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LMP90100 Family -- Sensor Diagnostic Flags

Paul Augenstein
Prodigy 100 points
Other Parts Discussed in Thread: LMP90100, LMP90098

Configuring a LMP9098 to scan a single channel using ScanMode3 does not appear to apply the burnout in the background..

How should the A/D configured to scan a single channel with a background burnout. 

Does it require setting up a second channel?

  • 0
    TI__Expert 3350 points
    Please make sure that the BURNOUT_EN =1 ( see section 9.3.8.2.1 Burnout Current Injection on page 29 of LMP90100/LMP9009x datasheet).
  • 0 in reply to Bahram Mirshab
    Prodigy 100 points

    I have a possible application for the LMP90098. One of its functions would be to determine if a thermocouple has degraded, resulting in a high resistance (1 to 2k Ohm).  Using the LMP90100 EVB it appears the sensor diagnostics, with burnout enabled, only reveals very high impedances.  Is this expected?

     

    Also I'm wondering if the following approach has merit:

    Use Scan Mode 3 scanning two channels both using the same inputs. One channel is configured with burnout enabled. Its output is used for the actual temperature measurement. The second channel is configured without burnout, however, the 10uA burnout current appears to be present. The difference between the two channel outputs could be used to determine the thermocouple resistance.  It appears this scheme works but does it have flaws?

  • 0 in reply to Paul Augenstein
    TI__Expert 3350 points

    Correct, very high impedance is expected to detect degraded sensor, “floating inputs", otherwise the internal comparator will not flag near rail condition.

    Regarding your second question: since you are using two thermocouples in parallel configuration, in mode 3 the current injection in the channel that is not being sampled would affect the accuracy of the measurement in the sampling channel. The device should be operated in two states; sampling state and diagnostic state.  In the sampling state the burnout current should be turned off. In the diagnostic state, the burnout current should be turned on just checking for the diagnostic flag and discarding the data.

  • 0 in reply to Bahram Mirshab
    Prodigy 100 points

    Thanks for the response.

    Actually the scheme I proposed involves only one thermocouple with two channels sampling (using Scan Mode3). I’m assuming the channel, with burn-out enabled, would not be affected by the injected 10uA.  The second channel (burn-out not enabled) would.  Hence the difference between the A/D outputs of both channels would be due to the thermocouple resistance.

    Related questions for either scheme:

    Is the over-all measurement time the sum of the periods (based on the ODRs) configured for each channel? If so, what are the effects of having different ODRs for the two channels?

    Will background calibration work?

    Will 60/50 Hz reduction work and if using different ODRS what is the maximum ODR for each channel.

    Finally is there a timing diagram available showing the sequence and duration of input sampling, burnout application and sensor flags determination as a function of ODR and scan mode?

  • 0 in reply to Paul Augenstein
    TI__Expert 3350 points
    Since you are using only one thermocouple, in mode 3 of operation the burnout current injection would automatically connected to the non-sampling channel and it will not affect the accuracy of the measurement.
    Regarding your questions:
    1. In general the selected over-all output data rate will be divided by the numbers of selected channels.
    2. Cont. cal would function if enabled
    3. 50-Hz to 60-Hz line rejection at ODR ≤13.42 SPS
    4. I am afraid, currently not available