ADS 1248 - 4 Wire RTD - ADC Reading Changes day to day

Dear Joachim,

Thank you for your reply. I think that you are correct, I have been playing around with different resistors and it appears to alter the outcome. Do you have any suggestions about the appropriate resistor to use?

Kind Regards,

Shiv

Hi Joachim,

Thanks for your reply. I have used a higher power resistor and and I have reduced the current to now 100uA. Between yesterday and today the measured resistance of the fixed resistor changed from 164.0 ohms to 164.8 ohms. These resistors have a temperature coefficient of +350ppm per deg C and the room temperature is 1 degC lower today, so it is appears not an effect of temperature as it is in the wrong direction. I have changed the reference resistor to ~4.7K(+-5%) 0.5W resistor and the measurement resistor is ~160R 0.5W (+-5%).

My calculation of resistance is as follows : [(ADC Value) *Rref] / [ (2^(24-1)) * (PGA Value)]

Rref = 4700; PGA Value = 2; ADC Value is read from the ADS1248;

My code is arduino:

// To set up registers:

ADS.SetRegisterValue(MUXy1, BCS1_1 | MUX_SP2_AIN0 | MUX_SN2_AIN1);
delay(500);
ADS.SetRegisterValue(MUXy1, VREFCON1_ON | MUXCAL2_NORMAL | REFSELT1_REF0); // Set Internal Ref ON , Normal operation MUX; Set Reference to REFP0 and REFN0

delay(500);
ADS.SetRegisterValue(IDAC0, IMAG2_100); // IDAC at 100uA current
delay(500);
ADS.SetRegisterValue(IDAC1, I1DIR_IEXT1 | I2DIR_OFF); // IDAC1 Current output ON IEXC1, IDAC2 OFF
delay(500);
ADS.SetRegisterValue(SYS0, PGA2_2 | DOR3_5); // PGA set at 2, 5SPS

//Read from chip

void loop() {

adz0 = ADS.GetConversion();

adz1 = (adz0*4700)/8388608/2;

Serial.print("Resistance = ");
Serial.println(adz1, DEC);

}

// C functions from flydroid for ADS1248

// function to get a 3byte conversion result from the adc

long ads12xx::GetConversion() {
int32_t regData;
//waitforDRDY(); // Wait until DRDY is LOW
SPI.beginTransaction(SPISettings(SPI_SPEED, MSBFIRST, SPI_MODE1));
digitalWrite(_CS, LOW); //Pull SS Low to Enable Communications with ADS1247
delayMicroseconds(10); // RD: Wait 25ns for ADC12xx to get ready
SPI.transfer(RDATA); //Issue RDATA
delayMicroseconds(10);
regData |= SPI.transfer(NOP);
//Serial.println(regData, HEX);
delayMicroseconds(10);
regData <<= 8;
regData |= SPI.transfer(NOP);
//Serial.println(regData, HEX);
delayMicroseconds(10);
regData <<= 8;
regData |= SPI.transfer(NOP);
Serial.println(regData, HEX);
delayMicroseconds(10);
digitalWrite(_CS, HIGH);
SPI.endTransaction();

// function to write a register value to the adc
// argumen: adress for the register to write into, value to write
void ads12xx::SetRegisterValue(uint8_t regAdress, uint8_t regValue) {
#ifdef ADS1248
if (regAdress == IDAC0) {
regValue = regValue | IDAC0_ID; // add non 0 non-write register value IDAC0_ID
}
#endif
uint8_t regValuePre = ads12xx::GetRegisterValue(regAdress);
if (regValue != regValuePre) {
//digitalWrite(_START, HIGH);
delayMicroseconds(10);
waitforDRDY();
SPI.beginTransaction(SPISettings(SPI_SPEED, MSBFIRST, SPI_MODE1)); // initialize SPI with SPI_SPEED, MSB first, SPI Mode1
digitalWrite(_CS, LOW);
delayMicroseconds(10);
SPI.transfer(WREG | regAdress); // send 1st command byte, address of the register
SPI.transfer(0x00); // send 2nd command byte, write only one register
SPI.transfer(regValue); // write data (1 Byte) for the register
delayMicroseconds(10);
digitalWrite(_CS, HIGH);
//digitalWrite(_START, LOW);
if (regValue != ads12xx::GetRegisterValue(regAdress)) { //Check if write was succesfull
Serial.print("Write to Register 0x");
Serial.print(regAdress, HEX);
Serial.println(" failed!");
}
SPI.endTransaction();

}

}

I have not included all the code as it is quite long, but i you need any more info please let me know.

Thank you I really appreciate your help.

My schematic is :

hi Shiv,

Have you fixed this problem?I also meet this issue,can you give me some suggestions?