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Original question:

ADS1248: Data converters forum

ADS1248: Lightning protection on analog inputs

Chuck Wong
Genius 3940 points
Part Number: ADS1248
Other Parts Discussed in Thread: STRIKE, , ADS124S08

Dear Bryan,

With reference to our previous discussions on this topic, I appreciate if you could clarify some information from the device datasheet:

The specified device input current (except to the power supply pins) is ±10mA continuous and ±100mA momentary. Could you please define the timing of "momentary"?

Thank you for the continuous support.

Chuck!

  • 0
    TI__Mastermind 43021 points

    Hi Chuck Wong,

    Unfortunately we don't have a time over which that "momentary" current is specified. You will see that that specification has been removed from more recent devices as a result

    The 10mA continuous spec is the spec we actually design to, so I would use that specification as the guideline for your design

    Apologies if this created any confusion

    -Bryan

  • 0 in reply to Bryan Lizon86
    Genius 3940 points

    Oh I see, so I think that could explain why we always had the ADS1248 devices blown up, when an applied waveform contains a high-power element of around 100mA that last around a couple of µs, before converging to under 10mA for the rest of the strike. In fact, we applied waveforms with 750V on the analog inputs and have a 7.5kΩ inline, so the max current would be 100mA before the TVS kicks in.

    Do you think that could be the failure source in our design?

    So your recommendation is to design around the ±10mA max guidelines, which could be very challenging for 750V and 1500V strikes (DO-160G), am I correct?

    I cannot find it now, but I believe that I've read ±2.5mA (instead of ±10mA) in our previous exchanges. Is that remained true?

    Thank you.

  • 0 in reply to Chuck Wong
    TI__Mastermind 43021 points

    Hi Chuck Wong,

    I think it's possible that the ADC could survive 100mA for several microseconds. We actually performed IEC-61000 testing on a different ADC recently (the ADS124S08, which is the next gen version of the ADS1248), and that device survived EFT and surge testing up to 1kV or higher with just TVS and current limiting resistors. And the current limiting resistors were sized such that they limited the current after the TVS kicked in (approximately 30V). I.e. they were not 100k resistors or something like that, which would have caused serious offset due to leakage currents, as well as noise.

    I just don't have any data to share with respect to how much current the device can handle with respect to time.

    The +/-2.5mA was just an example of not necessarily using all of the available 10mA, since this is an absolute maximum rating. Sustained current draw at these levels could cause issues with the device eventually, and operating at the abs max ratings does not guarantee the performance specified in the Recommended Operating Conditions

    -Bryan

  • 0 in reply to Bryan Lizon86
    Genius 3940 points
    • The ADS124S08 is interesting. Is it form, fit and fonction compatible thus could be a drop-in replacement of the existing ADS1248?

    Any data to share so far?

  • 0 in reply to Chuck Wong
    TI__Mastermind 43021 points

    Hi Chuck Wong,

    The ADS124S08 is not pin or software compatible with the ADS1248, so it will not be a trivial replacement. But the ADS124S08 is smaller, lower power, lower noise, includes diagnostic features, samples faster, has more channels, and offers better performance (DC, drift, VREF, IDACs, etc.), all at approximately the same cost as the ADS1248

    If you can consider a new ADC for your application, the ADS124S08 is a great choice

    -Bryan

  • 0 in reply to Bryan Lizon86
    Genius 3940 points

    Hi Bryan,

    It seems that the existing ADS1248 that we're using in our design was first introduced since 2008. Given the fact that the next generation of this same device is already available (ADS124S08), could TI give us an idea on the possible life cycle or roadmap of the previous generation A/D?

    Please note that our design with the ADS1248 is already completed and the hardware qualification is nearly completed, with the exception of lightning pin injection test on some of the analog inputs that we're having some difficulties, so a swap to the new A/D would be significant in term of efforts.

    Thank you again!

  • +1 in reply to Chuck Wong
    TI__Mastermind 43021 points

    Hi Chuck Wong,

    I can tell you right now that we don't have any current plans to stop manufacturing the ADS1248, and we will generally continue selling a product as long as there is demand. However, I cannot guarantee you that the device will be available 10 years from now for example.

    -Bryan

  • 0 in reply to Bryan Lizon86
    Genius 3940 points

    Thank you so much.