Hi Jonas, Most likely you are seeing analog settling. I would suggest reviewing 'A Basic Guide to RTD Measurements': http://www.ti.com/lit/an/sbaa275/sbaa275.pdf It would be very helpful to see your schematic and all resistance and capacitance of the sensor path. There are a number of ways to configure a 3-wire RTD. It is not clear how you are compensating the lead resistance with a single IDAC source. If you are using a design similar to Figure 10 in the above referenced document, I would highly recommend that you do not drive current out the input pin (AIN5) for the 3-wire measurement. You will need some form of filtering at least for anti-aliasing. I also see that you are changing the default setting for the start of conversion of a register write from the default of 14 tmod periods to 1 tmod. The default setting delay will cover most input settling issues. You can also increase as needed which will be a delay internal to the ADS124S08 and removes a micro timer requirement between measurements. A 20ms delay for IDAC settling is pretty long. Every time you switch between sensors and measurements there are analog settling delays as there is always capacitance in filters, traces and cables and the resistance is not fixed. 5k of resistance and 1uF of capacitance has a time constant of 5ms. 5 time constants of 5ms is 25ms. However, as the measurement is ratiometric (excitation is same for both the reference and sensor) even if the reference hasn't fully settled you should still see good results. Where you will most likely see an issue is a non-linearity when switching from 500uA to 250uA as there will still be residual charge at the reference and not at the sensor. I don't know what inputs or combinations of inputs you are using, but there are 2 reference inputs on the ADS124S08 . It would be possible to have 2 separate paths where the RTDs could each be powered simultaneously. I would suggest that you use 250uA for the magnitude and set IDAC1 to the 3-wire RTD and IDAC2 to the 4-wire RTD. You would need to resize the 3-wire reference resistor to match the current, and possibly increase the gain from 8 to 16. In this way you could have both RTDs powered at the same time which would remove the IDAC path settling delay. Best regards, Bob B
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