Improve ADC mesurements

[stbalestrini]

Hello, so im in the middle of a personal project where i need to meassure 1 to 4 4-20ma pressure sensors (Wika A10 0-6 bar). Since everything will be powered from a farm tractor which has voltage variations im using a LM2596 Step Down circuit to regulate the voltage to flat 9v and power the arduino and a XL6009 Step Up circuit to go back up to flat 12v and power the sensors. Im using an Arduino nano with a 20x4 LCD display with an I2C module.
My problem is that when i connect the sensors and read the analog pins the ADC values are around 150 to 200 for each connected sensor, im wondering if i can get that value lower and/or why this happans.
Here is the code:

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

#define BUTTON_PIN 3

LiquidCrystal_I2C lcd(0x27, 20, 4);

const int NUM_SENSORS = 4;
const int SENSOR_PINS[NUM_SENSORS] = {A0, A1, A2, A3};
const int NUM_MEASUREMENTS = 100;
const float MAX_PRESSURE = 6.0;

volatile bool checkCalibration = false;
int conSensors;

float sensorsADC[NUM_SENSORS];
int sensorData[NUM_SENSORS][NUM_MEASUREMENTS];
float medianData[NUM_SENSORS];
float previousPressureValues[NUM_SENSORS];

void setup() {
  Serial.begin(9600);
  Wire.begin();
  lcd.init();
  lcd.clear();
  lcd.backlight();
  pinMode(BUTTON_PIN, INPUT_PULLUP);
  attachInterrupt(digitalPinToInterrupt(BUTTON_PIN), calibrationCall, RISING);
  conSensors = 4;//checkConnectedSensors();
  lcd.clear();
  lcd.setCursor(0, 0);
  lcd.print("Connected Sensors: ");
  lcd.print(conSensors);
  delay(3000);
  lcd.clear();
}

void loop() {
  if (checkCalibration) {
    checkCalibration = false;
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("CALIBRATING");
    delay(1000);
    for (int i = 0; i < conSensors; i++) {
      calibrateSensor(i);
    }
    lcd.clear();
    lcd.setCursor(0, 0);
    lcd.print("Done!");
    delay(1000);
    lcd.clear();
    displaySensorValues();
  }
  for (int j = 0; j < conSensors; j++) {
    for (int i = 0; i < NUM_MEASUREMENTS; i++) {
      sensorData[j][i] = analogRead(SENSOR_PINS[j]);
    }
    delayMicroseconds(20);
  }
  for (int i = 0; i < conSensors; i++) {
    quicksort(sensorData[i], 0, NUM_MEASUREMENTS - 1);
    medianData[i] = sensorData[i][NUM_MEASUREMENTS / 2];
    Serial.print("ADC S");
    Serial.print(i);
    Serial.print("= ");
    Serial.println(medianData[i]);
    float pressureValue = map(medianData[i], sensorsADC[i], 1012, 0, MAX_PRESSURE);
    pressureValue = constrain(pressureValue, 0, MAX_PRESSURE);
    if (pressureValue != previousPressureValues[i]) {
      previousPressureValues[i] = pressureValue;
      lcd.setCursor(0, i);
      lcd.print("Sensor ");
      lcd.print(i + 1);
      lcd.print(": ");
      lcd.print(pressureValue, 2);
      lcd.print(" bar");
    }
  }
}

void calibrationCall() {
  checkCalibration = true;
}

void calibrateSensor(int sensorIndex) {
  float sensorSum = 0.0;
  for (int i = 0; i < NUM_MEASUREMENTS; i++) {
    sensorSum += analogRead(SENSOR_PINS[sensorIndex]);
    delayMicroseconds(20);
  }
  sensorsADC[sensorIndex] = sensorSum / (float)NUM_MEASUREMENTS;
}

int checkConnectedSensors() {
  int temp = 0;
  for (int i = 0; i < NUM_SENSORS; i++) {
    int j = analogRead(SENSOR_PINS[i]);
    if (j < 200) {
      temp++;
    }
  }
  return temp;
}

void quicksort(int arr[], int low, int high) {
  if (low < high) {
    int pivotIndex = partition(arr, low, high);
    quicksort(arr, low, pivotIndex - 1);
    quicksort(arr, pivotIndex + 1, high);
  }
}

int partition(int arr[], int low, int high) {
  int pivot = arr[high];
  int i = low - 1;
  for (int j = low; j <= high - 1; j++) {
    if (arr[j] < pivot) {
      i++;
      swap(arr[i], arr[j]);
    }
  }
  swap(arr[i + 1], arr[high]);
  return i + 1;
}

void swap(int& a, int& b) {
  int temp = a;
  a = b;
  b = temp;
}
void displaySensorValues() {
  for (int i = 0; i < conSensors; i++) {
    lcd.setCursor(0, i);
    lcd.print("Sensor ");
    lcd.print(i + 1);
    lcd.print(": ");
    lcd.print(previousPressureValues[i], 2);
    lcd.print(" bar");
  }
}

Here is the circuit diagram:

I only draw 1 sensor but each of the 4 possible sensors has its own 4.7kOhm resistor to its analog pin and its own 250Ohm resistor to ground.

[camsysca]

So 4-20 mA through 250 ohms gives you a 1-5V signal range, which corresponds to roughly 205 to 1023 counts on the ADC. This is to be expected. Without a preamp to remove the 1V and apply a 1.2 gain factor, you're stuck.

[stbalestrini]

Oh damn, so i forgot the basics of electronics. Thank you very much