Error/Warning in DC convergence in Cadence Spectre

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theguardian2001

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Hi everyone! I can say that I am relatively new in Cadence and I am currently migrating there from Spice. I was trying to simulate a dif. amplifier in gpdk90nm and have run into some problems. First of all, when I tried to simulate the schematic it has run into convergence errors and inability to compute an operating point (please see spectre_1.out in attachment). I am also attaching a log file (Job5) in case it is needed.

After several attempts I created a cellview from my schematic and provided power and input connections "on top" of it (by placing the created cell into a new cellview). The same simulation completed without any errors but the warnings appeared are crucial for the design:
Warning from spectre.
WARNING (CMI-2477): I0.PM0: `Rds' = 433.574 nOhm is less than 0.001. Set to 0.
WARNING (CMI-2477): I0.PM1: `Rds' = 15.9184 uOhm is less than 0.001. Set to 0.
WARNING (CMI-2477): I0.PM8: `Rds' = 867.631 nOhm is less than 0.001. Set to 0.
This basically implies that PMOSes does not contribute to output impedance thus ruining the overall gain. (Please see spectre_2.out). I would really appreciate any help and hints which can help me (and probably others in the future) to resolve the issue. The schematic is attached as a picture as well (Constant voltage sources were placed instead of a full bias circuitry - which has been tested without errors but with the same warnings - to reduce the complexity of the netlist file).

In case if anybody knows what causes Spectre to run the simulations with a symbol but not the schematic and why do only 3 PMOSes suffer from from low output impedance - I would also highly appreciate any explanation or advice regarding this issue to be able to resolve it in future.

Thanks everyone in advance
 

Attachments

  • spectre_1.txt
    1.6 MB · Views: 13
  • Job5.txt
    5 KB · Views: 13
  • spectre_2.txt
    11.5 KB · Views: 14

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I suppose you have solved the issue already.

For future interaction: I guess we need to see the test bench with the symbol option too. A netlist in addition to the run logs would also make sense.
What are the variables, Width and Length, for example, in the two simulations.

I also suspect that something is set up wrong with units/voltages. The transient time, for example, indicates Ts (as in 10^12 s) run times.
 

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Hi, jjx! Thanks for your reply.

Unfortunately, I a still struggling with this issue and as before would appreciate any reasonable comments under this thread. I got some feedback regarding the model files from cadence community: https://community.cadence.com/caden...-2477-rds-is-less-then-0-001-gpdk90nm/1401359

I also took a closer look at a log file and convergence errors simulator is facing. It somehow comes with voltages far above the supply and GAmpers currents through the constant dc sources which are connected to the gates

Code: 
Top 10 Solution too large Convergence failure:
    I(V6:p) = 585.468 MA
        update too large:  | 1.21597 GA | > 2.92734 MA + 1 nA
    I(V1:p) = 1.80144 GA
        update too large:  | 1.21597 GA | > 9.0072 MA + 1 nA
    I(V2:p) = 169.938 mA
        update too large:  | -81.8793 mA | > 849.69 uA + 1 nA
    V(PM10:int_s) = 4.22995 V
        update too large:  | -600 mV | > 21.1497 mV + 1 uV
    V(PM8:int_s) = 4.22995 V
        update too large:  | -600 mV | > 21.1497 mV + 1 uV
    V(PM9:int_d) = 5.43115 V
        update too large:  | -300 mV | > 27.1558 mV + 1 uV
    V(PM7:int_d) = 5.43115 V
        update too large:  | -300 mV | > 27.1558 mV + 1 uV
    V(NM2:int_s) = 13.8894 V
        update too large:  | -600 mV | > 69.4468 mV + 1 uV
    V(NM0:int_s) = 13.8894 V
        update too large:  | -600 mV | > 69.4468 mV + 1 uV
    V(in) = 13.8895 V
        update too large:  | -600 mV | > 69.4476 mV + 1 uV
 Top 10 Residue too large Convergence failure:
    V(PM0:int_d) = 52.4731 V
        residue too large: | -2.83001 GA | > 14.15 MA + 1 nA
    V(net01) = 34.2983 V
        residue too large: | 124.068 A | > 620.342 mA + 1 nA
    V(PM10:int_d) = 10.2595 V
        residue too large: | -82.8239 A | > 414.119 mA + 1 nA
    V(PM8:int_d) = 10.2595 V
        residue too large: | -82.8239 A | > 414.119 mA + 1 nA
    V(Voutn) = 2.63062 V
        residue too large: | 50.6215 A | > 253.108 mA + 1 nA
    V(Voutp) = 2.63062 V
        residue too large: | 50.6215 A | > 253.108 mA + 1 nA
    V(PM7:int_d) = 5.43115 V
        residue too large: | -32.9025 A | > 164.512 mA + 1 nA
    V(PM9:int_d) = 5.43115 V
        residue too large: | -32.9025 A | > 164.512 mA + 1 nA
    V(net027) = 2.56811 V
        residue too large: | 17.4373 A | > 87.1863 mA + 1 nA
    V(net029) = 2.56811 V
        residue too large: | 17.4373 A | > 87.1863 mA + 1 nA
(I assume that MOSFET model treats any gate as a high impedance node). I rearranged the symbols in the schematic so now it is probably little bit clearer which symbols have been used (Width and lengths for transistors are scaled up to 100nm as could be seen from the attached testbench). I am also attaching testbech with a netlist together with a log file part which prevents the conversion.

Again, as something which is not clear to me why do this errors do not appear for the cellview test (which can also be found in the attachments) where all the parameters and nodes stay the same without any modification.

I would assume that there is some simulation option which I probably have to set or reset to force the circuit into some "adequate" operation mode.
 

Attachments

  • Cellview.png
    28 KB · Views: 6
  • Convergence.png
    82.5 KB · Views: 7
  • schematic.png
    30.5 KB · Views: 6
  • testbench.png
    57.4 KB · Views: 7

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GA or MA or kA are incredibly high and need to be brought down to realistic values. Either by installing safety resistors or by reducing amplitude of voltage sources. Afterward neighboring circuitry should behave.

Certain simulators might respond differently to these situations with different alerts or none at all.
 

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Intriguing... can you post the input.scs netlist too, please?
(display netlist and grab the text from there...)
 

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