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LMH6518: Offset problem with LMH6518

Olaf Kaluza
Prodigy 10 points
Part Number: LMH6518


Tool/software:

Hi there,

In  LMH6518: The potential root causes of the output offset drift issue in LG mode TI had to confess the LMH6518

has a huge offset problem with when using with DC Level above 400mV. They also said they will update the datasheet

about this. But this never happens.

Why?

Did you repair the mask of the LMH6518? Or is the bug still there?

I am asking because I think R&S has a problem with there RTB2004, because it develope a huge offset after measuring

a fullscreen DC signal for some time.

Diskussion about it also here: 

https://www.eevblog.com/forum/testgear/new-killer-scope-a-true-game-changer-from-rs-rtb2002-rtb2004/msg6047785/#msg6047785

Olaf

  • 0
    TI__Expert 3070 points

    Olaf,

    Let me check and get back to you on this by tomorrow.

    Regards

    Anant

  • 0
    TI__Mastermind 23859 points

    Hello Olaf,

    I am one of the team members who worked on investigating this device, as well as the datasheet revision.  It may be beneficial to create a pinned e2e post or find another way to explain the device behavior, as much as I can discuss with you all.

    However, I can say the +/- 400mV DC input spec did not make it into the Absolute Maximum Ratings table for a singular reason: our investigation resulted in the decision to include a different specification revision in the Absolute Maximum Ratings table.

    The new spec is: "Maximum dc output value(2) 1700 mVPP", with a small footnote (2) and the root cause of the offset being specified by the output voltage, not the input voltage.  I did communicate and discuss an input voltage limit with multiple customers & on e2e, such as a steady-state or DC version of the existing "Differential input signal voltage ±1 V" spec.  My work resulted in the working guidance of 400mV, which you mention and various engineers have now become aware.

    The reason the steady-state 400mV limit (as a working/pending value) did not make it into the datasheet is tied to our investigation results.  The mechanism which contributes to the offset behavior is not tied to the input stage, as the working/pending value would wrongly suggest and infer.  It was decided to specify with respect to the output voltage, to prevent confusion on situations where the 400mV 'limit' was observed BUT the offset increase STILL OCCURS.  I know this may be confusing (in an unintended sense) regarding conversations and customer accounts of our use of 400mV as a working/pending input limit for steady-state (DC) signals.

    Please understand the revised abs max table does include a relevant limiting specification, but the selected specification is more specific to the output behavior of the device than a strict input limit.  This decision captures more nuance and reduces the number of exceptions from customers who 'follow 400mV or lower steady-state' but still see increased offset voltage.

    I hope this was beneficial.  If the community and customers need more information, we are able to support and provide assistance directly (as customer circuits & implementations vary).

    Best,

    Alec