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RTD sensor detection using burnout current source

Cecil
Intellectual 345 points

Other Parts Discussed in Thread: ADS1248

Hi,

I am making a 3-wire RTD measurement with the ADS1248 ADC. I have my set up wired exactly the same as in the TI Application report (SBAA180 January 2011). Please refer to Figure 2, page 3: Three wire RTD Application Example.

I want to be able to detect when there is no sensor attached to the device and I do this by enabling the burnout current sources in the MUX0 register. When I have no sensor attached Lines 1, 2 and 3 are all missing so I don't have any reference voltage but according to the ADS1248 datasheet, "a full-scale reading may indicate an open circuit in the front-end sensor, or that the sensor is overloaded. It may also indicate that the reference voltage is absent". So I am expecting the full-scale reading when I have nothing connected but this is not what I read from the ADC.

For sensor detection I just set the burnout current source value and the +ve and -ve inputs in the MUX0 register. My ADC reference input is set to external (VREF1P/VREF1N) as indicated in Figure 2. Then I take the reading just as I would when the sensor is present. Is there anything else that I must do to read in full-scale as described in the datasheet. I don't use a bias voltage for these measurements.

Also can I leave the burnout current source on for all measurements I take even when the RTDs are present? I think if I do that it will bump the values I read from the ADCs up by (current value * RTD resistance) but if I have 1mA from the internal current sources vs 0.5uA from the burnout current source this error may be insignificant of course depending on the RTD resistance value.

Thanks

Cecil

  • 0
    TI__Guru** 113765 points

    Cecil,

    Basically you are duplicating efforts by using both the IDAC current source and the burnout source.  As you have surmised, using both will introduce and error.  You really don't need to use the burnout source.  Use the IDAC, but instead of using the reference voltage generated from the bias resistor, use the internal reference voltage for the sensor detect.  Using the internal reference should give you full scale readings if the sensor is open.

    Best regards,

    Bob B

     

  • 0 in reply to Bob Benjamin
    Intellectual 345 points

    Hi Bob,

    Thanks for the quick response. Are you saying that the burnout current sources are the same as the IDAC sources and so I have to use one or the other at the same time? The main thing I want to do is to detect whether I have a sensor connected (or whether a reference voltage is present) by reading a full-scale voltage when open circuit and near-zero when shorted.

    Can you refer to page 29 of the ADS1248 datasheet on Sensor Detection and tell me how those measurements are to be made? There's not much explanation for that section.

    Thanks

  • 0 in reply to Cecil
    TI__Guru** 113765 points

    Cecil,

    You can have both the IDAC and the burnout sources on at the same time, but depending on the lead resistances involved you will most likely introduce an error.  What I'm saying is what is the point of having them on at the same time?  They are both current sources and the ADC will respond similarly.  The burnout source is usually a very small current and is meant to be used for those sensors that are not excited by current, such as a thermocouple.  Though you could use it with the RTD, you can used the IDAC for a similar purpose.  If you get a full scale reading it could mean an open by connection to the sensor or to the reference.  Either way, you will need to use the internal reference, or a different external reference to measure accurately to know the difference.  The reason being, if the sensor is open no current will flow through the reference resistor to establish the reference voltage so then what are you really measuring? 

    One advantage of the burnout source is for measuring for a short circuit detection, and that is the bigger advantage.  Figure 51 shows the location of the burnout sources relative to the mux inputs.  When the current source (burnout or IDAC) is open so that no current is flowing, the input from which the current is being sourced will be pulled toward the supply.  As the supply voltage is greater than the internal reference of 2.048V, the reading will be full scale. 

    Another method is for you to use the System Monitor and measure the voltage at the reference input.  Again you will need to use the internal reference voltage to make the measurement.  If the reference is 0V, you have an open for the sensor where no current is flowing through it.  If the reference is half of what is expected, then one leg is open.

    As you can see there are many ways to do this.  If you do use the burnout sources, follow the circuit diagram of Figure 51.  If the sensor is open, AINP will be pulled high, and AINN will be pulled low and this will produce the full scale reading.  If the sensor is shorted, then current will flow so that AINP and AINN are at the same voltage and the reading will be zero, assuming that the sensor is not at a resistance reading near zero.

    If you choose the burnout source method, I suggest keeping the measurements separate.  At power up you could do the sensor detect measurement, and either periodically thereafter, or if you see radical behaviour in your temperature measurement.

    Best regards,

    Bob B

     

     

  • 0 in reply to Bob Benjamin
    Intellectual 345 points

    Hi Bob,

    Bob Benjamin said:
    If you get a full scale reading it could mean an open by connection to the sensor or to the reference.

    This statement is rather contradictory and it is the same thing mentioned in the datasheet. You will only get a full-scale reading when you have no reference but you have an input voltage - a situation which is possible depending on which leg of the rtd is disconnected. If nothing is connected however, both input and reference voltages are missing and the ADC output swings (noise). From reading the datasheet, I was expecting a full-scale reading when I didn't have a reference because I had turned on the burnout current source. Now I understand this is not possible because I don't have any input voltage either. It will help if the sensor detection part of the datasheet is explained further.

    What I really wanted to avoid was having to interrupt my periodic measurements for sensor detection (eg. if any leg of a 4-wire RTD is disconnected it affects the measurement). It looks like I have to include periodic sensor detection in my code as well.

    Thanks for the explanation.

    Cecil

  • 0 in reply to Cecil
    TI__Guru** 113765 points

    Cecil,

    I think that I may have confused you even more by my explanation.  Let me go back to the original question about the burnout source.  On the AINP side is a source current and on the AINN side is a sink current.  They are of the same value.  If there is an open you will see a full scale reading because the AINP side will approach the AVDD and the AINN side will approach AVSS.  It becomes full scale because the difference exceeds the reference.  It this point it doesn't matter what the reference source is (either internal or external) as the reference will be exceeded.  So, if there is an open there will always be full scale.

    But here is where the second part comes in to play.  What happens if the reference is zero volts?  In your case it will be zero volts because of the Rbias resistor that establishes the reference will pull REFP to ground.  If the sensor is connected between AINP and AINN some small voltage will develop, but because the reference connection is open you will still see full scale.  Though it may seem contradictory it really isn't as you will need to test which case is creating full scale.

    What I advise customers to do is check the sensor on power up, and if they see a full scale reading check the sensor, send an alarm, or whatever.  You really don't need to do a periodic check unless you see radical changes between readings.  In your process loop just compare previous reading to current reading (or series of readings) and look for the radical behaviour.  If you really want to leave the BCS on, then you will need to characterize your system over temperature to see if the error is acceptable and within your design limits.

    Best regards,

    Bob B