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SN74AHCT244-EP: Chi-square number inquiry

Art Mecina
Genius 16585 points
Part Number: SN74AHCT244-EP

Hello Guys,

Good day.

I am not sure if this is the appropriate channel for this but our customer is referring to the document below regarding SN74AHCT244-EP FIT ( Failure-In-TIme). He was not able to find the chi-square numbers for CL=60% with different number of failures. Could you help provide the numbers for failure = 0 and 1, 2 (or df = 2, 4 and 6) at CL=60%?

www.ti.com/.../spraby3.pdf

Thanks in advance!

Art

  • 0
    TI__Guru* 96681 points

    Hi Art,

    I'll check with our quality team and see if this is something we can provide -- I will get back to you with some information within one or two business days.

  • 0 in reply to Emrys Maier
    TI__Genius 16585 points

    Hi Emrys,

    Any update?

    Thanks!

    Art

  • 0 in reply to Art Mecina
    TI__Guru* 96681 points

    Hey Art,

    No, I don't have any useful update yet.

  • 0
    TI__Guru* 96681 points

    Hey Art,

    I think I have what you're looking for.  The chi-square numbers just come from a statistics function -- in Excel there's a function "chisquare.inv" that provides this directly. Inputs are confidence level and # of fails + 2.  I built a table of values for quick reference:

    # Fails 0 1 2 3 4
    %CL
    60% 1.832581 4.044626 6.210757 8.350525 10.47324
    70% 2.407946 4.878433 7.231135 9.524458 11.78072
    90% 4.60517 7.77944 10.64464 13.36157 15.98718
    95% 5.991465 9.487729 12.59159 15.50731 18.30704
    99% 9.21034 13.2767 16.81189 20.09024 23.20925

    This is then combined with the acceleration factor (AF), number of samples tested, and time tested to directly give the FIT rate.

    The equation is:  FIT = ( chi-square * 10^9 ) / ( 2 * hours * samples *  AF)

    Using the provided values on our quality page for the device:

    I got these values (to two decimal places):

    Confidence Level FIT rate
    60% 1.08
    70% 1.42
    90% 2.72
    95% 3.54
    99% 5.44

  • 0 in reply to Emrys Maier
    TI__Guru* 96681 points

    Hey Art,

    My apologies - I got carried away with the confidence level and didn't give you what you asked for.  I'm not sure why this data is required, since our tests didn't have any failures. Usually you would use the actual test results and then select the desired confidence level.

    With 60% confidence level and different # of failures:

    # Fails FIT rate
    0 1.08
    1 2.39
    2 3.67