ADuCM360 ADC0 Counts Decresses Gradually

Dear Associates,

We want best accuracy & steady results of ADC.

We are executing ADC sample code with below settings as inetrnal reference & ADC filter for 3.75Hz update rate, but observed ADC counts has 16 bit accuracy only & remaining 8 bit continuously decressing. 

void ADC0INIT(void)

{

   AdcMski(pADI_ADC0,ADCMSKI_RDY,1);            

// Enable ADC ready interrupt source

   AdcFlt(pADI_ADC0,124,14,FLT_NORMAL|ADCFLT_NOTCH2|ADCFLT_CHOP);

// ADC filter set for 3.75Hz update rate with chop on enabled

   AdcRng(pADI_ADC0,ADCCON_ADCREF_INTREF,ADCMDE_PGA_G1,ADCCON_ADCCODE_INT);

// Internal reference selected, Gain of 1, Signed integer output

AdcBuf(pADI_ADC0,ADCCFG_EXTBUF_OFF,ADCCON_BUFBYPN|ADCCON_BUFBYPP|ADCCON_BUFPOWP|ADCCON_BUFPOWN);

// Turn off input buffers to ADC and external reference

   AdcPin(pADI_ADC0,ADCCON_ADCCN_AIN1,ADCCON_ADCCP_AIN0);

// Select AIN0 as postive input and AIN1 as negative input

}

Below is the Circuit diagram of our Customized Board.

Please let me know if there is any problem in Hw or in Sw?