ADS8887: Low Resolution Problem During ADC Read

Hello,

 Me and my colleagues are struggling with  ADS8887 18-BIT ADC Component (ADS8887IDRCR). We have desired to read the voltage value with 0.1 mV error tolerance.Therefore, we chose this component but unfortunately we only achieved to get adc results with 7-8 bit resolution even though we apply the same signals (manually)  which demonstrated on the datasheet (4-WIRE OPERATION) .I am going to share my main code , raw data and oscilloscope results.

Every feedback and  advise would be so helpful.

Here are some raw data examples =>     0.0 Volts->75760

                                                                          0.1 Volts->75776 

                                                                          0.5 Volts->79856
                                                                         1.0 Volts->84976
                                                                         2.4 Volts->98288
                                                                         3.8 Volts->112624

Figure shows the Raw ADC Values we have received

(PS: These values doesn't change even one integer)

Here are Oscilloscope Screen shots =>

MAIN CODE/////////////////////////////////

#define CS 47    //PL2(DIN)
#define SCK 52   //PB1
#define MISO 50  //PB3
#define M0SI 51  //PB2
#define CONVST 45 //PL4

byte val;
void setup() {

  pinMode(39,OUTPUT);         //adc ch1 power enable
  digitalWrite(39,HIGH);
  Serial.begin(115200);

  DDRB = DDRB | B00000010;     //SCK is output
  DDRL = DDRL | B00010100;     //DIN and CONVST are output

  DDRB = DDRB & B11110111;
 


  PORTL = PORTL & B11101011; //CS and CONVST LOW
  PORTB = PORTB & B11111001;  //SCK LOW


 
  }
  unsigned long MISO_;
  unsigned long clk=0,clk2=0;     //these two parameters are used in delay operation
 
void loop() {
 
  delay(10);
  PORTL = PORTL | B00000100;        //CS HIGH
  delayMicroseconds(1);
  PORTL = PORTL | B00010000;        //CONVST HIGH
  MISO_ = 0;
  delayMicroseconds(1);
  for(int i=0;i<50;i++)              // 5 cycle boyunca SCK
  {
    
    PORTB = PORTB | B00000010;  //SCK HIGH
    
    while(micros-clk <1)
    {
      clk=micros();                 //OR AND Operation takes different amount of time so we make them equal
    }

    PORTB = PORTB & B11111101;  // SCK LOW
  }
 

 delayMicroseconds(10);
  PORTL = PORTL & B11111011;     //CS LOW
   //delayMicroseconds(100);
  byte val = 0;
  MISO_ = 0;
 
   for(int i=0;i<18;i++)            
  {
     PORTB = PORTB | B00000010;     //SCK HIGH
      delayMicroseconds(1);
     
     val = (byte)PINB & B00001000;      //   READ DOUT PIN
     val = val >> 3;                    // shift it to LSB
     val = val & B00000001;
 
     MISO_ = (MISO_<< 1) | val ;                     //Shift the value 1 bit left to get next bit
     /*
    Serial.print(i);
    Serial.print(" = ");
    Serial.println(val);
    */
    /*
    while(micros-clk2 <1)
    {
      clk2=micros();                       
    }
    */
    PORTB = PORTB & B11111101;     //SCK LOW
    delayMicroseconds(1);
   
  }
 
  PORTL = PORTL & B11101111;                  // CONVST LOW


delayMicroseconds(50);                   
Serial.println(MISO_);
}