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ADS1247 with RTD

Madhuri Akkenepalli
Expert 1215 points
Other Parts Discussed in Thread: ADS1247, ADS1248

 We have a new project which will use ADS1247/8 to measure temperatureusing the PT-100.  The application note from the datasheet is particular for the temperature range 0 –50C.  How can we select Rbias and Rcomp  if the measure range is from -200C  to +400C. 

 Also, in this project, we will use 8 channels PT-100 measurement. Dowe have to use 8pcs of the ADS1247/8?

Thanks for your help.

  • 0
    TI__Guru** 113765 points

    Hi Madhuri,

    The RTD response will be 18.49 ohms (-200 degC) to 274.04 ohms (400 deg C).  You will need a large enough current to successfully measure the lower resistance, but yet small enough to measure the larger resistance while maintaining enough headroom for IDAC voltage compliance and the reference voltage.  The total current will also depend on the number of wires required for your RTD (2, 3, or 4-wire RTD).  From you question regarding Rcomp, I will assume that you want to use a 3-wire RTD.

    The 3-wire RTD uses two current sources to cancel lead resistance so it is not included in the value of the measurement.  The Rcomp sets the zero volt point at the input that is measured across AINP and AINN.  For example, at 0 deg C the RTD will be 100 ohms.  If Rcomp is set to be 100 ohms, then the voltage drop across the RTD will be equal and opposite to the voltage drop across Rcomp which will then cancel so that the voltage at the ADC inputs is zero.  Any ADC result that is positive is an above 0 deg C reading and below 0 deg C is a negative reading.  You can set the value of Rcomp to any value that makes sense to you throughout the desired temperature range.  Keep in mind that any drift or inaccuracy of Rcomp will affect the accuracy of the measurement.

    The value of Rbias (reference resistor) will depend on the current, the desired reference voltage and any PGA gain to be applied that would affect the common mode input range.  The smallest allowable reference voltage is 0.5V.  As an example we will use an arbitrary value of 1.5V for the reference and 1mA for the total current (or 500uA for each input leg of the RTD).  For 1.5V and 1mA of current you will need 1500 ohms for Rbias.

    As to the number of RTDs that can be connected to either the ADS1247 or ADS1248 will depend on the method the IDACs are sourcing the current.  One method that allows for the maximum number of connected devices is to route the current through the mux directly to the analog inputs being measured.  The downside of this method is any input filters used will directly impact the measurement.  Using this method you can connect 4, 3-wire RTDs to the ADS1248 and 2, 3-wire RTDs to the ADS1247.

    The second method of routing the currents would be around any input filters used.  To do this you would need to use some of the analog inputs just for routing currents.  This reduces the number of avaible RTD inputs to 2, 3-wire RTDs for the ADS1248 and a single 3-wire RTD for the ADS1247.

    Best regards,

    Bob B