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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 IC126.96.36.1990.13 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
Thanks in advance for any helpful answer.
PS. It is urgent for me since tape out date is close!!
Hi jimitoIf you simulate just a small cell (e.g. a current mirror block), do you have the same problem?Best regardsQuek
In reply to Quek:
It seems that the circuit "burns" the desired value of total current
(the same with the value i get when i simulate the schematic with the
symbols not as av_extracted views but as schematic views) but when i add
all currents "by hand" from all branches i am far away from this
annotated value on the VDD symbol of schematics L.After some observation
on the annotated values i noticed that the indicated current on the
schematics L after the dc analysis is the current that flows into the
one finger of a transistor even if it has multiplicity factor (bus more
exactly) or not,that's why i wrote in my previous post that all values
are divided by the product (m*nf).
On the other hand,i simulated a test case with the main opamp only and
biases from ideal sources and an ideal cmfb circuit that i have created
and the problem still remains when av_extracted view is the simulated...
Another colleague that works in my lab a few months ago had created a test case with a simple resistively loaded common source topology and we tested my problem on it and it was simulated correctly after the RC extraction.We just divided the transistor into 2 parallel fets with bus and some fingers and connected it with the resistor with metal1 just to make the test,not anything important from layout aspect!
Do you suspect something that causes this strange problem or you think that IBM will give the answer?
Thanks a lot in advance.
In reply to jimito13:
Hi JimitoSorry for the late reply. Actually I think this looks like a setup issue that might has a reasonable explanation if there is a testcase. It would be good for your Prof to help contact Cadence or IBM support for this issue.Best regardsQuek
Well we contacted IBM and they admitted that there is an error while the total current is calculated when the av_extracted view is post simulated.They didn't give us any solution instead of proposing us to change all fets from my design to nfet_rf and pfet_rf (now i use the regular fets-nfet,pfet and the respective lvt versions)...Only in this case they said that the problem is solved...Anyway,this is the least annoying since if i multiply the multiplicity factor with the number of fingers and the simulation current i get the total correct current that i expected.So,i suppose that i am ok with it.But...
There is another more difficult issue that hasn't been solved up to now even from IBM's support (they proposed me the same solution i wrote above for this issue as well !!).After simulating my opamp i notice severe voltage mismatches at various symmetrical branches (20mV the one and 700mV for example the other !!!).We checked the layout with my professor and he admitted as well that everything is ok and that those mismatches cannot be explained...We made all the checks (parasitic resistances,widh of interconnection cables,symmetries,distances etc...) that must be done and we didn't find any problem to my layout.My professor insists that i must change all fets to RF fets for a cell and post simulate it,if problem still exists then he says that we must report back to IBM.This is a little bit difficult to be done.Have you ever seen something like that happening during post simulation with a specific design kit and in this case what did you do to solve it??
Thanks in advance,
Hi JimitoJust to be sure that those voltage mismatches are not due to a bad extracted view, you can try the following:a. In the terminal window, enter the following:terminal>setenv DBGRCX_CONNECTIVITY Yterminal>icfb &b. Run Assura LVS and QRC extraction to generate an rc extracted_viewAt the end of the extraction, there should be connectivity checking using gemini checker. This is to confirm that the netlist is still correct after insertion of rc parasitics.There are indeed some old cases in which we see bad simulation results due to broken nets in the extracted view. I think sending a testcase to IBM would be a good idea.Best regardsQuek
Thanks for your replies.I have a couple of question before i try your suggestions.
1.Where the environment variable DBGRCX_CONNECTIVITY will be stored after the execution of the command setenv DBGRCX_CONNECTIVITY Y via linux terminal?
2.Will this command work with virtuoso instead of icfb,thus with command virtuoso &? (What is "&" after virtuoso?)
3.What is gemini checker?Is a tool that comes with assura?
We have sent a testcase to IBM's support and they are working on it,we still wait for their final response...They suggested me to try the LVS_extracted view and run "R only" and "C only" extraction runs.I did it and the problem happens only to R extraction...
Sorry to ask so much questions but i want to go in-depth with the things i deal with :-)
Thank you in advance.
Hi Jimitoa. This is a shell variable. It will be active in the terminal window in which the command has been executed.b. Actually it is not a matter of whether the variable works with IC61 or IC51 because Virtuoso is not the tool that is using the variable. QRC is the tool. So it does not matter whether you use "virtuoso" or "icfb". The "&" is to send the process to the background. This means that Virtuoso can continue to run even if you accidentally close the terminal window. If you do not use "&", Virtuoso will crash if you close the terminal windowc. Gemini is a connectivity checker that comes with EXT package. It used to come from Assura package. It is common to have problem for only r-only or rc-extraction and not for c-only extraction because the network is altered more when there is resistance fracturing. Using gemini saves you the simulation step. My guess is that if you try gemini with r-only extraction, you will get mismatch reported. There is no way to resolve gemini errors except to upgrade the version of QRC. I think it would be good if you can upgrade to the latest EXT913HF2.Best regardsQuek
Thanks again for your clear answers :-) . Well i did what you proposed me with gemini checker combined with R-only extraction and the result from QRC report is the following :
#==========================================================## Verify netlist connectivity#==========================================================#cat <<ENDCAT> gemfilterfileF model=C 2 X XENDCATgemini -V -S -sd -w -td 0.01 -flt gemfilterfile -ac"C" -prr_c '#' -f_name_io \ s#vdbToRcx.sp s#/users/jimphd/Dimitris/ota_main/LVS/LVS/extview.tmpINFO (GEMINI-113014): gemini: Declaring node 0 as global - default.INFO (GEMINI-113014): gemini: Declaring node 0 as global - default.Removed parasitics in ota_main (resistors: 7855 -> 0, L-inductors: 0 -> 0, VCCS: 0 -> 0)THE DESCRIPTIONS MATCHED COMPLETELY, ota_main & ota_mainGEMINI COMPARISON CONCLUDES: The circuits match completely.INFO (GEMINI-113292): Gemini: Reading files tookINFO (GEMINI-113184): 0.08 user, 0.00 sys, 0.00 elapsed, 3526.7 KbytesINFO (GEMINI-113293): Gemini: Comparison tookINFO (GEMINI-113184): 0.01 user, 0.00 sys, 0.00 elapsed, 3813.4 Kbytes
What can i conclude from the above statements?
And a final question..if i put the setenv command i run before in my .cshrc file,i will be able to load gemini everytime i access assura,right?If not how can i have permanent access to gemini for every active assura session?
Thanks a lot for your precious help.
Hi JimitoThis seems quite strange. You mentioned that simulation using r-only extracted view gives wrong results but yet gemini is reporting that the extracted view is ok. Could it be that your circuit really has some problems after the addition of the resistors? Perhaps more checking is needed.You can put the cmd in your .cshrc file so that gemini is enabled automatically every time you run QRC. Best regardsQuek
To understand better..gemini checks only connectivity problems or something else additionaly in my layout during extraction procedure?e.g. does it check for parasitic resistor mismatch to symmetrical branches of a circuit,let's say drain net of nmos transistors in a differential input pair of an amplifier?
You said that : Could it be that your circuit really has some problems after the addition of the resistors?
Do you mean layout problems or testbench setup problems?
Thanks a lot in advance :-)
Hi JimitoHere is what happens during QRC parasitic extraction:a. QRC reads in gds or DFII layout data and creates a netlist (let's call it "netlist_pre")b. RC parasitics are calculated for each netc. QRC writes out the final netlist that has RC parasitics (let's call it "netlist_post")Gemini opens all parasitic caps and shorts parasitic all resistors/inductors in netlist_post and compares it with netlist_pre. The 2 netlist must be equal. Otherwise it means that QRC has accidentally modified netlist_pre wrongly when inserting the parasitic RCs. Gemini does not check for parasitic resistor mismatches in symmetrical nets. This is not the purpose of gemini checking.I am not refering to layout or testbench problems. I am refering to design problems. I am wondering if the circuit is robust enough to within the addition of some parasitics. E.g. suppose I design an amplifier and get a very nice flat low-pass ac response. Layout and extracted view are then created and I proceed to do post layout sim on the extracted view. If my ac response now shows some peaking (e.g. 1dB), naturally I cannot conclude that QRC has given me a bad extracted view. It is simply because my amplifier is not robust enough to with the parasitics. For example, I might need to trade off some bandwidth for stability by increasing the compensation cap a little.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. I think your next step should be to simply try the latest version of QRC in EXT913HF2. If the simulation problems are resolved using this version, then it simply means that it is a bug and that it has been fixed in the latest version.Best regardsQuek
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 :-)
QuekYou 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.
Hi JimitoIt looks like you have done quite an amount of testing yourself. : ) Just a small request, if you ever get an answer, please post it so that everybody can benefit from your answer. : ) It is very important that we enrich each other with our experience.Best regardsQuek