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LMT70: Output is oddly sensitive to probing, and also it drifts

Scott Brenneman
Genius 3265 points
Part Number: LMT70
Other Parts Discussed in Thread: TLA2022

When using a 'scope to probe the LMT70's output, the voltage drops somewhat.  We can determine this because we have an ADC (TLA2022) and firmware reporting the output.

The probe is 10 Meg, 13 pF, and yet the drop is about 100 mV out of, say, 800 mV.  We're also seeing the output drift slowly over time, on the order of 10 C or more in either direction, in a quiescent system without significant changes in the room temperature. 

Is there any reasonable explanation for this, or do we have damaged ICs?  Are they unusually sensitive to handling or soldering?

Thanks,
Scott

  • 0
    TI__Mastermind 35835 points

    Hi Scott,

    DSBGA devices are sensitive to handling and soldering. The packaging almost doesn't exist compared to other packages where the die is completely encapsulated in plastic. For this reason, they can be chipped. Any BGA can also have issues with flux residue trapped beneath the device. This flux provides a strong leakage path, which could explain the problem you're seeing. 

    The LMT70, and analog temp sensors in general, doesn't have really robust output drivers. It's certainly better than not having a buffered output, but the LMT70 is specified with only +/-5uA of source/sink. It's not unreasonable to overpower it inadvertently, and microamps of leakage thru uncleaned flux is a possibility. Despite the weak drive strength, the output impedance and max cap load (28ohm typ and 1100pf max) aren't explicitly broken by your configuration. 

    Do you have bypass capacitance next to LMT70?

    How are you grounding your scope probe?

    Is there a long distance between LMT70 and your ADC? Are you using cabling?

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Genius 3265 points

    It appears that we have a soldering problem -- checked more boards and quite a few of the devices could be swept off the board with the slightest touch of a fingernail!

    Scott