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Simulators >> Circuit Simulators >> DC Resistance vs ron in Nmos
https://designers-guide.org/forum/YaBB.pl?num=1500479438

Message started by kabir_fakir on Jul 19^th, 2017, 8:50am

Title: DC Resistance vs ron in Nmos
Post by kabir_fakir on Jul 19^th, 2017, 8:50am

I wanted to design a switch for switch cap circuit and hence wanted to determine the on resistance of the switch. I know this question has been already asked in the previous threads like

http://www.designers-guide.org/Forum/YaBB.pl?num=1176261122

After reading this thread I made a testbench to get the feeling of ron. but still I get confused between the DC resistance (say large signal: here I say it R_nmos) and ron(given by model). In the testbench R_nmos is given by:
R_nmos=((v("/dc" ?result "dc") - v("/v_dco" ?result "dc")) / i("/R1/PLUS" ?result "dc")).
Then I made parametric simulation to see the ron. What I have found that R_nmos is different than the ron. Could you explain me why it is like so?
Is there is something wrong in testbench? or am I missing some fundamentals?

PS:Here I am using ams high voltage transistor in 350nm. And the bias resisto R1=1G(ideal)

Title: Re: DC Resistance vs ron in Nmos
Post by kabir_fakir on Jul 19^th, 2017, 8:55am

I swept the dc voltage , connected to the drain of the Nmos , to calculate R_nmos and then made parametric simulation to plot ron. As its clear from the curve that both resistance is different

Title: Re: DC Resistance vs ron in Nmos
Post by Ken Kundert on Jul 19^th, 2017, 10:18am

You can't try to measure 300Ω while it is in series with 1GΩ and expect any kind of accuracy.

And where is C8? Is it a capacitor? Why are you measuring the current through a capacitor in a DC analysis, and dividing by it of all things?

This is not a good testbench for measuring the on resistance of a switch.

-Ken

Title: Re: DC Resistance vs ron in Nmos
Post by kabir_fakir on Jul 19^th, 2017, 10:42am

Firstly, thanks for a quick response.
Secondly, there is a typo error so its R1 instead of C8. Is there any better test bench which I can refer to for switch design or whatcan I improve in this existing test bench?
As i have seen so many papers and articles, so get a bit confused. I understood the theory part of switch but struggling with the test bench.

Title: Re: DC Resistance vs ron in Nmos
Post by Maks on Jul 19^th, 2017, 10:58am

Just replace 1GOhm resistor with 1 mOhm resistor, to improve your accuracy (as Ken suggested), us max (nominal) Vgs, and small value (like 0.1 V) for Vds.

Title: Re: DC Resistance vs ron in Nmos
Post by kabir_fakir on Jul 19^th, 2017, 11:07am

Maks wrote on Jul 19^th, 2017, 10:58am:

Just replace 1GOhm resistor with 1 mOhm resistor, to improve your accuracy (as Ken suggested), us max (nominal) Vgs, and small value (like 0.1 V) for Vds.

but am i not suppose to sweep the vds for larger range , because in my design drain terminal is changing from 0 volt to 9 volt. So I dnt understand why small value of vds?

In fact, I am asked to measure both ac resistance (small signal i.e ron) and also the slope of the line connecting (0,0) with (Vds,Ids) (I call it large resistance)

Title: Re: DC Resistance vs ron in Nmos
Post by Maks on Jul 19^th, 2017, 12:33pm

The switch has two states - on and off.
In off state, it will see the full voltage VDD, but Vgs is zero and its the switch resistance is huge.
In on state, Vgs is maximum, and Vds will be (should be) small, otherwise you will burn out your transistor. In on state, its resistance Rdson is small.

Title: Re: DC Resistance vs ron in Nmos
Post by DanielLam on Jul 19^th, 2017, 4:26pm

I'm copying and pasting a post I made about the Ron testbench.
( http://www.designers-guide.org/Forum/YaBB.pl?num=1443544276 )

The oregon 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: DC Resistance vs ron in Nmos
Post by kabir_fakir on Jul 20^th, 2017, 1:46am

DanielLam wrote on Jul 19^th, 2017, 4:26pm:

Thanks for the reply. Interestingly I have already found this test bench but I did nt get the purpose of switch( I think its sp2switch) in the test bench.

Title: Re: DC Resistance vs ron in Nmos
Post by DanielLam on Jul 20^th, 2017, 11:47am

I don't think that's the s2pswitch. I think it's just a schematic symbol they made.

That sim has 3 parallel resistance measurements, you only need one.

Title: Re: DC Resistance vs ron in Nmos
Post by Ken Kundert on Jul 20^th, 2017, 6:04pm

Replace the resistor between the source and ground with a voltage source. You can sweep the DC voltage of this source to get an idea of how the resistance of the switch changes with bias.

Replace the voltage source that connects between the drain and ground with a voltage source that connects between the drain and the source. This source would be 0V DC and 1V AC. The switch's on conductance is then equal to the AC current through this second source.

The AC analysis would be configured to sweep the DC voltage on the first source while setting the AC frequency to a constant 0 Hz.

-Ken

Title: Re: DC Resistance vs ron in Nmos
Post by kabir_fakir on Jul 26^th, 2017, 5:40am

DanielLam wrote on Jul 20^th, 2017, 11:47am:

I don't think that's the s2pswitch. I think it's just a schematic symbol they made.

That sim has 3 parallel resistance measurements, you only need one.

As per discussion I have understood what you tried to say me. Here is the updated testbench

Title: Re: DC Resistance vs ron in Nmos
Post by kabir_fakir on Jul 26^th, 2017, 5:43am

@Daniel
I have plotted the following curves which looks fine. Thanks for the support.

P.S: Please note that these transistors are high voltage!

Title: Re: DC Resistance vs ron in Nmos
Post by DanielLam on Jul 26^th, 2017, 12:29pm

Nicely done. Thanks for showing people the plots. I hope you can help others in the future. :)