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Transient and DC operating point are different

BaaB

Hi,

I just run DC and transient analyses to find the operating point. However, the result is different. 

Transient analysis will perform DC analysis first, so should they be equal?

I tried the two separately and the results are different. 

Here is the netlist (if necessary).

// Generated for: spectre
// Generated on: Apr 9 00:29:19 2016
// Design library name: Huan
// Design cell name: Differential_Amp
// Design view name: schematic
simulator lang=spectre
global 0 vdd!
include "/root/Desktop/SaveOpPoint.scs"
parameters x=1
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=dio3
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=dio_dnw
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=dio
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfres_sa
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfmvar
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfind
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rtmom
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_bbmvar
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfesd
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_mim
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfrtmom
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfres_hri
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfres_rpo
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_3vna
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_m
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfmos
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfmim
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfjvar
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_3m
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_res
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_bip3
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_bip
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_3v
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfmos33
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_na
include "/root/TSMC018/tsmc18rf/../models/spectre/rf018.scs" section=tt_rfsbd
include "/root/Desktop/SaveOpPoint.scs"

// Library name: Huan
// Cell name: Differential_Amp
// View name: schematic
R4 (vdd! VP) resistor r=1K
R3 (vdd! VN) resistor r=1K
V3 (net08 net07) vsource dc=600.0m type=dc
V2 (net016 0) vsource dc=x type=dc
V0 (vdd! 0) vsource dc=1.8 type=dc
V6 (Vin2 net016) vsource mag=1 type=sine ampl=1m sinephase=180 freq=1M
V5 (Vin net016) vsource mag=1 type=sine ampl=1m sinephase=0 freq=1M
I11 (VS 0) isource dc=1.6m type=dc
M3 (net015 net08 net07 net07) nch l=180n w=2u m=50 ad=5.4e-13 as=5.568e-13 \
pd=2.54u ps=2.6368u nrd=0.135 nrs=0.135
M1 (VP Vin2 VS VS) nch l=180n w=2u m=50 ad=5.4e-13 as=5.568e-13 pd=2.54u \
ps=2.6368u nrd=0.135 nrs=0.135
M0 (VN Vin VS VS) nch l=180n w=2u m=50 ad=5.4e-13 as=5.568e-13 pd=2.54u \
ps=2.6368u nrd=0.135 nrs=0.135
simulatorOptions options reltol=1e-3 vabstol=1e-6 iabstol=1e-12 temp=27 \
tnom=27 scalem=1.0 scale=1.0 gmin=1e-12 rforce=1 maxnotes=5 maxwarns=5 \
digits=5 cols=80 pivrel=1e-3 sensfile="../psf/sens.output" \
checklimitdest=psf
dcOp dc write="spectre.dc" maxiters=150 maxsteps=10000 annotate=status
dcOpInfo info what=oppoint where=rawfile
dc dc param=x start=0 stop=1.8 lin=200 oppoint=rawfile maxiters=150 \
maxsteps=10000 annotate=status
tran tran stop=300u write="spectre.ic" writefinal="spectre.fc" \
annotate=status infotimes=[0] maxiters=5 infoname=tran_Info
finalTimeOP info what=oppoint where=rawfile
tran_Info info what=oppoint where=rawfile
modelParameter info what=models where=rawfile
element info what=inst where=rawfile
outputParameter info what=output where=rawfile
designParamVals info what=parameters where=rawfile
primitives info what=primitives where=rawfile
subckts info what=subckts where=rawfile
saveOptions options save=allpub

Parents

  • The transient and DC operating points will be the same if the DC value of the sources is the same as the time zero value of the source. That appears to be the case here.

    However, do note that the dc solution for transient will start from the values at the end of the previous simulation - and so that would be the value of x=1.8 (because of the dc sweep). If the circuit has more than one stable operating point, then that could influence the result.

    However, in this case I don't know what you're looking at to determine that they are different. I checked the node voltages and currents and they all seem to be the same (comparing the spectre.ic and spectre.dc files). 

    What are you looking at which shows you there's a difference?

    Regards,

    Andrew.

Reply

  • The transient and DC operating points will be the same if the DC value of the sources is the same as the time zero value of the source. That appears to be the case here.

    However, do note that the dc solution for transient will start from the values at the end of the previous simulation - and so that would be the value of x=1.8 (because of the dc sweep). If the circuit has more than one stable operating point, then that could influence the result.

    However, in this case I don't know what you're looking at to determine that they are different. I checked the node voltages and currents and they all seem to be the same (comparing the spectre.ic and spectre.dc files). 

    What are you looking at which shows you there's a difference?

    Regards,

    Andrew.

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