Error on 3-wire measure with the EVAL-AD7124-8SDZ

Question

I have an AD7128-8 conected to a 3-wire measure circuit to read from a PT1000. I am following the steps of the RTD Configurator and error budget calculator.

My problem is that the measure i'm getting is: 
 
 Digital sample: 6919806.000000 RTD: 552.944214 Digital sample: 6919790.000000 RTD: 552.942932 Digital sample: 6919734.000000 RTD: 552.938477 Digital sample: 6919654.000000 RTD: 552.932068 Digital sample: 6919639.000000 RTD: 552.930908 
 
 When i should be getting around 1110 ohms 
 
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 The steps i'm following on the code to measure the RTD are:

//1. Setting the 50uA [AIN0+ (IOUT0) / AIN1- (IOUT1)] 
 ad7124_regs[ AD7124_IOCon1 ]. value =0x910;

//2. Analog input [AIN2+ / AIN3-] 
 ad7124_regs[ AD7124_Channel_0 ]. value =0x8043;

//3. Configuring the PGA to 16, the unipolar mode, and the external references [REFIN1+ / REFIN1-] 
 ad7124_regs[ AD7124_Config_0 ]. value =0x1E4;

//4. Writing of the registers 
 ad7124_write_register(ad7124_handler, ad7124_regs[ AD7124_IOCon1 ]); 
 ad7124_write_register(ad7124_handler, ad7124_regs[ AD7124_Channel_0 ]); 
 ad7124_write_register(ad7124_handler, ad7124_regs[ AD7124_Config_0 ]);

//5. Waiting for conversion 
 ad7124_wait_for_conv_ready(ad7124_handler, timeout);

//Rref= 21500ohms 1%, on the polimeter it reads 21460ohms

ad7124_read_data(ad7124_handler, &sample);

RTD =(sample * 21460)/(16* pow (2.0, 24)); //UNIPOLAR 
 printf ( "RTD: %f 
" , RTD);

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This are the schematics:

I am working at the moment with the EVAL-AD7124-8SDZ, and these are the conexions on the plaque.

RAdante · Accepted Answer

Hi blnc , 
 I am getting your expected value through the RTD Configurator and error budget calculator. You may refer to the image below: 
 
 I reviewed your code and 
 how the RTD resistance is calculated given your RTD configuration setup. 
 blnc said: RTD =(sample * 21460)/(16* pow (2.0, 24)); //UNIPOLAR 
 For "Alt_3wire" RTD configuration, wherein this configuration, the reference resistor is connected to the low side of the RTD which means both IOUT currents passes through the reference resistor. Moreover, taking into account that 2IOUT passes the reference resistor RREF, your calculation should now be: 
 RTD =(sample * 21460 *2 )/(16*pow(2.0, 24)) 
 Thanks and regards, 
 Rod)

blnc · Answer

Hi, thank you for your answer. 
 
 You are right, the calculations should be taking into account the two currents circulating through the Rref. 
 
 I have changed the equation and the measurement is a lot closer than before. Although i am still getting an accuracy error. 
 
 I have measured some simple resistors to make a test on how much it is deviating, and it seems like the error is exponential. 
 1002 ohms ---- reading is 995.750916 
 502 ohms ---- reading is 497.855164 
 199.7 ohms ---- reading is 196.463806 
 99.4 ohms ---- reading is 98.355583 
 49.7 ohms ---- reading is 49.040466 
 
 I am doing this internal calibrations to the ad7124:

// Reset offset register 
 ad7124_regs[ AD7124_Offset_0 ]. value = 0x800000; 
 ad7124_write_register(ad7124_handler, ad7124_regs[ AD7124_Offset_0 ]); 
 
 // Calibrations ADC Offset 
 ad7124_regs[ AD7124_ADC_Control ]. value = 0x14; 
 ad7124_write_register(ad7124_handler, ad7124_regs[ AD7124_ADC_Control ]); 
 ret = ad7124_wait_for_conv_ready(ad7124_handler, timeout); 
 
 // Calibrations ADC Gain 
 ad7124_regs[ AD7124_ADC_Control ]. value =0x8; 
 ad7124_write_register(ad7124_handler, ad7124_regs[ AD7124_ADC_Control ]); 
 ret = ad7124_wait_for_conv_ready(ad7124_handler, timeout);

// Calibrations ADC Continuous convert 
 ad7124_regs[ AD7124_ADC_Control ]. value =0x0; 
 ad7124_write_register(ad7124_handler, ad7124_regs[ AD7124_ADC_Control ]); 
 ret = ad7124_wait_for_conv_ready(ad7124_handler, timeout);

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Error on 3-wire measure with the EVAL-AD7124-8SDZ