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ADS1148: Wire Break detection while using Ratiometric measurment

SimonF
Expert 3410 points
Part Number: ADS1148


Hi Team,

If we choose to do ratiometric measurements what will be the effect of a wire break then?

Both the RTD signal and the reference voltage will/should converge to some value close to zero.

Will the conversion result in just any value in the overall range or will the ADC return some defined value if e.g. its reference voltage gets lower than some threshold?

Best regards

Simon

  • 0
    TI__Guru 56222 points

    Hi Simon,

    When using ADS1248 in a 4-wire RTD application you have - besides others - the following options to detect an open wire:

    • When one of the two excitation wires is broken you will in many cases read a +FS or -FS value.
      In addition you can detect this condition by measuring back the voltage across the external bias resistor (using the reference voltage monitor), which should be close to 0V in that case.
      Some customers have also leveraged the GPIO input on REFP0 to detect if the voltage is above or below the logic level threshold. In that case you would configure the REFP0 pin as a GPIO input during your diagnostic cycle.
    • In order to properly detect a broken sense line you would need to pull the ADC inputs into a state which is outside the normal measurement range. You could do that by enabling the integrated burnout current sources (BOCS) during a diagnostic cycle. Those current sources would then pull one or both ADC inputs to the positive or negative supply rails. You should then read a FS value. You need to make sure the burnout current sources can charge the external capacitors of the RC filters quickly enough during your diagnostic cycle.
      Another option would be to route the excitation current source (IDAC) to different analog inputs during the diagnostic cycle to force certain conditions in case of a broken wire.

    I can expand on that for a 3-wire RTD application as well. But the detection method depends a little on how you implement the measurement (do you use two IDACs or only one, do you use a bias resistor on the high or low side).

    Last year we released the ADS124S08, which is the next generation of the ADS1248. We integrated a few diagnostic features in this device to help with open sensor wire detection.
    We can for example detect if the external reference voltage dropped below two different thresholds without having to measure back the reference voltage. The first threshold could detect if the reference voltage halved, the second threshold could detect if the reference voltage is completely missing.
    We also integrated monitors that check if the PGA outputs are railed to the supplies. This can be very helpful as well to detect exactly which wire is broken.

    You could experiment how all this works using the ADS124S08EVM. This evaluation board was designed for RTD (and other sensor) measurements.