Stats

Locked Locked
Replies 5
Subscribers 125
Views 3285
Members are here 0

This discussion has been locked.

You can no longer post new replies to this discussion. If you have a question you can start a new discussion

PSS convergence issue when Chopper(implented using NMOS) kept at low impedance node

When CH2 and CH3 are implemented using NMOS transistors, I am facing convergence issue while doing PSS with the schematic given below. Whereas when CH2 and CH3 are implemented using ideal switch there is no convergence issue. Also it doesnt matter whether CH1 is implemented with NMOS or with ideal switches.

Even tried with gmin = 0;

Output Log

**************************************************
Periodic Steady-State Analysis `pss': fund = 1 kHz
**************************************************

=============================
`pss': time = (0 s -> 1.5 ms)
=============================
Important parameter values in tstab integration:
    start = 0 s
    outputstart = 0 s
    stop = 1.5 ms
    period = 1 ms
    step = 1.5 us
    maxstep = 40 us
    ic = all
    useprevic = no
    skipdc = no
    reltol = 10e-06
    abstol(V) = 1 uV
    abstol(I) = 1 pA
    temp = 27 C
    tnom = 27 C
    tempeffects = all
    method = trap
    lteratio = 3.5
    relref = sigglobal
    cmin = 0 F
    gmin = 0 S

    pss: time = 37.5 us      (2.5 %), step = 1.32 ns       (88 u%)
    pss: time = 112.5 us     (7.5 %), step = 6.828 ns     (455 u%)
    pss: time = 187.5 us    (12.5 %), step = 26.08 ns    (1.74 m%)
    pss: time = 262.5 us    (17.5 %), step = 10.86 ns     (724 u%)
    pss: time = 337.5 us    (22.5 %), step = 13.2 ns      (880 u%)
    pss: time = 412.5 us    (27.5 %), step = 12.92 ns     (861 u%)
    pss: time = 487.5 us    (32.5 %), step = 51.82 ns    (3.45 m%)
    pss: time = 562.5 us    (37.5 %), step = 1.655 ns     (110 u%)
    pss: time = 637.5 us    (42.5 %), step = 7.035 ns     (469 u%)
    pss: time = 712.5 us    (47.5 %), step = 21.96 ns    (1.46 m%)
    pss: time = 787.5 us    (52.5 %), step = 10.61 ns     (707 u%)
    pss: time = 862.5 us    (57.5 %), step = 13.28 ns     (885 u%)
    pss: time = 937.5 us    (62.5 %), step = 13.14 ns     (876 u%)
    pss: time = 1.013 ms    (67.5 %), step = 53.25 ns    (3.55 m%)
    pss: time = 1.088 ms    (72.5 %), step = 2.05 ns      (137 u%)
    pss: time = 1.163 ms    (77.5 %), step = 5.909 ns     (394 u%)
    pss: time = 1.238 ms    (82.5 %), step = 21.67 ns    (1.44 m%)
    pss: time = 1.313 ms    (87.5 %), step = 10.64 ns     (709 u%)
    pss: time = 1.388 ms    (92.5 %), step = 12.91 ns     (861 u%)
    pss: time = 1.463 ms    (97.5 %), step = 13 ns        (866 u%)
Conv norm = 796e+03, max dV(CLK) = 1.8 V, took 52.75 s.

Important parameter values in pss iteration:
    start = 500 us
    outputstart = 0 s
    stop = 1.5 ms
    period = 1 ms
    steadyratio = 10e-03
    step = 1.5 us
    maxstep = 5 us
    ic = all
    useprevic = no
    skipdc = no
    reltol = 10e-06
    abstol(V) = 1 uV
    abstol(I) = 1 pA
    temp = 27 C
    tnom = 27 C
    tempeffects = all
    errpreset = conservative
    method = trapeuler
    lteratio = 3.5
    relref = alllocal
    cmin = 0 F
    gmin = 0 S

================================
`pss': time = (500 us -> 1.5 ms)
================================
    pss: time = 525 us       (2.5 %), step = 9.694 ns     (969 u%)
    pss: time = 575 us       (7.5 %), step = 4.509 ns     (451 u%)
    pss: time = 625 us      (12.5 %), step = 13.56 ns    (1.36 m%)
    pss: time = 675 us      (17.5 %), step = 8.069 ns     (807 u%)
    pss: time = 725 us      (22.5 %), step = 13.69 ns    (1.37 m%)
    pss: time = 775 us      (27.5 %), step = 22.24 ns    (2.22 m%)
    pss: time = 825 us      (32.5 %), step = 4.157 ns     (416 u%)
    pss: time = 875 us      (37.5 %), step = 14.25 ns    (1.42 m%)
    pss: time = 925 us      (42.5 %), step = 777.3 ps    (77.7 u%)
    pss: time = 975 us      (47.5 %), step = 17.21 ns    (1.72 m%)
    pss: time = 1.025 ms    (52.5 %), step = 455.7 ps    (45.6 u%)
    pss: time = 1.075 ms    (57.5 %), step = 28.66 ns    (2.87 m%)
    pss: time = 1.125 ms    (62.5 %), step = 9.93 ns      (993 u%)
    pss: time = 1.175 ms    (67.5 %), step = 37.33 ns    (3.73 m%)
    pss: time = 1.225 ms    (72.5 %), step = 14.36 ns    (1.44 m%)
    pss: time = 1.275 ms    (77.5 %), step = 42.25 ns    (4.23 m%)
    pss: time = 1.325 ms    (82.5 %), step = 19.62 ns    (1.96 m%)
    pss: time = 1.375 ms    (87.5 %), step = 22.16 ns    (2.22 m%)
    pss: time = 1.425 ms    (92.5 %), step = 5.579 ns     (558 u%)
    pss: time = 1.475 ms    (97.5 %), step = 9.724 ns     (972 u%)
Conv norm = 632e+03, max dV(net0121) = 1.47342 V, took 64.31 s.

================================
`pss': time = (500 us -> 1.5 ms)
================================
    pss: time = 525 us       (2.5 %), step = 14.9 ns     (1.49 m%)
    pss: time = 575 us       (7.5 %), step = 455.2 ps    (45.5 u%)
    pss: time = 625 us      (12.5 %), step = 25.68 ns    (2.57 m%)
    pss: time = 675 us      (17.5 %), step = 6.903 ns     (690 u%)
    pss: time = 725 us      (22.5 %), step = 33.65 ns    (3.37 m%)
    pss: time = 775 us      (27.5 %), step = 12.16 ns    (1.22 m%)
    pss: time = 825 us      (32.5 %), step = 40.61 ns    (4.06 m%)
    pss: time = 875 us      (37.5 %), step = 17.65 ns    (1.76 m%)
    pss: time = 925 us      (42.5 %), step = 35.71 ns    (3.57 m%)
    pss: time = 975 us      (47.5 %), step = 11.25 ns    (1.13 m%)
    pss: time = 1.025 ms    (52.5 %), step = 38.2 ns     (3.82 m%)
    pss: time = 1.075 ms    (57.5 %), step = 15.05 ns     (1.5 m%)
    pss: time = 1.125 ms    (62.5 %), step = 42.7 ns     (4.27 m%)
    pss: time = 1.175 ms    (67.5 %), step = 17.43 ns    (1.74 m%)
    pss: time = 1.225 ms    (72.5 %), step = 20.74 ns    (2.07 m%)
    pss: time = 1.275 ms    (77.5 %), step = 5.657 ns     (566 u%)
    pss: time = 1.325 ms    (82.5 %), step = 9.427 ns     (943 u%)
    pss: time = 1.375 ms    (87.5 %), step = 4.585 ns     (458 u%)
    pss: time = 1.425 ms    (92.5 %), step = 9.026 ns     (903 u%)
    pss: time = 1.475 ms    (97.5 %), step = 3.022 ns     (302 u%)
Conv norm = 775e+03, max dV(M28:int_d) = 1.75482 V, took 63.6 s.

================================
`pss': time = (500 us -> 1.5 ms)
================================
    pss: time = 525 us       (2.5 %), step = 15.29 ns    (1.53 m%)
    pss: time = 575 us       (7.5 %), step = 8.704 ns     (870 u%)
    pss: time = 625 us      (12.5 %), step = 9.639 ns     (964 u%)
    pss: time = 675 us      (17.5 %), step = 4.218 ns     (422 u%)
    pss: time = 725 us      (22.5 %), step = 12.89 ns    (1.29 m%)
    pss: time = 775 us      (27.5 %), step = 7.303 ns     (730 u%)
    pss: time = 825 us      (32.5 %), step = 14.38 ns    (1.44 m%)
    pss: time = 875 us      (37.5 %), step = 22.43 ns    (2.24 m%)
    pss: time = 925 us      (42.5 %), step = 4.2 ns       (420 u%)
    pss: time = 975 us      (47.5 %), step = 14.96 ns     (1.5 m%)
    pss: time = 1.025 ms    (52.5 %), step = 1.02 ns      (102 u%)
    pss: time = 1.075 ms    (57.5 %), step = 18.54 ns    (1.85 m%)
    pss: time = 1.125 ms    (62.5 %), step = 365 ps      (36.5 u%)
    pss: time = 1.175 ms    (67.5 %), step = 29.42 ns    (2.94 m%)
    pss: time = 1.225 ms    (72.5 %), step = 9.917 ns     (992 u%)
    pss: time = 1.275 ms    (77.5 %), step = 35.87 ns    (3.59 m%)
    pss: time = 1.325 ms    (82.5 %), step = 14.35 ns    (1.43 m%)
    pss: time = 1.375 ms    (87.5 %), step = 42.7 ns     (4.27 m%)
    pss: time = 1.425 ms    (92.5 %), step = 19.68 ns    (1.97 m%)
    pss: time = 1.475 ms    (97.5 %), step = 24.69 ns    (2.47 m%)
Conv norm = 506e+03, max dV(M28:int_d) = -1.77118 V, took 63.87 s.

================================
`pss': time = (500 us -> 1.5 ms)
================================
    pss: time = 525 us       (2.5 %), step = 21.8 ns     (2.18 m%)
    pss: time = 575 us       (7.5 %), step = 3.258 ns     (326 u%)
    pss: time = 625 us      (12.5 %), step = 31.36 ns    (3.14 m%)
    pss: time = 675 us      (17.5 %), step = 11.27 ns    (1.13 m%)
    pss: time = 725 us      (22.5 %), step = 38.21 ns    (3.82 m%)
    pss: time = 775 us      (27.5 %), step = 15.82 ns    (1.58 m%)
    pss: time = 825 us      (32.5 %), step = 35.69 ns    (3.57 m%)
    pss: time = 875 us      (37.5 %), step = 15.26 ns    (1.53 m%)
    pss: time = 925 us      (42.5 %), step = 20.73 ns    (2.07 m%)
    pss: time = 975 us      (47.5 %), step = 6.629 ns     (663 u%)
    pss: time = 1.025 ms    (52.5 %), step = 8.492 ns     (849 u%)
    pss: time = 1.075 ms    (57.5 %), step = 1.219 ns     (122 u%)
    pss: time = 1.125 ms    (62.5 %), step = 15.28 ns    (1.53 m%)
    pss: time = 1.175 ms    (67.5 %), step = 11.68 ns    (1.17 m%)
    pss: time = 1.225 ms    (72.5 %), step = 10.02 ns       (1 m%)
    pss: time = 1.275 ms    (77.5 %), step = 23.43 ns    (2.34 m%)
    pss: time = 1.325 ms    (82.5 %), step = 989.1 ps    (98.9 u%)
    pss: time = 1.375 ms    (87.5 %), step = 19.76 ns    (1.98 m%)
    pss: time = 1.425 ms    (92.5 %), step = 391.7 ps    (39.2 u%)
    pss: time = 1.475 ms    (97.5 %), step = 25.69 ns    (2.57 m%)
Conv norm = 503e+03, max dV(M27:int_d) = 1.14399 V, took 77.24 s.

Andrew Beckett over 9 years ago

There's not enough information here to really go on. You seem to have some slightly odd settings - I think you've picked errpreset=conservative but then picked method=trap (why?). Which version of the simulator are you using?

Can you post the netlist (as an attachment - the input.scs) and then I could take a look?

Regards,

Andrew.
Cancel
Vote Up 0 Vote Down

Cancel
meraj025 over 9 years ago

input.scs.zip

spectre version :- 12.1.0.272

The actual schematic is shown above. Here MOS Pseudo Resistor(~1TOhms) is used to establish the DC at the output. In other words it acts as a internal CMFB.

Regards,

Meraj
Cancel
Vote Up 0 Vote Down

Cancel
meraj025 over 9 years ago

Hi Andrew,

I have attached two input.scs file. One is with ideal switches and other is with NMOS switches. The amplifier with ideal switch converges, but the the amplifier with NMOS switch does not converge.

Please look into it and give your valuable suggestions.

Regards,

Meraj

input.scs_nmos_switch.zip

input.scs_ideal_switch.zip
Cancel
Vote Up 0 Vote Down

Cancel
MB13 over 5 years ago in reply to Andrew Beckett

Hi Andrew

I could not make a new question in forum and ask my question in here.

I am simulate a dc dc converter using periodic steady state tab. I set tstab such that the converter settles in steady state. but after just one iteration simulation stops withno error and warning
Cancel
Vote Up 0 Vote Down

Cancel
Andrew Beckett over 5 years ago in reply to MB13

I don't see why you can't just post a new question by hitting the Blue new button at the top of the list of posts in each forum (you might also want to read the forum guidelines which ask you not to post on old threads, and to provide sufficient information to enable somebody to stand a chance of being able to answer; so far you've given none of that). In fact it's rather amusing that you've posted a reply to my reply saying that there was not enough info to go on, including the simulator version and so on (as you've provided none of this).

You should be able to find the blue "New" button at the top here: https://community.cadence.com/cadence_technology_forums/f/custom-ic-design

Andrew
Cancel
Vote Up 0 Vote Down

Cancel

Community Guidelines

The Cadence Design Communities support Cadence users and technologists interacting to exchange ideas, news, technical information, and best practices to solve problems and get the most from Cadence technology. The community is open to everyone, and to provide the most value, we require participants to follow our Community Guidelines that facilitate a quality exchange of ideas and information. By accessing, contributing, using or downloading any materials from the site, you agree to be bound by the full Community Guidelines.