Published November 18, 2017

Arduino Variable Gain Amplifier

Use an Arduino board to control different gain coefficients from an operational amplifier.

IntermediateShowcase (no instructions)12,053

Things used in this project

Hardware components

Arduino UNO

Texas Instruments General Purpose Quad Op-Amp

Resistor 10k ohm

Resistor 4.75k ohm

Resistor 2.21k ohm

Resistor 1k ohm

Resistor 100 ohm

Software apps and online services

Arduino IDE

Story

About

In signal condition domain, a critical functional block is a VGA or Variable Gain Amplifier. It is used in measurement devices like: the oscilloscope, spectral analyzer, network analyzer and so on.

A simple VGA consists of a few resistors with different values, an operational amplifier and components that switch between these resistors. It can be a solenoid switch or an IC electronic switch. In this case, a SN74HC4066 is a suitable device. It have 4 switches with individual control. A good feature for use with the Arduino board.

So, by making a big voltage divider and choosing between nodes, we can modify the gain equation.

Gain Coefficient

RF stands for resistor feedback. If S1 is close and the rest are open according to OP, the operation gain coefficient will be:

G1 = 1 + RF/(R1+R2+R3+R4)

For S2 closed and the rest open:

G2 = 1 +(RF+R1)/(R2+R3+R4)

And so on:

G3 = 1 +(RF+R1+R2)/(R3+R4)
G4 = 1 +(RF+R1+R2+R3)/(R4)

Therefore, we can obtain 4 gain coefficients. But you may wonder if we can make combinations between gain in order to achieve other equations. Yes, we can and there is more detailed information are here.

So, with this concept in mind, we can make connections between the Arduino board, resistors, IC switch and OP AMP like in the bottom schematic. Pins 2/3/4/5 are connected to Arduino digital pin 2/3/4/5.

Arduino code is very easy because it contains digitalWrite commands and a serial communication to modify gain from the serial monitor.

Code

vga.ino

void setup() {
  pinMode(2, OUTPUT);// to pin 1C
  pinMode(3, OUTPUT);// to pin 2C
  pinMode(4, OUTPUT);// to pin 3C
  pinMode(5, OUTPUT);// to pin 4C
  Serial.begin(9600);
}

char cmd;

void loop() {
  while (Serial.available() > 0) {
    cmd = char(Serial.read());
    
    if (cmd == '0') {
      digitalWrite(2, 1);
      digitalWrite(3, 0);
      digitalWrite(4, 0);
      digitalWrite(5, 0);
    }
    if (cmd == '1') {
      digitalWrite(2, 0);
      digitalWrite(3, 1);
      digitalWrite(4, 0);
      digitalWrite(5, 0);
    }
    if (cmd == '2') {
      digitalWrite(2, 0);
      digitalWrite(3, 0);
      digitalWrite(4, 1);
      digitalWrite(5, 0);
    }
    if (cmd == '3') {
      digitalWrite(2, 0);
      digitalWrite(3, 0);
      digitalWrite(4, 0);
      digitalWrite(5, 1);
    }
    if (cmd == '4') {
      digitalWrite(2, 1);
      digitalWrite(3, 0);
      digitalWrite(4, 0);
      digitalWrite(5, 1);
    }
    if (cmd == '5') {
      digitalWrite(2, 1);
      digitalWrite(3, 0);
      digitalWrite(4, 1);
      digitalWrite(5, 0);
    }
    if (cmd == '6') {
      digitalWrite(2, 1);
      digitalWrite(3, 1);
      digitalWrite(4, 0);
      digitalWrite(5, 0);
    }
    if (cmd == '7') {
      digitalWrite(2, 0);
      digitalWrite(3, 1);
      digitalWrite(4, 0);
      digitalWrite(5, 1);
    }
    if (cmd == '8') {
      digitalWrite(2, 0);
      digitalWrite(3, 1);
      digitalWrite(4, 1);
      digitalWrite(5, 0);
    }
    if (cmd == '9') {
      digitalWrite(2, 0);
      digitalWrite(3, 0);
      digitalWrite(4, 1);
      digitalWrite(5, 1);
    }
  }
}

Credits

lucian_vdo

1 project • 12 followers

Arduino Variable Gain Amplifier

Things used in this project

Hardware components

Software apps and online services

Story

About

Gain Coefficient

Schematics

capture_UnjjlKWDLA.JPG

Code

vga.ino

Credits

lucian_vdo

Comments

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Arduino Variable Gain Amplifier

Arduino Variable Gain Amplifier

Things used in this project

Hardware components

Software apps and online services

Story

About

Gain Coefficient

Schematics

capture_UnjjlKWDLA.JPG

Code

vga.ino

Credits

lucian_vdo

Comments

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