Plotting in cadence

EDAboard.com Forum Index -> Analog IC Design & Layout -> Plotting in cadence

Author

Message

ccw27

Joined: 13 Oct 2004
Posts: 292
Helped: 8

06 Dec 2004 4:53 how to plot vout vs vin cadence

Hi,

I am sweeping the input Vgs and would like to plot Vgs-Vt on the
x-axis instead of only Vgs. How do I do this in Cadence?

Thanks

Fom

Joined: 10 Mar 2004
Posts: 778
Helped: 57
Location: Taiwan

06 Dec 2004 5:41 20 log(vout/vin) using cadence

You already asked this question 3 days ago. Did you tried that?
http://www.edaboard.com/viewtopic.php?t=99528&highlight=

yaxazaa

Joined: 13 Nov 2004
Posts: 119
Helped: 3

06 Dec 2004 7:02 vg-vth plot

Just use the cacultor to minus the vth from vgs and plot.

ccw27

Joined: 13 Oct 2004
Posts: 292
Helped: 8

06 Dec 2004 9:41 plotting in cadence

Hi,

Not sure how your method work, but i used the lshift function to shift the x-axis (Vgs) by -Vth using OP function. Now, if I wanted to times by a constant then I am not sure how to do it. Any ideas? Maybe interchange y and x axis but not sure how to it either.

Cheers

Fom

Joined: 10 Mar 2004
Posts: 778
Helped: 57
Location: Taiwan

06 Dec 2004 12:20 cadence waveform axis plot vs not

It works, because I verified that before posting.

ccw27

Joined: 13 Oct 2004
Posts: 292
Helped: 8

07 Dec 2004 1:58 Re: Plotting in cadence

Hi,

I tried your method, but it seems that it will plot Vs-Vg-Vth against Vgs, it doesn't change the x-axis variable. Am I doing anything wrong?

Thanks

chinito

Joined: 05 Jul 2004
Posts: 133
Helped: 4

17 Dec 2004 2:12 Re: Plotting in cadence

use calculator --> OP (operating point) --> select Vgs from drop down and plot it

srivatsan

Joined: 04 Aug 2004
Posts: 183
Helped: 4

17 Dec 2004 3:37 Re: Plotting in cadence

I want to plot (dB scale) Vout/Vin (obviously gain...) in spice (stand alone and not with cadence or other software).
How to do it?
thanks a lot (in advance)
regards,
srivatsan

Hughes

Joined: 10 Jun 2003
Posts: 718
Helped: 87

17 Dec 2004 5:32 Re: Plotting in cadence

srivatsan wrote:

I want to plot (dB scale) Vout/Vin (obviously gain...) in spice (stand alone and not with cadence or other software).
How to do it?
thanks a lot (in advance)
regards,
srivatsan

Set the AC magnitude of Vin to 1.0V for ac analysis, then the gain can be obtained:
.probe ac VDB(out)

Google
AdSense
Google Adsense

17 Dec 2004 5:32 Ads

srivatsan

Joined: 04 Aug 2004
Posts: 183
Helped: 4

17 Dec 2004 18:47 Re: Plotting in cadence

If I set it at 1v and have a gain of 10000; then it does not make sense. If I am wrong, let me know.
- vats

Hughes

Joined: 10 Jun 2003
Posts: 718
Helped: 87

18 Dec 2004 0:49 Re: Plotting in cadence

srivatsan wrote:

If I set it at 1v and have a gain of 10000; then it does not make sense. If I am wrong, let me know.
- vats

It does make sense. The ac signal value in ac analysis can be very large. The output is proportional to the input provided the DC operating point does not change. If the gain is 10000 and you set the AC value of the input to 1uV, you get 10mV AC value at the output. If the input is 1V, the output will be 10000V. In other words, the gain is 10000 when the input ac magnitude is 1uV, the gain will remain to be 10000 when the input ac magnitude changes to other value. The AC simulation results will not saturated at the power supply limited.

srivatsan

Joined: 04 Aug 2004
Posts: 183
Helped: 4

18 Dec 2004 1:56 Re: Plotting in cadence

WEll, I know that if the gain is 10000, then i cannot go beyond (VDD + abs(VSS))/gain for the input as it does not make sense.

What I was asking is the plot stuff when you have vin less than 1v and i want to plot 20*log(vout/vin).

let me know.
thanks.

- vats

Hughes

Joined: 10 Jun 2003
Posts: 718
Helped: 87

18 Dec 2004 6:31 Re: Plotting in cadence

srivatsan wrote:

WEll, I know that if the gain is 10000, then i cannot go beyond (VDD + abs(VSS))/gain for the input as it does not make sense.

It is nearly true for transient analysis, but ac analysis is another matter of fact. Both input and output can go beyond (VDD + abs(VSS)) in magnitude (of course only for ac value). You may have a try.

srivatsan wrote:

What I was asking is the plot stuff when you have vin less than 1v and i want to plot 20*log(vout/vin).

If you do prefer this method for gain measurement, you may use:
.probe ac gain=par('VDB(vout)-VDB(vin))

srivatsan

Joined: 04 Aug 2004
Posts: 183
Helped: 4

18 Dec 2004 8:46 Re: Plotting in cadence

I will try those.
thanks.
srivatsan

lakeoffire

Joined: 10 Dec 2002
Posts: 54
Helped: 2

19 Dec 2004 7:55 Re: Plotting in cadence

ccw27 wrote:

Hi,

I tried your method, but it seems that it will plot Vs-Vg-Vth against Vgs, it doesn't change the x-axis variable. Am I doing anything wrong?

Thanks

Plot Vgs-Vth vs. Vgs, then click on the x-axis on the waveform window, and in the pop-out window, select the Vgs-Vth as the x-axis instead of the independent variable. Hope it helps.

Added after 47 seconds:

ccw27 wrote:

Hi,

I tried your method, but it seems that it will plot Vs-Vg-Vth against Vgs, it doesn't change the x-axis variable. Am I doing anything wrong?

Thanks

Plot Vgs-Vth vs. Vgs, then click on the x-axis on the waveform window, and in the pop-out window, select the Vgs-Vth as the x-axis instead of the independent variable. Hope it helps.

ccw27

Joined: 13 Oct 2004
Posts: 292
Helped: 8

22 Dec 2004 6:22 Re: Plotting in cadence

Quote:

Plot Vgs-Vth vs. Vgs, then click on the x-axis on the waveform window, and in the pop-out window, select the Vgs-Vth as the x-axis instead of the independent variable. Hope it helps.

I tried but somehow the plot disappears when I do that.

Fom

Joined: 10 Mar 2004
Posts: 778
Helped: 57
Location: Taiwan

22 Dec 2004 7:02 Re: Plotting in cadence

Quote:

I tried but somehow the plot disappears when I do that.

May be you need fit the plot to the window. Try press "f" when the waveform window is active.

ccw27

Joined: 13 Oct 2004
Posts: 292
Helped: 8

23 Dec 2004 1:32 Re: Plotting in cadence

Yea I did that but still no plot. So I plot it again and now it plots y vs. y. Anyway I decided to export into Matlab and do the plot there.

Thanks

Fom

Joined: 10 Mar 2004
Posts: 778
Helped: 57
Location: Taiwan

23 Dec 2004 2:33 Re: Plotting in cadence

Y vs. Y because you plot only one variable Vgs-Vth. You get it as X variable. What do you want to be as Y variable? If you plot that in the same window and hide variable Vgs-Vth (just switch off not delete) you will get plot you wanted.