CCS/MSP430FG6626: Results of msp430fg662x_ctsd16_04.c example

Tool/software: Code Composer Studio

Hi,

my question is on the CTSD16 of MSP430FG6626.

For testing purposes I used the given example code of msp430fg662x_ctsd16_04.c (Single Conversion on a Single Channel, Poll IFG) from the TI product page.

According to the data sheet the differential input voltage range for the internal voltage reference 1.16mV and a gain factor of 1 is 928mV.

When I apply a constant voltage of e.g. 500mV between pins P6.4 and P6.5 I'd expect a repetitive constant value of 500mV / 928mV x 32767 = 17654 for the results variable when I debug the program several times in a row.

Instead I get alternating values of e.g. 42677, 9307, 5959, 5087, 42669, 13748, ...

#include <msp430.h>

unsigned int  results;                          // CTSD16 Conversion Results

void main(void) {
    WDTCTL = WDTPW | WDTHOLD;                   // Stop WDT

    P6SEL |= BIT4 | BIT5;                       // Select AD0+/- analog input pins

    CTSD16CTL = CTSD16REFS;                     // Internal ref
    CTSD16CCTL0  |= CTSD16SNGL | CTSD16DF;      // Single sample, 2's complement
    CTSD16INCTL0 |= CTSD16INCH_9;				// Select AD0+ / AD0- for CH1

    __delay_cycles(2000);                   // Delay ~120us for 1.2V ref to settle

    while(1) {
        CTSD16CCTL0 |= CTSD16SC;                // Set bit to start conversion
        while(!(CTSD16IFG & CTSD16IFG));      // Poll IFG until conversion completes
        results = CTSD16MEM0;                   // Save CTSD16 conversion results
        __no_operation();                       // SET BREAKPOINT HERE
    }
}

Am I getting something wrong here? Please help! Thanks!