Problem with ADC module in Microchip Explorer 16 Kit

#include <p24fj128ga010.h>
#include <stdio.h>

_CONFIG1(JTAGEN_OFF & FWDTEN_OFF)

//ADC
#define VINPUT    3                                             // Voltage signal from photodiode connected to AN3 input. Not using AN0 or AN1 as they are combined with PGC and PGD-not sure if can be used, AN2-USB.
#define AINPUTS 0xfff7                                        // Analog input to AN3 - in Explorer16 PICTailPlus 13th position


//LCD
#define LCDDATA 1
#define LCDCMD  0
#define PMDATA  PMDIN1                                         // PMDIN1 8-bit Parallel Port Date-In Register

#define LCDbusy() LCDread( LCDCMD) & 0x80                     // Bitwise '&' LCDread( LCDCMD) output with 0x80...where 0x80 is from Pg 130-Read busy-flag Instr-BF is 1 if internal op  in progress

#define putLCD( d)  LCDwrite( LCDDATA, (d))                     // Send ASCII data to LCD
#define LCDsetC( a) LCDwrite( LCDCMD, (a & 0x7F) | 0x80)     // To go to second line of LCD-starting at 0x40

//Timer1
int TimerCount = 0;
int ButtonPush = 0;
//------------------------------- LCD ---------------------------------


void LCDinit( void)
{
                                // PMP Initialization
    PMCON = 0x83BF;                // Enable the PMP, PMWREN & PTRDEN enabled, active highs, polaritys, long waits
    PMMODE = 0x3FF;                // Master Mode 1; 4TCY, 15TCY, 4TCY wait
    PMAEN =    0x0001;                // PMA0 enabled
   
    T1CON = 0x8030;                // Fosc/2 (osc=8MHz), prescaled 1:256, 64us/tick
    TMR1 = 0;                     
    while( TMR1<500)            // 500 x 64us = 32ms. Need >30ms for LCD internal initialization

                                // LCD Initialization
    PMADDR = LCDCMD;            // command register
    PMDATA = 0b00111000;        // 8-bit interface, 2 lines, 5x7..Pg 130-Function Set
    TMR1 = 0;
    while( TMR1<1);             // 1 x 64us = 64us. Need >40us for above operation. See "exec time" on Pg 130-Function Set 
   
    PMDATA = 0b00001100;        // Pg 129-Display on/off control...display ON, cursor off, blink off
    TMR1 = 0;
    while( TMR1<1);               // 1 x 64us = 64us. Need >40us for above operation. See "exec time" on Pg 129-Display ON/OFF    
   
    PMDATA = 0b00000001;        // Pg 129-Clear Display...clear display.
    TMR1 = 0;
    while( TMR1<25);            // 25 x 64us = 1.6ms. Need about 1.65ms for above operation. See "exec time" on Pg 129-Clear display
   
    PMDATA = 0b00000110;        // Pg 129-Entry mode set..increment cursor, no shift
    TMR1 = 0;
    while( TMR1<100);             // 1 x 64us = 64us. Need >40us for above operation. See "exec time" on Pg 129-Entry Mode Set       
                                // or 1.6ms??????
} // initLCD


// See if/Wait for PMP to be ready. Then look for LCD to be ready
char LCDread( int addr)
{
    int dummy;
    while( PMMODEbits.BUSY);    // wait for PMP to be available
    PMADDR = addr;              // select the command address
    dummy = PMDATA;             // initiate a read cycle, dummy read
    while( PMMODEbits.BUSY);    // wait for PMP to be available
    return( PMDATA);            // read the status register
   
} // LCDread


void LCDwrite( int addr, char c)   
{
    while( LCDbusy());            // Wait till LCD is available
    while( PMMODEbits.BUSY);    // wait for PMP to be available
    PMADDR = addr;
    PMDATA = c;
} // writeLCD
   

void putsLCD( char *s)
{
    while( *s)
    {
        putLCD( *s++);
    }
} //putsLCD


//------------------------------- ADC ---------------------------------


// initialize the ADC for single conversion, select Analog input pins
void initADC( int amask)
{
    AD1PCFG = amask;      // select analog input pins
    AD1CON1 = 0x00E0;     // data output format-integer, auto convert after end of sampling. No need to include a delay loop to provide time for completion of sampling
    AD1CSSL = 0;          // no scanning required
    AD1CON3 = 0x1FFF;     // Use internal clock, max sample time avail is 31Tad, conversion time Tad = 128*Tcy = 128*(2/Fosc) = 256/8MHz = 32us which is > (required)75ns ); 31 Tad = 31*32us = 992us is the sample time. About 100samples/sec
    AD1CON2 = 0;          // use MUXA, AVss and AVdd are used as Vref+/-
    AD1CON1bits.ADON = 1; // turn on the ADC
} //initADC


int readADC( int ch)
{
    AD1CHS  = ch;               // select analog input channel
    ADC1BUF0 = 0x0000;
    AD1CON1bits.SAMP = 1;       // start sampling, automatic conversion will follow at end of sampling   
    while (!AD1CON1bits.DONE);  // wait to complete the conversion. DONE will be set as soon as conversion ends
    return ADC1BUF0;            // read the conversion result
} // readADC


//------------External Interrupt 0-------------

void _ISR __attribute__((no_auto_psv)) _INT0Interrupt (void)
{
    ButtonPush = 1;
    _INT0IF = 0;
}

//------------------- TMR1 --------------------

void _ISR __attribute__((no_auto_psv)) _T1Interrupt (void)
{
    TimerCount++;
    _T1IF = 0;
}

//------------------- main ---------------------

int main ()
{
   
    // initializations
    LCDinit();                   // initialize the PMP and LCD
    initADC( AINPUTS);         // initialize the ADC for the Explorer16 analog inputs
    TRISB= 0x0008;                // RB3 from PORTB set as input since AN3 being used as analog input
    TRISD= 0x0040;             // Button S3 is RD6. Set RD6 as input.

    _NSTDIS = 0;               // Enable Interrupt nesting. Button push interrupt priority should be higher than TMR1.
    _IPL = 0;                  // Processor priority level. Default 0.
   

    T1CON = 0x8030;               // TMR1 is ON, Fosc/2 (osc=8MHz), prescaled 1:256, 64us/tick, for 1 sec - post delay of 15625 -1.
    PR1 = 15625 - 1;           // period register
    _T1IP = 4;                 // set TMR1 interrupt priority. 4 is default
    _T1IF = 0;                   // clear TMR1 flag
    _T1IE = 1;                 // set TMR1 enable

    _INT0IP = 5;               // External interrupt 0 priority set to 5. Higher than TMR1 and processor
    _INT0IF = 0;               // Clear Ext interrupt 0 flag
    _INT0IE = 1;               // Set Ext interrupt 0 enable bit
   
    float a=0, sum=0, n=0, avg=0;
    int up=0, counter=0, FASamp = 0;

    float OneBeatTime = 0, FourBeatTime = 0;
    static int BeatsPerMin =0; 
    char s[10];
  
    while(1)
    {  
        if(PORTDbits.RD6 == 0)  // S3 button connected to RD6 is an active low pin.
        {                         
            _INT0IF = 1;
            FASamp = 1;
         }
          
        while(ButtonPush == 1)     
        {   
            if(FASamp == 1)
            {
                FASamp = 0;
                a = 0; sum = 0; avg = 0; up = 0; counter = 0; n = 0;
                TimerCount = 0;
                TMR1 = 0;
            }
            a = readADC(VINPUT);  // Sample the Voltage Signal and convert - 10 bit result
           
            sum = sum + a;        // running average
            ++n;
               avg = sum/n;
      
               if (a > avg*1.5)      // current value > 1.5 of average. If yes, a peak has been detected. Increment UP
                   up++;
            else
                   up=0;
          
            if (up == 1)          // If one peak has already been detected, to ignore rest of the a values > avg in the same peak - increment another counter to count one heart beat
                counter++;
   
            if(counter == 4)                               // After four heart beats have been detected
            {
               
                FourBeatTime = (TMR1*256)/4000000 + TimerCount;
                OneBeatTime = FourBeatTime/4 ;         // divide by 4 to get time for one beat.
                BeatsPerMin = 60/OneBeatTime;
                LCDsetC(0x00);
                putsLCD( "Beats Per Minute");             // put a title on the first line
                LCDsetC(0x40);                              // Goto second line on LCD
                sprintf(s, "%i", BeatsPerMin);           // convert bpm from integer to string type     
                putsLCD(s);            

                ButtonPush = 0;  
                PORTDbits.RD6 = 1;      // reset button to high to check for another button press in future                                                            
            } 
                
        } // end button push - while loop

    } // end main while

}// end main