ADS1118EVM: Way to get accurate value in both modes

//
//This program is used for interfacing ADS1118 temperature sensor to TM4C1294NCPDT microcontroller.
//TM4C1294NCPDT microcontroller provides Quad Synchronous Serial Interface(SSI).
//
#include <stdbool.h>
#include <stdint.h>
#include "inc/hw_memmap.h"
#include "driverlib/gpio.h"
#include "driverlib/pin_map.h"
#include "driverlib/ssi.h"
#include "driverlib/sysctl.h"
#include "driverlib/uart.h"
#include "uartstdio.h"

#define NUM_SSI_DATA 2

//
//UART0 is configured to get debug logs.
//
void InitConsole(void)
{
//
// Enable GPIO port A which is used for UART0 pins.
//
SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);

//
// Configure the pin muxing for UART0 functions on port A0 and A1.
//
GPIOPinConfigure(GPIO_PA0_U0RX);
GPIOPinConfigure(GPIO_PA1_U0TX);

//
// Enable UART0 to configure the clock.
//
SysCtlPeripheralEnable(SYSCTL_PERIPH_UART0);

//
// Using the internal 16MHz oscillator as the UART clock source.
//
UARTClockSourceSet(UART0_BASE, UART_CLOCK_PIOSC);

//
// Select the alternate (UART) function for these pins.
//
GPIOPinTypeUART(GPIO_PORTA_BASE, GPIO_PIN_0 | GPIO_PIN_1);

//
// Initialize the UART for console I/O.
//
UARTStdioConfig(0, 115200, 16000000);
}

int main(void)
{
uint32_t ui32SysClock;
uint16_t config_reg[NUM_SSI_DATA];
uint16_t conv_reg[NUM_SSI_DATA];
uint16_t dummy;
uint16_t temp;
int index;
int i = 1;
uint16_t raw_data = 0;
uint16_t bit_test;

config_reg[0] = 0xbd0b;
config_reg[1] = 0x8d1b;

//
//Sets clock source for SSI module.
//
ui32SysClock = SysCtlClockFreqSet((SYSCTL_XTAL_25MHZ |
SYSCTL_OSC_MAIN |
SYSCTL_USE_OSC), 25000000);

//
// Sets the serial console to use for displaying messages.
//
InitConsole();

//
// Display the setup on the console.
//
UARTprintf("SSI ->\n");
UARTprintf(" Mode: SPI with Polarity 0 and Phase 1\n");
UARTprintf(" Data: 16-bit\n\n");


SysCtlPeripheralEnable(SYSCTL_PERIPH_SSI2);


SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOD);

GPIOPinConfigure(GPIO_PD3_SSI2CLK);
GPIOPinConfigure(GPIO_PD2_SSI2FSS);
GPIOPinConfigure(GPIO_PD0_SSI2XDAT1);
GPIOPinConfigure(GPIO_PD1_SSI2XDAT0);

GPIOPinTypeSSI(GPIO_PORTD_BASE, GPIO_PIN_3 | GPIO_PIN_2 | GPIO_PIN_1 | GPIO_PIN_0);

SSIConfigSetExpClk(SSI2_BASE, ui32SysClock, SSI_FRF_MOTO_MODE_1,
SSI_MODE_MASTER, 2000000, 16);

SSIEnable(SSI2_BASE);

for(index = 0; index < NUM_SSI_DATA; index++){
    while(SSIDataGetNonBlocking(SSI2_BASE, &conv_reg[index]))
    {
    }
}

SSIDataPut(SSI2_BASE, config_reg[0]);
UARTprintf("ADC_MODE config register sent\n\n");
while(SSIBusy(SSI2_BASE))
{
}

while(1){

SysCtlDelay(5750000);

if (i == 1) {
SSIDataGet(SSI2_BASE, &dummy);
UARTprintf("dummy data received\n");
i--;
}
SSIDataPut(SSI2_BASE, config_reg[1]);
UARTprintf("TEMP_MODE config register is sent\n\n");

SSIDataGet(SSI2_BASE, &conv_reg[0]);
UARTprintf("ADC_MODE data received is %x\n\n",conv_reg[0]);

//conv_reg[0] = ~(conv_reg[0] - 1);
temp = (256 * conv_reg[0]) / 32768;
UARTprintf("Thermocouple voltage difference value is %dmV\n\n", temp);

while(SSIBusy(SSI2_BASE))
{
}

SysCtlDelay(5750000);

SSIDataPut(SSI2_BASE, config_reg[0]);
UARTprintf("ADC_MODE configured\n");

SSIDataGet(SSI2_BASE, &conv_reg[1]);
UARTprintf("TEMP_MODE data received is %x\n\n",conv_reg[1]);

raw_data = conv_reg[1];
UARTprintf("raw data received as integer is %x\n", raw_data);
raw_data = raw_data >> 2;
UARTprintf("raw data received as integer is %x\n", raw_data);

bit_test = raw_data;

if((bit_test & 0x2000) == 1){
raw_data = raw_data - 0x01;
raw_data = ~(raw_data);
temp = (raw_data * (-0.03125));
UARTprintf("value in celsius is %d\n", temp);
} else {
temp = (raw_data * 0.03125);
UARTprintf("value in celsius is %d\n", temp);
}

}
}