Adding a sign

float _raw;

float _data;
bool sign;
float Volt;

const int polarity = 7;

void setup() {
  Serial.begin(115200);
  pinMode(polarity, INPUT);
  pinMode(LED_BUILTIN, OUTPUT);
}

//TODO two's compliment in asm

void loop() {

  sign = digitalRead(polarity);
    _raw = (float) analogRead(A0);

  if(sign) {
    //_data = _raw * -1.0;
    Volt = _raw * (-5.0 / 1023.0);
    digitalWrite(LED_BUILTIN, HIGH);
  } else {
    //_data = _raw * 1.0;
    Volt = _raw * (5.0 / 1023.0);
    digitalWrite(LED_BUILTIN, LOW);
  }
  Serial.println(Volt);
}

--

Increasing Vertical resolution:

#include

#define ADC1 (0x90 >> 1)
#define ADC2 (0x92 >> 1)

byte v0,v1;

int adcSum;

void setup() {
  Serial.begin(9600);
  Wire.begin();
 
  Wire.beginTransmission(ADC1);
  Wire.write(0x40);
  Wire.endTransmission(false);

  Wire.beginTransmission(ADC2);
  Wire.write(0x40);
  Wire.endTransmission(false);
}

void loop() {  
  Wire.requestFrom(ADC1, 1);
  v1 = Wire.read();
    Wire.requestFrom(ADC2, 1);
  v2 = Wire.read();

  adcSum = v1 + v2;
  Serial.print(v1);Serial.print(",");
  Serial.print(v2);Serial.print(",");
  Serial.println(adcSum);
}