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maximun number of rtd connection

krishna prasad
Prodigy 80 points
Other Parts Discussed in Thread: ADS1248, ADS124S08

hi,

i would like to know how many 3 wire RTDs can be connected to one ADS1248 Chip?

  • 0
    TI__Guru* 94285 points
    Krishna,


    The maximum number of rtds you could connect to the ADS1248 would be four. For this configuration, you would use two channels apiece for the the sensing of the RTD, and with matched IDAC value driving each input. The third wire for each rtd would connect to a precision resistor which would sink the IDAC currents and be used for the reference input.

    The one thing that I would warn you about would be that if you drive the IDACs to the AINx pins, often you will have some sort of filtering with extra resistance. the mismatch of the IDAC currents and the extra filtering resistance will give you additional error in the measurement.

    Some will use the external IDAC pins to drive the rtd outside of the filtering so the current bypasses the filter resistances. This may take extra circuitry (like a mux) to implement.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Prodigy 10 points
    Would you be so kind to add a schematic drawing how to connect the multiple RTD's to one reference ?
  • 0 in reply to Klaas Jelle Bakker
    TI__Guru* 94285 points

    Klaas,

    I think the original way that I had drawn up the schematic was for four 3-wire RTD measurements like this:

    The 15Ω shown are basically lead resistance on the RTD. Note that there is no input filtering with this topology, and that if you were to insert the input filtering, the input resistor mismatch would add to the error. If you need input filtering, you would need a separate set of inputs just for the IDAC currents and I think that would drop you to being able to measure three 3-wire RTDs.


    Joseph Wu

  • 0 in reply to Joseph Wu
    TI__Mastermind 22315 points

    Hi Klaas,


    as Joseph mentioned at the end of his post, we usually recommend to place an RC filter on every analog input. Please see the application section of the ADS1248 datasheet as an example. The input filter serves two purposes:

    1. Antialiasing filter
    2. Over-voltage protection (limiting the currents into the analog inputs to save levels in case of an over-voltage event)

    Depending on the over-voltage protection level you need to select the series resistor value. For example to protect against over-voltages up to 30V you would choose a resistor value >3kOhm to limit the input current to <10mA.

    With such large input filter resistors it is not recommended to route the IDACs to the same analog input where you are measuring because the voltage drop across the series resistor would be too large. In that case you would use a dedicated analog input as the IDAC output and connect it to the RTD before the RC filter as shown in the ADS1248 application section. Unfortunately the ADS1248 will then not have enough analog inputs anymore to measure four 3-wire RTDs.
    However you could use an external Dual 4:1 MUX to route the IDACs from the IEXC1 and IEXC2 pins to the RTDs then.

    If you don't like additional external MUXs then you could try to measure up to four 3-wire RTDs using our new ADS124S08, which offers more inputs than ADS1248. Below is an example implementation. In that case you would not make use of the two IDACs for automatic lead wire compensation. Instead you would measure the lead resistance of each RTD in a 2nd measurement and then subtract the lead resistance from your RTD measurement. In this example you would measure the  lead resistance of RTD1 between AIN1 and AINCOM. And for the other RTDs accordingly between AIN3/5/7 and AINCOM.
    The disadvantage of this implementation is that you need to take two measurements for each RTD reading. But it is a very common approach.
    The benefit is that you do not need to worry about the matching of the two IDACs because only one is used.

    There are many ways how multiple RTD measurements can be implemented with one of our ADCs. This should just give you some ideas.
    We will be happy to review your circuit once you have drawn something up and provide further guidance.

    Regards,