How to plot Vgs-Vt on the x axis in Cadence?

[ccw27]

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]

20 log(vout/vin) using cadence

You already asked this question 3 days ago. Did you tried that?

 

[yaxazaa]

vg-vth plot

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

 

[ccw27]

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]

cadence waveform axis plot vs not

It works, because I verified that before posting.

 

[ccw27]

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]

Re: Plotting in cadence

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

 

[srivatsan]

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]

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

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

 

[srivatsan]

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]

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

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]

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]

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.

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.

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]

Re: Plotting in cadence

I will try those.
thanks.
srivatsan

 

[lakeoffire]

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

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:

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]

Re: Plotting in cadence

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]

Re: Plotting in cadence

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]

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]

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.