Flash ADC-based Analog to Digital Converter.

Flash ADC-based Analog to Digital Converter using 741 Op-Amps and 74LS148 Encoder

Introduction

Analog-to-Digital Converters (ADCs) are essential components in embedded systems for converting real-world analog signals into digital form for processing. In this project, I designed and implemented a Flash ADC (Parallel ADC) using eight 741 operational amplifiers (Op-Amps) and a 74LS148 priority encoder to encode the analog input into a 3-bit digital output. The encoded data is processed by an Arduino Uno, which displays the corresponding voltage value on an I2C-based LCD.

Components Used

  • Operational Amplifier (LM741) x 8: Used as voltage comparators.

  • Resistors (10KΩ) x 9: Used for voltage division and signal processing.

  • 74LS148 (8-to-3 bit Priority Encoder): Encodes 8-bit data into 3-bit binary output.

  • Arduino Uno: Processes digital signals and displays results.

  • Liquid Crystal Display (LCD 16x2 with I2C module): Displays the digital output and equivalent analog voltage.

  • Power Supply (+12V and -12V): Required for the operational amplifiers.

  • Various input voltage sources (1V to 4V for testing).

Circuit Simulation


Working Principle

The Flash ADC (Parallel ADC) is one of the fastest ADC architectures, as it uses multiple comparators to compare the input voltage with reference voltages. Here’s how our system works:

  1. Voltage Division: A resistor network creates different reference voltage levels.

  2. Op-Amps as Comparators: Each of the eight 741 op-amps compares the input voltage against the corresponding reference voltage.

  3. Binary Encoding with 74LS148: The 8-bit comparator outputs are fed into the 74LS148 priority encoder, which converts the 8-bit data into a 3-bit binary output.

  4. Microcontroller Processing: The Arduino reads the 3-bit digital output and maps it to the corresponding voltage.

  5. LCD Display: The final voltage value is displayed on the I2C LCD as per the mapping:

Digital Output
Analog Voltage
0
0.0V
1
1.0V
2
1.5V
3
2.0V
4
2.5V
5
3.0V
6
3.5V
7
4.0V

Applications of Flash ADCs

Flash ADCs are widely used in applications requiring high-speed conversions, such as:

  • Digital Oscilloscopes

  • Video Signal Processing

  • Radar and Communications Systems

  • High-Speed Data Acquisition Systems

Advantages of Flash ADC

  • Extremely Fast: Provides instant conversion without needing successive approximation.

  • Simple Design: No need for complex control logic.

  • Ideal for High-Speed Applications.

Limitations of Flash ADC

  • Power Consumption: Requires multiple comparators, leading to high power usage.

  • Increased Hardware Complexity: More components increase circuit complexity.

  • Expensive: Due to the large number of comparators required.

Arduino Code for Displaying ADC Data

#include <Wire.h>
#include <LiquidCrystal_I2C.h>

//pins
int lsb_pin = 5;
int mid_pin = 6;
int msb_pin = 7;

//variables
uint8_t lsb = 0;
uint8_t mid = 0;
uint8_t msb = 0;
uint8_t _3bit_adc_data = 0;


LiquidCrystal_I2C lcd(0x27, 16, 2);

void setup() {
  lcd.init();
  lcd.backlight();  // Turn on the backlight

  pinMode(lsb_pin, INPUT);
  pinMode(mid_pin, INPUT);
  pinMode(msb_pin, INPUT);
}

void loop() {

  lsb = digitalRead(lsb_pin);
  mid = digitalRead(mid_pin);
  msb = digitalRead(msb_pin);

  lcd.setCursor(0, 0);lcd.print(msb);
  lcd.setCursor(1, 0);lcd.print(mid);
  lcd.setCursor(2, 0);lcd.print(lsb);
  lcd.setCursor(5, 0);lcd.print("=>");

  _3bit_adc_data = ((lsb) | (mid << 1) | (msb << 2));

  lcd.setCursor(9, 0);
  lcd.print(_3bit_adc_data);

  float data = datamap(_3bit_adc_data);

  lcd.setCursor(0, 1);lcd.print("Analog:");
  lcd.setCursor(8, 1);lcd.print(data);
  lcd.setCursor(13, 1);lcd.print("v");
}

float datamap(int _3bit_adc_data) {
  if(_3bit_adc_data==0){return 0.0;};
  if(_3bit_adc_data==1){return 1.0;};
  if(_3bit_adc_data==2){return 1.5;};
  if(_3bit_adc_data==3){return 2.0;};
  if(_3bit_adc_data==4){return 2.5;};
  if(_3bit_adc_data==5){return 3.0;};
  if(_3bit_adc_data==6){return 3.5;};
  if(_3bit_adc_data==7){return 4.0;};
}

Conclusion

This project successfully demonstrates the working of a Flash ADC using op-amps and a priority encoder. The Arduino Uno processes the 3-bit output and accurately maps it to the corresponding voltage, which is displayed on the LCD. Despite its advantages in speed, the Flash ADC's complexity and power consumption make it more suitable for high-speed applications rather than general-purpose use.

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