ADS122U04EVM: Values of R77 and R79 for a PT1000 RTD

The ADS122U04EVM was designed to be a generic solution to show many different sensor measurements.  It may not be the best solution for you to use in your end system.  The EVM implementation for 3-wire RTDs is showing a high-side reference that requires 2 measurements.  The first is AIN0/AIN3 which is the RTD + Lead Resistance and the second is AIN0/AIN2 which is just Lead Resistance.  The codes from the second measurement should be subtracted from the first measurement and then converted to resistance.

There are also other forms of RTD configuration and measurement possible and they are discussed in A Basic Guide to RTD Measurements.  The configurations also include a high-side reference measurement using 2 IDACs (automatic lead cancellation), and low-side reference measurements.

Now to attempt to answer your specific question.  Again, the EVM is a generic solution and when evaluating different options and then choosing values you need to make some decisions on how you would intend on operating the ADC.  This would include temperature range of measurement, IDAC current output value, and analog operating voltage.  The decisions made are dependent on each other.  For example, the IDAC current has a compliance voltage (to maintain constant current) where the total voltage drop of the current path must be less than AVDD - 0.9V.  So when using 5V you can have a much greater voltage drop in the current path as opposed to 3.3V.

Related to the current path is the temperature range of operation.  A maximum resistance could be around 3.9k if the full-range of the temperature is to be used (850 deg C).  This resistance would drop considerably if the max temperature is 200 deg C (1.785k).

The reference resistor (R77) value will depend on the IDAC current, the total resistance in the path, and if gain is to be used in the measurement.  The R79 value is chosen so that the RTD is centered closely to mid-AVDD supply to achieve an RTD measurement within the center of the input range of the ADC. If, for example, AVDD is 5V, the compliance voltage using the existing resistors on the EVM with a maximum RTD of 3.9k would have a maximum resistance of 12k and when using 250uA of IDAC current would result in a 3V drop across the resistance.  This works for 5V but not for 3.3V AVDD.  Here you would be limited to a gain of 1, but could use the entire temperature range for the RTD and still use the existing R77 and R79 values.

The interaction of the various settings and values makes a difference and really depends on your intentions for the system you intend to use (or evaluate).  I would highly recommend reviewing the information in the basic guide.  If you then can give me some information as to what you intend to do with respect to AVDD supply voltage, temperature range of measurement, how much PGA gain (if required) and desired IDAC current I can be more helpful with values.

You may also find the ADC-RTD-CONFIG-CALC tool helpful as well.  However I don't recall if the specific high-side implementation is given in the calculator tool that is on the EVM.

Best regards,

Bob B