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Design >> Analog Design >> npoly vs ppoly resistor
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Message started by casual on Feb 25^th, 2010, 11:23pm

Title: npoly vs ppoly resistor
Post by casual on Feb 25^th, 2010, 11:23pm

For a nmos CML logic eg: typical CML latch, should I pick up npoly or ppoly as a resistor loads? The voltage swing is from Vdd to Vdd-swing, Does it have any difference to use npoly or ppoly if Rsheet is about the same

Title: Re: npoly vs ppoly resistor
Post by ywguo on Feb 26^th, 2010, 12:53am

Hi Casual,

If Rsheet is different, I suggest that you choose the resistor with larger Rsheet. Because it means smaller area and parasitic cap. If Rsheet is about equal to each other, I don't know what is the difference between the two.

Yawei

Title: Re: npoly vs ppoly resistor
Post by casual on Feb 26^th, 2010, 10:52pm

I think npoly is much better than ppoly because npoly sit at p-substrate, thus has larger reverse bias voltage (nmos is reference to vdd swing) results in smaller parasitic cap. is it correct?

Title: Re: npoly vs ppoly resistor
Post by ywguo on Feb 27^th, 2010, 5:56am

supermoment wrote on Feb 26^th, 2010, 10:52pm:

I think npoly is much better than ppoly because npoly sit at p-substrate, thus has larger reverse bias voltage (nmos is reference to vdd swing) results in smaller parasitic cap. is it correct?

Casual,

I am sorry that I don't understand what you said. -- Yawei

Title: Re: npoly vs ppoly resistor
Post by casual on Feb 27^th, 2010, 9:39pm

sorry i think i had made a mistake in previous statement.
npoly is sitting at tox and not on the diffusion, so there is no diffusion cap as in nplus resistor

So is there any big difference between npoly or ppoly resistor? I use it as a load in nmos CML latch.

Title: Re: npoly vs ppoly resistor
Post by thechopper on Feb 28^th, 2010, 11:34am

Hi

Since both resistors have different type of implants, they will also have different temperature coefficients.
If TC is not an issue I agre with Yawei: use the one with highest sheet rho.
If they are similar and TC is something you care about for your circuit you might want to select the one that gives behavior over temp suited for your application.

Tosei

Title: Re: npoly vs ppoly resistor
Post by casual on Feb 28^th, 2010, 8:35pm

thank you very much