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Weird ADS Simulation behaviour of LMH6881 in combination with another FDA

Ki Ka
Prodigy 10 points
Other Parts Discussed in Thread: LMH5401, LMH6881, LMH6554, TINA-TI

i want to use the LMH6554/LMH5401 in combination with the lmh6881. independently they work as they should but the lmh6881's output is blowing up as seen in the attached video when used in combination with LMH6554.
i can fix this either by having an open load or playing with the 2xOCM tuning and it somehow fixes itself but is unstable as seen in the attached video and pictures .

  • 0
    TI__Expert 3620 points

    Hi Kivanc,

    What is your intended output common mode? It looks like you have 5V going to the OCM pin. Keep in mind this pin has a 2x gain internally, so for an output common mode of 2.5V you would need to supply it with 1.25V.

    Best,

    Sam

  • 0 in reply to Sam Burnett
    Prodigy 10 points

    Hello Sam, that unfortunately is the problem when setting it to 1.25V you get the result of the Video(OCM = 100+V.. When tuning the value up and down(Window is not shown in the video ) to e.g. 5V i get the correct x2 gain and set to 10 when tuning up and down from there I get the correct answer when tuned down to 1.25V. But running the simulation again gives me the huge offset

  • 0 in reply to Ki Ka
    TI__Expert 3620 points

    Hi Kivanc,

    Did you have any netlist syntax issues when importing the model to ADS? It seems to be behaving in SPICE. If you would, try bypassing the OCM pin with a capacitor to ground. This not acceptable for the actual device but it looks like the model should default to mid-supply, which could be a workaround for you in this case.

    Best,

    Sam

  • 0 in reply to Sam Burnett
    Prodigy 10 points

    Hi Sam,

    thanks for the quick response:

    Yes in Tina-Ti everything is working fine for me aswell the LMH6881 independently is working fine in ADS too.

    Just in Combination with another Component it has this behaviour (including your Workaround):

    And if i deactivate the LMH6554 component for example (it can be any other FDA this behaviour is always the same):

    I checked the Top Design netlist but couldn't find any potential cross references which could anyhow interfere with eachother.. :/

  • +1
    TI__Expert 3620 points

    Hi Kivanc,

    Unfortunately I do not have access to ADS so I cannot try to replicate your issue...  I am curious what happens if you ground-reference the OUTP and OUTN pins separately through 100 Ohm resistors like I have done above. Seems like it might be having trouble establishing a DC operating point. I am sorry that I cannot be more helpful. It is strange that they work separately.

    Best,
    Sam

  • +1 in reply to Sam Burnett
    Prodigy 10 points

    The ground-referencing didn't make it work either.(edit: just forgot to activate the other component earlier)

    I guess i have to find some setting in DC operating point calculation settings.

    Thank you for your help,

    Kivanc

  • +1 in reply to Ki Ka
    Guru 140200 points

    Hi Kivanc,

    in TINA-TI I do have the same problem from time to time: Each individual circuit runs correctly, but when combining them in one big simulation the simulation fails and errors are displayed and/or the DC bias operating point cannot be found.

    I think this is typical to all or a least very most simulators and is the result of heavily increasing complexity when increasing the number of subcircuits. Keep in mind that the complexity is not linearily increasing with the number of components and subcircuits but exponentially.

    Well, there's a simple remedy: Divide the big circuit into subcircuits and simulate them individually. Assume the input signals of the subcircuits to be well known and model them as simple signals coming from a signal generator instead from the output of another subcircuit being simulated at the same time.

    Divide and conquer Relaxed

    Kai