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AD1248 Burnout current functionality??

Sandeep NS
Intellectual 400 points

Hi,

Am in a design stage of Ad1248 based project. In this project i need to detect sensor quality status (Open or short)

For this i am using sensor burn out functionality. I have following questions regarding this

1, Burnout currnt and IDAC currents are different sources?

2. Whether I have to turn off the other currents IDAC before enabling burnout current?? If its happened like that any damage will happen to ADC??

3,I have a requirement to check all 4 channels sensor status(Open or short). So how much time will take to complete all for channel status scanning???

Thanks and Regards

Sandeep 

  • 0
    TI__Guru* 98025 points
    Sandeep,


    Let me start with a quick explanation and then I'll get to the answers for your questions. The purpose of the burnout current sources are to pull apart the inputs if the sensor has burned out and created an open circuit. In this case, the ADC positive input pulled up by having current sourced into it, while the ADC negative input is pulled down by having current sunk from it.

    1. The burnout current and the IDAC current come from different sources.

    2. This depends on how you have your circuit set up. Often you would keep your normal configuration and add the burnout current sources to see if the sensor is open. However, if you are using the IDAC at the same time (as in an RTD ratiometric measurement, then you may not need the burnout current sources at all. If the RTD is burned out, the IDAC would pull the positive input up because it's an open circuit. The connection to the opposite RTD end goes to the reference resistor would be connected to ground, pulling the negative input down. In this case, the burnout measurement is built in.

    3. No, you do not have to turn off the IDAC currents to use the burnout current. It will not damage the part.

    4. Normally, you would set up the device to turn on the burnout current sources, and then select the channel to take a read. Then do this for the next channel, until you read all four channels. The time it takes would depend on the data rate that you are using, but if you used the fastest data rate, you could be done in just over 2 ms.


    Joseph Wu
  • 0 in reply to Joseph Wu
    Intellectual 400 points
    Dear Joseph Wu,

    Regarding the second answer,

    I think it is explained in the basis of 2 wire mode RTD connection. How we can detect it in 3 wire mode RTD connection with ratiometric measurement connection?? Then there will be two currents IDAC1 in AIN+ and IDAC 2 in AIN-. So the AIN- channel have one pull up and one pull down (Reference resistor) connection . So it will be act as a voltage divider. Please let me know my understanding is correct or not??

    Sandeep
  • 0 in reply to Sandeep NS
    TI__Guru* 98025 points

    Sandeep,


    This shouldn't be a problem. In a 3-wire RTD system, the reference voltage would drop to one half of the expected value because only one of the two excitation currents would reach the reference resistor. This would at least lower the AINN input to half the expected value. The other input would still be pulled up to AVDD. The combination of the two will still over-range the input.

    I'll use the setup in shown in one of the TIPD write-ups as an example:
    www.ti.com/.../slau520a.pdf

    Here the RTD is a PT100, this uses 1 mA as the two excitation currents, with a reference resistor of 820 Ohms. If the RTD is in place, the max voltage is about 400mV (at the hottest temperature), while the reference voltage is 1.64V (2*1mA*820). The PGA is 4, but this still works for PGA of 1 where it is the hardest case for the burnout test. A higher gain requires the inputs need to go apart less to see the burnout.

    If the RTD is burned out, one current source pulls the higher node of the RTD to near AVDD, while the bottom node is set by the other current source going to the reference resistor. Even if AVDD is the lowest it could possibly be at 2.7V, the reference voltage would be at 0.820V. That means the RTD would have 1.88V across it, while the reference is much smaller at 0.820V. Even if the compliance of the IDAC current source goes to only 0.7V to AVDD, the ADC would still be over-ranged and you'd still see a full scale reading.


    Joseph Wu