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Design >> Analog Design >> Basic Question : NMOS Switch on-resistance Versus Input Voltage Figure
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Message started by allenhan on Sep 29^th, 2015, 9:31am

Title: Basic Question : NMOS Switch on-resistance Versus Input Voltage Figure
Post by allenhan on Sep 29^th, 2015, 9:31am

It is quite a basic question, I wanna draw the nmos switch on-resistance versus the input voltage figure. Here is what I have done so far. nmos gate is 3.3V, and the input voltage is a DC sweeping voltage ( I suppose it is from 0 to 3.3V), and the at the output node of the nmos, I put a holder cap to the ground also. So based on this model I wanna draw the Ron VS. Input Voltage figure.

I tried to use the XL calculator to calculate it, like, Ron =( Vin-Vout )/In, but compared to the real value in the DC operating point report, the calculator doesnt give a good number. and also which is strange, the when input is 500mV, for example, I plot the S node and D node current of nmos, but the currents are not same.....confuse me.

Any way, I think I saw some options on web, but not really accurate and neat, so I want to ask again, is there a good way, in cadence to draw the continuous figure line Ron VS. Vin. Thanks.

Title: Re: Basic Question : NMOS Switch on-resistance Versus Input Voltage Figure
Post by RobG on Sep 30^th, 2015, 7:48am

Are you applying an "In" or just looking at leakage? How much is the difference between the source and drain current?

I apply an Iin using a DC current source so that Vds is ~10mV then plot Vds/Iin. You could also do a parametric sweep and plot gds using the OP function in the calculator.

Title: Re: Basic Question : NMOS Switch on-resistance Versus Input Voltage Figure
Post by DanielLam on Oct 1^st, 2015, 11:36pm

This page has the setup for Ron. I've used it hundreds of times. Look for on-resistance of a switch. You can do a parametric sweep of W/L versus Vcm.

If you get an impedance, take the real part of it to get resistance.

http://web.engr.oregonstate.edu/~moon/kaj/cadence.html

Title: Re: Basic Question : NMOS Switch on-resistance Versus Input Voltage Figure
Post by sheldon on Oct 5^th, 2015, 7:54pm

Why don't you just plot input voltage against the gds value, the
real value, from the dc operating point? Right now, you are just
dividing 0/0.

Title: Re: Basic Question : NMOS Switch on-resistance Versus Input Voltage Figure
Post by AnilReddy on Oct 6^th, 2015, 1:38am

Hi,

Can you give a drawing of what you are trying to do? i am not getting where is Vin , Vout and all those notations you have used.

Regards
Anil Reddy