = 0, LoFlag = 0; int key = 0;

float HiTemp, LoTemp, Temp; long int HiCount, LoCount;

clrscr();

while(!Quit)

{

Adc.ADConvert();

gotoxy(10,10);

if((HiFlag == 1) && (LoFlag == 1))

{

Temp = LoTemp+(HiTemp-LoTemp)* (Adc.GetADCValue()-LoCount)/(HiCount-LoCount);

cprintf("The temperature is: %6.1f (deg)", Temp);

}

else

cprintf("The ADC Value is: %3u",

(int)Adc.GetADCValue());

if(bioskey(1)!=0)

{

key = bioskey(0);

switch(key)

{

case 0x2d00 : /* Alt-X */ Quit = 1;

break;

case 0x4800 : /* Up Arrow */ gotoxy(10,5);

cout << "Enter Upper Calibration Temp."; cin >> HiTemp;

11 ANALOG-TO-DIGITAL CONVERSION 361

HiCount = Adc.GetADCValue();

HiFlag = 1; break;

case 0x5000 : /* Down Arrow */ gotoxy(10,6);

cout << "Enter Lower Calibration Temp."; cin >> LoTemp;

LoCount = Adc.GetADCValue();

LoFlag = 1;

}

}

}

}

Executable File Generation

The wiring connections need to be changed slightly for this program to operate. We do not use the DAC to provide the analog voltage. Instead we use the thermistor circuit to generate a voltage that represents the temperature of the thermistor. The output of the thermistor circuit is connected to the ADC analog input (VIN) as given in Table 11-6. The remaining connections for the ADC and the MUX are shown in Figure 11-19.

Table 11-6 Thermistor circuit connections to the ADC.

It has been rather easy for us to change our program that used the VCO to now operate in conjunction with the ADC. We have been careful to be consistent in developing our classes so that minimum changes will be needed if they are later

362 11 ANALOG-TO-DIGITAL CONVERSION

modified for new or existing programs. These examples are typical of the ease with which object-oriented programs can be maintained and upgraded.

11.8 Summary

This chapter described the principles of operation and use of an analog-to-digital converter. The more popular types of analog-to-digital converters and their various methods of conversion have been explained. This was followed by a discussion of the importance of a sample and hold circuit and the effects of aliasing that occurs when signals are sampled too slowly.

We used our now familiar object-oriented approach to develop software for interfacing the parallel port of the PC with the ADC. We developed a new object class named ADC using an approach consistent with that of Chapter 10 when the VCO class was developed. This object-oriented approach has allowed us to develop the voltage and temperature measuring programs that used the ADC by making minor changes to the programs written for the VCO.

11.9 Bibliography

Fluke, The ABC’s of Oscilloscopes, Fluke Corporation, 1997.

Horowitz, P. and Hill, W., The Art of Electronics, Cambridge University Press, Cambridge, 1989.

Loveday, G., Microprocessor Sourcebook, Pitman Publishing Limited, London, 1986.

NS DATA CONVERSION/ACQUISITION Databook, National Semiconductor Corporation, 1984.

Stiffler, K., Design with Microprocessors for Mechanical Engineers, McGrawHill, 1992.

Webb, R.E., Electronics for Scientists, Ellis Horwood, New York, 1990. Wobschall, D., Circuit Design for Electronic Instrumentation, McGraw-Hill, 1987. Van Gilluwe, F., The Undocumented PC, Addison Wesley, 1994.

Winston, P.H., On to C++, Addison Wesley, 1994.

12

Data Acquisition with Operator Overloading

Inside this Chapter