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Design >> Analog Design >> Resistance Pins
https://designers-guide.org/forum/YaBB.pl?num=1148493346

Message started by Faisal on May 24^th, 2006, 10:55am

Title: Resistance Pins
Post by Faisal on May 24^th, 2006, 10:55am

Hi,

I intend to use a nwell resistor in my circuit. In my symbol library, I see three pins for a such a resistor and I am not clear about the function of the third pin. Is this (probably) some nwell connection ?? Do I need to connect this "extra" terminal to 5V or 0V ?? (This is no PMOS implemented in nwell)

I viewed the layout and I can figure out only two connections there (The nwell resistance is in nwell and there are two n+ contacts for the two terminals).

Title: Re: Resistance Pins
Post by Raul on May 24^th, 2006, 12:31pm

The third is probably for the backgate. This is probably because you may have DEEP-N and NBL(n buried layer) available in your process. These last two diffusions would give you the possibility of isolating your resistors from the global p-substrate if you wanted to do this. That way you know that for sure your diffusion resistors won't interact with anything else and cause latchup. Especially PMOS devices in the vicinity of the Nwell resistor.

Title: Re: Resistance Pins
Post by mikki33 on May 25^th, 2006, 2:19am

The third terminal is for P-Bulk connection. The NWell-resistor model takes in to account NWell-P-Bulk diode and parasitic capacitances to the P-Bulk as well.

I think you should have a very good reason to use NWell-resistor in your design (temperature sensor???). This resistor is very bad. It has very bad matching properties. It has very high parasitic capacitance. The sheet resistance variates +/- 40% with the process, it also variates 100% with temperature (0C-100C), it depends also on N-Well - P-Bulk voltage (non-linear), the profile of conductive layer is changing with the voltage across the length and it gives a good chance to get into the latch up (if 1 terminal of this resistor is connected to VSS).

I suggest to use non-salisided POLY resistor instead.

Title: Re: Resistance Pins
Post by Faisal on May 25^th, 2006, 2:53am

Hello,

Thanks for the details. I had figured this out that connecting this 3rd terminal to "0 volts" gives me correct results, but was unable to figure out the reason.

Faisal