the adc14 sampling noise problem of msp432 launchpad

hi, all. i got the adc14 sampling problem again .

i tried the adc sampling code( C code and driverlib code from TI) and found both results exceed my expection.

the C code is the single channel repeat mode and another is multi channels repeat mode

the difference between the max and min of raw data, which i got from ADC14MEM0, can be larger than 30 counts, even up to 70 counts.

why the noise level is so high??

someone helps!

the C code listed as follows

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//******************************************************************************
// MSP432P401 Demo - ADC14, Repeated Sequence of Conversions
//
// Description: This example shows how to perform a repeated sequence of
// conversions using "repeat sequence-of-channels" mode. AVcc is used for the
// reference and repeated sequence of conversions is performed on Channels A0,
// A1, A2, and A3. Each conversion result is stored in ADC14MEM0, ADC14MEM1,
// ADC14MEM2, and ADC14MEM3 respectively. After each sequence, the 4 conversion
// results are moved to A0results[], A1results[], A2results[], and A3results[].
// Test by applying voltages to channels A0 - A3. Open a watch window in
// debugger and view the results. Set Breakpoint1 in the index increment line
// to see the array values change sequentially and Breakpoint2 to see the entire
// array of conversion results in A0results[], A1results[], A2results[], and
// A3results[]for the specified Num_of_Results.
//
// Note that a sequence has no restrictions on which channels are converted.
// For example, a valid sequence could be A0, A3, A2, A4, A2, A1, A0, and A7.
// See the User's Guide for instructions on using the ADC14.
//
// MSP432P401
// -----------------
// /|\| |
// | | |
// --|RST |
// | |
// Vin0 -->|P5.5/A0 |
// Vin1 -->|P5.4/A1 |
// Vin2 -->|P5.3/A2 |
// Vin3 -->|P5.2/A3 |
// | |
//
// Wei Zhao
// Texas Instruments Inc.
// June 2014
// Built with Code Composer Studio V6.0
//******************************************************************************
#include "msp.h"
#include <stdint.h>

#define Num_of_Results 8

volatile uint16_t A0results[Num_of_Results];
volatile uint16_t A1results[Num_of_Results];
volatile uint16_t A2results[Num_of_Results];
volatile uint16_t A3results[Num_of_Results];
static uint8_t index;

int main(void)
{
WDTCTL = WDTPW+WDTHOLD; // Stop watchdog timer
// Configure GPIO
P5SEL1 |= BIT5 | BIT4 | BIT3 |BIT2; // Enable A/D channel A0-A3
P5SEL0 |= BIT5 | BIT4 | BIT3 |BIT2;

__enable_interrupt();
NVIC_ISER0 = 1 << ((INT_ADC14 - 16) & 31);// Enable ADC interrupt in NVIC module

ADC14CTL0 = ADC14ON | ADC14MSC | ADC14SHT0__192 | ADC14SHP | ADC14CONSEQ_3; // Turn on ADC14, extend sampling time
// to avoid overflow of results

ADC14MCTL0 = ADC14INCH_0; // ref+=AVcc, channel = A0
ADC14MCTL1 = ADC14INCH_1; // ref+=AVcc, channel = A1
ADC14MCTL2 = ADC14INCH_2; // ref+=AVcc, channel = A2
ADC14MCTL3 = ADC14INCH_3+ADC14EOS; // ref+=AVcc, channel = A3, end seq.
ADC14IER0 = ADC14IE3; // Enable ADC14IFG.3

SCB_SCR &= ~SCB_SCR_SLEEPONEXIT; // Wake up on exit from ISR

while(1)
{
ADC14CTL0 |= ADC14ENC | ADC14SC; // Start conversion-software trigger
__sleep();
__no_operation(); // For debugger
}

}

// ADC14 interrupt service routine
void adc_isr(void)
{
if (ADC14IFGR0 & ADC14IFG3)
{
A0results[index] = ADC14MEM0; // Move A0 results, IFG is cleared
A1results[index] = ADC14MEM1; // Move A1 results, IFG is cleared
A2results[index] = ADC14MEM2; // Move A2 results, IFG is cleared
A3results[index] = ADC14MEM3; // Move A3 results, IFG is cleared
index = (index + 1) & 0x7; // Increment results index, modulo
__no_operation(); //Set Breakpoint1 here
}

}

void SysTick_ISR()
{

}