[FAQ] MSP430I2020: Measuring SD ADC noise

Hello Hung,

I'm attaching the code I used for this testing. Using a software oversampling (or averaging) factor of 16, I achieved less than 10uV noise between 100 samples on Sigma Delta ADC Channel 0 (shorted A0.0+ and A0.0- with jumper wire) using i2041 target socket board. I pause the code at the while(1) function, add the "results[]" array to the Watch window in CCS, copy the array into Excel, subtract the offset from the samples (I'm using bipolar format), and convert to voltages. Then, I find the maximum difference for the noise value.

msp430i20xx_sd24_03_modified_for_sw_oversampling.c

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//******************************************************************************
//  MSP430i20xx Demo - SD24, Continuous Conversion on a Single Channel
//
//  Description: This program uses the SD24 module to perform continuous
//  conversions on a single channel. A SD24 interrupt occurs when a conversion
//  has completed.
//
//  Test by applying voltages to the input channel and setting a breakpoint
//  at the indicated line. Run program until it reaches the breakpoint, then use
//  the debugger's watch window to view the conversion results.
//
//  Results (upper 16 bits only) are stored in the array "results"
//
//  ACLK = 32kHz, MCLK = SMCLK = Calibrated DCO = 16.384MHz, SD_CLK = 1.024MHz
//  * Ensure low_level_init.c is included when building/running this example *
//
//  Notes: For minimum Vcc required for SD24 module - see datasheet
//         100nF cap btw Vref and AVss is recommended when using 1.2V ref
//
//               MSP430i20xx
//             -----------------
//         /|\|                |
//          | |                |
//          --|RST             |
//            |                |
//   Vin1+ -->|A0.0+      VREF |---+
//   Vin1- -->|A0.0-           |   |
//            |                |  -+- 100nF
//            |                |  -+-
//            |                |   |
//            |           AVss |---+
//
//  T. Witt
//  Texas Instruments, Inc
//  September 2013
//  Built with Code Composer Studio v5.5
//******************************************************************************
#include "msp430.h"

/* Array to store SD24 conversion results */
volatile int j, index = 0;
volatile unsigned long sum = 0;
unsigned long results[100];                     // Final results from running average

/* Defines */
#define SW_OVER_SAMPLING_FACTOR                 16

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

    SD24CTL = SD24REFS;                         // Internal ref
    SD24INCTL0 = SD24GAIN_1;                    // Set gain on Channel 0
    SD24CCTL0 |= SD24IE;                        // Enable interrupt, OSR = 256 (default)

    __delay_cycles(3200);                       // Delay ~200us for 1.2V ref to settle

    SD24CCTL0 |= SD24SC;                        // Set bit to start conversion
    __bis_SR_register(GIE);                     // Enable interrupts

    while(1) {                                  // Loop endlessly, SET BREAKPOINT or PAUSE HERE
    }
}

#if defined(__TI_COMPILER_VERSION__) || defined(__IAR_SYSTEMS_ICC__)
#pragma vector=SD24_VECTOR
__interrupt void SD24_ISR(void)
#elif defined(__GNUC__)
void __attribute__ ((interrupt(SD24_VECTOR))) SD24_ISR (void)
#else
#error Compiler not supported!
#endif
{
    long temp = 0;
    switch (__even_in_range(SD24IV,SD24IV_SD24MEM3)) {
        case SD24IV_NONE: break;
        case SD24IV_SD24OVIFG: break;
        case SD24IV_SD24MEM0:

            // Capture ADC data
            SD24CCTL0 &= ~SD24LSBACC;           // Clear LSBACC bit
            temp = SD24MEM0;                    // Read upper 16 bits of SD24MEM0
            SD24CCTL0 |= SD24LSBACC;            // Set LSBACC bit
            temp = temp*256 + (SD24MEM0&0xff);  // Read lower 16 bits of SD24MEM0, combine with upper bits

            // Perform software oversampling
            sum+= temp;                         // Add result to running sum
            j++;
            if(j >= SW_OVER_SAMPLING_FACTOR) {  // Calculate and store average
                results[index++] = sum/SW_OVER_SAMPLING_FACTOR;
                sum = 0;
                j = 0;
            }

            if(index >=100) {
                index=0;
            }
            break;
        case SD24IV_SD24MEM1: break;
        case SD24IV_SD24MEM2: break;
        case SD24IV_SD24MEM3: break;
        default: break;
    }
}

Regards,

James

Hi Hung,

I'm sorry for the confusion. The factor of 16 in my code is the number of averaged samples (on software side). I'm summing the hardware samples then reporting the averaged result after a certain number of samples have been captured (here it's 16). My code is actually using a GAIN value of 1 (on hardware side). Does this make sense?

Regards,

James