Connecting 4 3-wire RTD sensors in one ADS1248

Dear all,

I am trying to design a circuit for connecting 4 3-wire RTDs using ADS1248

Since the RTDs will be in a noisy environment having 5-6 meters cabling we will use RC filtering in ADC and REFERENCE inputs.

Is it possible to connect more than 2 sensors (with RC filters) to one ADS1248?

Thank you in advance!

  • In reply to Mo.:

    Dear Mo,

    Thank you very much for your support!

    Indeed my design is very close with TIDU145 which includes a resistor in one of the input channels for hardware compensation. I was wondering for compensation resistor accuracy issues from the very beginning of the design and finally after and Mr Wu advice (latest post) I will remove the hardware compensation resistors (R4 & R94).

    I am planning to use 1mA IDAC currents (taking into consideration the temperature range 0-50oC). For this reason in the upd2 schematic the REF resistor reduced from 3K3 to 820R. Do you believe that I have to stay with the initial design using 3K3 REF resistor?

    Also I was confused with Vincenzo advice on the initial design "if no RTD is connected you have 0V reference so you can't sense the pH sensor as well". Do you believe that there is an issue in the upd2 design with VREF in pH measurement if  RTD's are not connected?

    Best regards,

    Nikos

  • In reply to Joseph Wu:

    Dear Joseph,

    Thank you very much for your support!

    I was wondering about the use of the two compensation resistors from the very beginning of the design and your advice really helps me to decide about removing them!

    The two inductors are used in TI design tidu145. Additionaly there are 10uF capacitors in parallel with 100nF decoupling capacitors in analog and digital power lines. Do you believe that its better the two inductors to be removed?

    Best regards,

    Nikos
  • In reply to Nikolaos Piliouras:

    Nikos,


    Since it's in the TI design, I would leave them in for the time being. I'm just concerned that with digital currents, you might see more supply spiking. You can still test it to see if it poses any problems. Later, it should be easy to alter the design to remove them.


    Joseph Wu
  • In reply to Joseph Wu:

    Dear Joseph,

    I wll keep it mind during the prototype testing.
    Thank you very much for yur support!

    Nikos
  • In reply to Nikolaos Piliouras:

    Hi Nikos,

    Nikolaos Piliouras
    I am planning to use 1mA IDAC currents (taking into consideration the temperature range 0-50oC). For this reason in the upd2 schematic the REF resistor reduced from 3K3 to 820R. Do you believe that I have to stay with the initial design using 3K3 REF resistor?

    Increasing the IDAC current to 1mA will not violate the IDAC compliance voltage @3.3V AVDD in your configuration. Below the Math:

    Vcomp = AVDD - 0.7V = 2.6V

    VIDAC1 = Vref + Vrtd(max) < 2.6V, with Rrtd(50°C) = 120 ohm, Vrtd(50°C) = 120 ohm x 1mA = 0.12V

    VIDAC1 = 820 ohm x 2 x 1mA + 0.12V = 1.76V < 2.6V, good!

    Now to the self-heating effect. Increasing the IDAC current will increase errors due to the self-heating of the RTD. The effects of self-heating on measurement accuracy are also based on the placement of the RTD in the application (open-air = insulates vs. moving fluid = dissipates). If this error -depending on your RTD type- is very little and negligible in your design and it can be tolerated that then that's okay!

    Nikolaos Piliouras
    Also I was confused with Vincenzo advice on the initial design "if no RTD is connected you have 0V reference so you can't sense the pH sensor as well". Do you believe that there is an issue in the upd2 design with VREF in pH measurement if  RTD's are not connected?

    If no RTD is connected then there will be no voltage created over Rref, hence no external Vref will be available. However, you can still use the internal generated Vref (2.048V) for your pH measurements.

    Kind Regards,

    Mo.

  • In reply to Mo.:

    Nikos,


    There are two things that I wanted to point out about Mo's comments. First, he is correct that you should watch the compliance of the output voltage of the IDAC current to AVDD. If you get to close to AVDD, then the IDAC current will start to decrease. Second, the compliance of 0.7V to AVDD is a bit optimisitic. This was in previous revisions of the datasheet, but in the current revision of the datasheet, we simply refer to Figures 41 and 42.

    I would also note that it likely will not be a problem. If there is no current leakage path out of the RTD and RREF measurement, then the ratiometric measurement still holds, regardless of the current. You should still be able to get a useful measurement out of the device.


    Joseph Wu
  • In reply to Joseph Wu:

    Dear Mo and Joseph,

    Thank you so much for your support!

    Nikos