ADS1248 ADC drift

Hi 

I have and ADS1248 connected to two 4 wire PT100 interfaces, Se attached schematic

i am successfully switching between the two PT100 resistors when both resistors are connected but as soon as one of the resistors is removed the other ADC value starts to drift. I do have a lot of capacitance on my lines due to transorbs and ESD protection so it may be that but i am unsure

My setup is as follows:

Temp_Data[0] = 0x4A; //[01][00 1][010] Burnout current 0.5uA, Positive input channel AIN1, Negative input channel AIN2
Temp_Data[1] = 0x00; //[xxxx][0000] Bias voltage register: No bias voltages
Temp_Data[2] = 0x20; //[0][01][0 0][000] Internal oscillator ON, Internal reference ON, Ref0 selected, Normal operation
Temp_Data[3] = 0x33; //[0][011] [0011] Always 0, Gain of the PGA, Output data rate 40(SPS)

Temp_Data[4] = 0x00;//SYSOCAL - OFFSET calculator
Temp_Data[5] = 0x00;
Temp_Data[6] = 0x00;


Temp_Data[7] = 0x00;//SYSGCAL - GAIN calculator
Temp_Data[8] = 0x00;
Temp_Data[9] = 0x40;

Temp_Data[10] = 0x07; //[0000] [0][111] Dout only, current = 1.5mA
Temp_Data[11] = 0x03; //[0000] [0011] Output pin for 1st IDAC: AIN0, output pin for 2nd IDAC AIN3

Temp_Data[12] = 0x00; //GPIO
Temp_Data[13] = 0x00;
Temp_Data[14] = 0x00;
START = 1;//Set start pin and keep it on

my servicing of the ADS1248 looks like this:

if(TEMP_READY == 0)
      {
        if(ADS.Which == 0)
        {
          Temp_Data[0]  = 0x65;       //[01][10 0][101]         Burnout current 0.5uA, Positive input channel AIN4, Negative input channel AIN5, No bias
          Temp_Data[11] = 0xC3;       //[0011] [1100]           Output pin for 1st IDAC: AIN0, output pin for 2nd IDAC AIN3
        }
        else
        {
          Temp_Data[0]  = 0x4A;       //[01][00 1][010]         Burnout current 0.5uA, Positive input channel AIN1, Negative input channel AIN2, No bias
          Temp_Data[11] = 0xC0;       //[0011] [0000]           Output pin for 1st IDAC: AIN0, output pin for 2nd IDAC AIN0
        }

        ADC_Reading = ADS1248_Write_Command(0, 12, (char *)&Temp_Data);//Write the data and return the ADC reading
}

The basics around the operation are as follows:

1. initialize ADS1248
2. wait for DRDY to go low
3. if low, save reading for active channel
4. Switch IDAC and analogue inputs to "other" channel
5. go to step 2

With this operation i have stable results when both PT100 resistors are connected but as soon as i remove one of the resistors the other starts to drift

I have managed to overcome this by adding a 100mS delay between step 5 and step 2 but i do not understand why this helps the situation

My understanding is that once the DRDY line goes low the data sampled should be stable

Any and all help is appreciated

Richard

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