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IBM cms9flp & Assura QRC strange problem

jimito13

Hello all,

I will try to describe my problem the better i can.I have an op-amp design that consists of several cells (main opamp,gain boosters,cmfb,biasing).I have created the layout of all the above cells and i pass successfully from DRC and LVS runs (clean runs,no errors no warnings).The last step is to perform the parasitics' extraction and run post layout simulations.Well,here things become a little bit strange...Before i describe my problem in depth i must say that my design kit is IBM's cms9flp and the versions of cadence virtuoso and assura are IC6.1.3.500.13 and 3.2_USR2_HF11 respectively.

I run LVS for each cell successfully (clean results) and then i run successfully as well the assura QRC run (no errors-no warnings) for all my cells and the result is the well known message of assura with the av_extracted view's name and location in the library.

Next step is to simulate the av_extracted views.For this i create a schematic with all the above cells interconnected as symbols and i use the av_extracted declaration under Environment submenu of ADE L.

I run DC analysis at first and here is my issue.The currents in all branches of the circuit are far away from the designed values.From a quick thought you can say that i made mistake in transistor sizes during layout but this should be reported by LVS but in any case i double checked this by myself to be absolutely certain.Second thought is that i have not calculated correct the widths of all metals that make the interconnections in the design,i double checked this as well according to pdk's rules for metals and viases.With a more careful view in the values of the currents that dc analysis resulted i noticed that all of them are divided by the product (m*nf),where m is multiplicity and nf number of fingers!Looking at the vdd symbol i see that my circuit consumes X mA but if i add all the currents from the subcells i do not ever reach X/4 mA,so i ask myself where is the rest of the current consumed?!Another strange thing is that even though that all currents are wrong calculated the biasing voltages from the current mirrors are exact to the design values...

Has anybody else come up with such an issue with this or another IBM's pdk??Do you think that i should report this straightly in IBM's support??


Thanks in advance for any helpful answer.


PS. It is urgent for me since tape out date is close!!

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  • Good Morning Quek,

    Before i say anything i must tell you a big thanks for your clear and helpful replies and for the time as well you spent to read my problem and contribute to a solution :-)

    Quek said:
    You can also try adding some small resistors from analogLib into your schematic and simulate your circuit again using the modified schematic. If there are no problems, then most likely the problem does not lie with your circuit.

    My testbench consists of a folded cascode OTA and it's ideal CMFB (ideal for simplicity).What i have already done is to inject a small resistance (i tried with 1Ω,3Ω,5Ω,10Ω) in to one of the output nodes of the amplifier.With this movement i manage to insert a manual parasitic resistance mismatch in my circuit.I run dc analysis and the output nodes are close to each other and almost close to my desired operating point,this is also valid for the rest of the nodes of my amplifier.Also,i have run a monte carlo mismatch analysis and i have concluded that the output dc voltages are insensitive to mismatches.

    At the moment it is a little bit difficult to upgrade to a newer version of EXT,but i will have a talk with my prof and i'll see what i can do.In the middle time,i am still waiting for IBM's verdict.

    Best Regards,

    Jimito13

Reply

  • Good Morning Quek,

    Before i say anything i must tell you a big thanks for your clear and helpful replies and for the time as well you spent to read my problem and contribute to a solution :-)

    Quek said:
    You can also try adding some small resistors from analogLib into your schematic and simulate your circuit again using the modified schematic. If there are no problems, then most likely the problem does not lie with your circuit.

    My testbench consists of a folded cascode OTA and it's ideal CMFB (ideal for simplicity).What i have already done is to inject a small resistance (i tried with 1Ω,3Ω,5Ω,10Ω) in to one of the output nodes of the amplifier.With this movement i manage to insert a manual parasitic resistance mismatch in my circuit.I run dc analysis and the output nodes are close to each other and almost close to my desired operating point,this is also valid for the rest of the nodes of my amplifier.Also,i have run a monte carlo mismatch analysis and i have concluded that the output dc voltages are insensitive to mismatches.

    At the moment it is a little bit difficult to upgrade to a newer version of EXT,but i will have a talk with my prof and i'll see what i can do.In the middle time,i am still waiting for IBM's verdict.

    Best Regards,

    Jimito13

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