ADS1256: Getting different readings when using differential mode vs common mode on ADS1256

Hi user,

Do you have a schematic to share that shows how you are connecting the inputs and supply voltages to the ADS1256? Can you also share your register settings? These might help diagnose the issue.

In general, differential inputs are recommended for common-mode noise rejection. But AINCOM is effectively just another analog input - it could just as easily be referred to as AIN8. You can see it has the same input structure as shown in Figure 6. If you are measuring between some voltage and GND, that is a single-ended measurement, regardless of the pins you choose. So there is no meaningful difference between the two measurements you described, at least from the ADC's perspective.

Let me know.

-Bryan

Hi Brian,

This is the setting I have. The first schematic is for the common mode reading and the second one is for the differential reading.

Here are my register settings:

/* gain channel  */
typedef enum
{
	ADS1256_GAIN_1			= (0),	/* GAIN   1 */
	ADS1256_GAIN_2			= (1),	/*GAIN   2 */
	ADS1256_GAIN_4			= (2),	/*GAIN   4 */
	ADS1256_GAIN_8			= (3),	/*GAIN   8 */
	ADS1256_GAIN_16			= (4),	/* GAIN  16 */
	ADS1256_GAIN_32			= (5),	/*GAIN    32 */
	ADS1256_GAIN_64			= (6),	/*GAIN    64 */
} ADS1256_GAIN_E;

/* Sampling speed choice*/
/* 
	0xF0  11110000 = 30,000SPS (default)
	0xE0  11100000 = 15,000SPS
	0xD0  11010000 = 7,500SPS
	0xC0  11000000 = 3,750SPS
	0xB0  10110000 = 2,000SPS
	0xA1  10100001 = 1,000SPS
	0xA2  10010010 = 500SPS
	0x82  10000010 = 100SPS
	0x72  01110010 = 60SPS
	0x63  01100011 = 50SPS
	0x53  01010011 = 30SPS
	0x43  01000011 = 25SPS
	0x33  00110011 = 15SPS
	0x23  00100011 = 10SPS
	0x13  00010011 = 5SPS
	0x03  00000011 = 2.5SPS
*/

typedef enum
{
	ADS1256_30000SPS = 0,
	ADS1256_15000SPS,
	ADS1256_7500SPS,
	ADS1256_3750SPS,
	ADS1256_2000SPS,
	ADS1256_1000SPS,
	ADS1256_500SPS,
	ADS1256_100SPS,
	ADS1256_60SPS,
	ADS1256_50SPS,
	ADS1256_30SPS,
	ADS1256_25SPS,
	ADS1256_15SPS,
	ADS1256_10SPS,
	ADS1256_5SPS,
	ADS1256_2d5SPS,
	ADS1256_DRATE_MAX
} ADS1256_DRATE;

void ADS1256_CfgADC(ADS1256_GAIN_E _gain, ADS1256_DRATE _drate)
{
	ADS1256_WaitDRDY();
	//g_tADS1256.Gain = _gain;
	//g_tADS1256.DataRate = _drate;


	uint8_t buf[4];		/* Storage ads1256 register configuration parameters */

		/*Status register define
			Bits 7-4 ID3, ID2, ID1, ID0  Factory Programmed Identification Bits (Read Only)
			Bit 3 ORDER: Data Output Bit Order
				0 = Most Significant Bit First (default)
				1 = Least Significant Bit First
			Input data  is always shifted in most significant byte and bit first. Output data is always shifted out most significant
			byte first. The ORDER bit only controls the bit order of the output data within the byte.
			Bit 2 ACAL : Auto-Calibration
				0 = Auto-Calibration Disabled (default)
				1 = Auto-Calibration Enabled
			When Auto-Calibration is enabled, self-calibration begins at the completion of the WREG command that changes
			the PGA (bits 0-2 of ADCON register), DR (bits 7-0 in the DRATE register) or BUFEN (bit 1 in the STATUS register)
			values.
			Bit 1 BUFEN: Analog Input Buffer Enable
				0 = Buffer Disabled (default)
				1 = Buffer Enabled
			Bit 0 DRDY :  Data Ready (Read Only)
				This bit duplicates the state of the DRDY pin.
			ACAL=1  enable  calibration
		*/
	//buf[0] = (0 << 3) | (1 << 2) | (1 << 1);// buffer ON, auto-cal ON
         buf[0] = (0 << 3) | (1 << 2) | (0 << 1);  // buffer OFF, auto-cal ON

        //ADS1256_WriteReg(REG_STATUS, (0 << 3) | (1 << 2) | (1 << 1));

	buf[1] = 0x08;	

		/*	ADCON: A/D Control Register (Address 02h)
			Bit 7 Reserved, always 0 (Read Only)
			Bits 6-5 CLK1, CLK0 : D0/CLKOUT Clock Out Rate Setting
				00 = Clock Out OFF
				01 = Clock Out Frequency = fCLKIN (default)
				10 = Clock Out Frequency = fCLKIN/2
				11 = Clock Out Frequency = fCLKIN/4
				When not using CLKOUT, it is recommended that it be turned off. These bits can only be reset using the RESET pin.
			Bits 4-3 SDCS1, SCDS0: Sensor Detect Current Sources
				00 = Sensor Detect OFF (default)
				01 = Sensor Detect Current = 0.5 uA
				10 = Sensor Detect Current = 2  uA
				11 = Sensor Detect Current = 10 uA
				The Sensor Detect Current Sources can be activated to verify  the integrity of an external sensor supplying a signal to the
				ADS1255/6. A shorted sensor produces a very small signal while an open-circuit sensor produces a very large signal.
			Bits 2-0 PGA2, PGA1, PGA0: Programmable Gain Amplifier Setting
				000 = 1 (default)
				001 = 2
				010 = 4
				011 = 8
				100 = 16
				101 = 32
				110 = 64
				111 = 64
		*/
	buf[2] = (0 << 5) | (0 << 3) | (_gain << 0);
		// ADS1256_WriteReg(REG_ADCON, (0 << 5) | (0 << 2) | (GAIN_1 << 1));	/* choose 1: gain 1 ;input 5V */

	buf[3] = ADS1256_DRATE_E[_drate];	// DRATE_10SPS;	

		CS_0();	// SPI CS = 0   (CS active low. Doc claims no setup time needed after this)

        //ADS1256_WaitDRDY();

	ADS1256_Send8Bit(CMD_WREG | 0);	/* Write command register, send the register address */
	bsp_DelayUS(10);
	ADS1256_Send8Bit(0x03);			/* Register number 4,Initialize the number   */
        bsp_DelayUS(10);
	ADS1256_Send8Bit(buf[0]);	/* Set the status register */
	bsp_DelayUS(10);
	ADS1256_Send8Bit(buf[1]);	/* Set the input channel parameters */
	bsp_DelayUS(10);
	ADS1256_Send8Bit(buf[2]);	/* Set the ADCON control register,gain */
	bsp_DelayUS(10);
	ADS1256_Send8Bit(buf[3]);	/* Set the output rate */
	bsp_DelayUS(10);

	CS_1();	/* SPI  CS = 1 */
bsp_DelayUS(10);

        // printf("drate: %d  Buf3 = %d, %02x\n",_drate,buf[3], buf[3]);

	//bsp_DelayMS(1);
}





This is how I'm currently reading the data:

static int32_t ADS1256_ReadData(void)
{
	
	 uint32_t read = 0;
         static uint8_t buf[3];

	CS_0();	/* SPI   CS = 0 */
	
	ADS1256_WaitDRDY();  // wait for ADC result to be ready
	ADS1256_Send8Bit(CMD_RDATA);	/* Write the register number */
	//ADS1256_WriteCmd(CMD_RDATA); // start continuous reading mode
	
   // ADS1256_WaitDRDY();  // wait for ADC result to be ready
    	
        ADS1256_DelayDATA();    /*delay time  */
	   //ADS1256_WriteCmd(CMD_RDATAC); // start continuous reading mode
           //bsp_DelayMS(20);
	   



	    
	    //ADS1256_WriteCmd(CMD_RDATAC); // start continuous reading mode
          // bsp_DelayMS(2);
	   
	



    // read the 3 byte ADC value
    buf[0] = ADS1256_Recive8Bit();
    //bsp_DelayMS(5);
    buf[1] = ADS1256_Recive8Bit();
    //bsp_DelayMS(5);
    buf[2] = ADS1256_Recive8Bit();
    bsp_DelayMS(2);

    // assemble 3 bytes into one 32 bit word
    read = ((uint32_t)buf[0] << 16) & 0x00FF0000;
    read |= ((uint32_t)buf[1] << 8);
    read |= buf[2];

    CS_1();	/* SPI CS = 1 */

	/* Extend a signed number*/
    if (read & 0x800000)
    {
	    read |= 0xFF000000;
    }
    
    //printf("read : %ld \n",read);
    //exit(0);

    return (int32_t)read;
} // end ADS1256_ReadData()

Thanks,

Hi user,

Do you have an actual schematic? That image is hard to read, and it's not clear what the voltage levels are or how they are derived. For example, what is the VREF voltage you are using, and how is it generated? What about the clocking scheme? This looks like it might be a pre-built board that you are using, so a user's guide or something similar would be helpful.

Can you read back the register settings used to take both sets of measurements and provide those to me? You mentioned you used 1000SPS in your original post, but your code shows 10SPS, so I am not sure which settings are actually valid for the issue you are seeing.

Are you really using 2 AA batteries as the input? You mentioned a DC power supply, so I imagined a bench top device. I just want to make sure I understand your actual setup.

How are you reading the voltage levels on the adjacent channels? You said there was 1.25V on adjacent inputs when you tied the DC GND to AINCOM - did you measure this with a DMM, or with the ADC?

-Bryan