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Design >> Analog Design >> Gnd and Vss
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Message started by georgejor on Feb 10^th, 2004, 2:58am

Title: Gnd and Vss
Post by georgejor on Feb 10^th, 2004, 2:58am

Dear all,

In analog design, I wonder whether the substrate contact of NMOS and the circuit ground should be connected to each other in "on chip" or "off chip"?
What is the trade off?

THANKS!

Best Regards,
George

Title: Re: Gnd and Vss
Post by Edward on Feb 10^th, 2004, 7:13pm

Hi George,

for NMOS, the substrate is a p-type. for PMOS, the substrate is a n-type. the substrate of NMOS and PMOS can form a parasitic diode, which is undesired.

therefore, in order to reverse bias the diode, the anode voltage of the diode should be lower than the cathode.

Hence, the substrate of NMOS is connected to the min potential, which is in this case a gnd or -vss.

Rgds
Edward

Title: Re: Gnd and Vss
Post by georgejor on Feb 10^th, 2004, 10:08pm

Dear Edward,
Thanks for your opinion. I understand that gnd and vss should be the min potential, but i think it is a problem of NOISE when the substrate ground(Vss) and circuit ground(gnd) are connected "on chip".
So, in some standard cell, there are two type, one is vss connects to gnd directly, another is vss and gnd separately.

Regards,
Raymond

Title: Re: Gnd and Vss
Post by Paul on Feb 11^th, 2004, 11:53pm

Hi George,

you should find out if your technology provides triple well or not. If so, the standard cells may be used with body biasing techniques to reduce leakage currents. In that case you can have different voltages on both nodes.

If the NMOS devices are in common substrate (not in a well), you must apply the same voltage to both nodes, as mentioned by Edward. Connecting it on-chip or off-chip is part of a global substrate noise reduction technique which you should consider for each design seperately. In order to avoid body effects, I would in most cases recommend to use the local NMOS source voltage to connect the substrate.

If you have substrate noise problems, use additional independent guard rings to pick it up. These should be connected to a seperate supply pin and only be taken to GND off-chip.

Paul

Title: Re: Gnd and Vss
Post by Edward on Feb 12^th, 2004, 2:03am

Hi,

What is the difference between ac ground and system ground?

What symbol do we use to differentiate them in the schematic?

Title: Re: Gnd and Vss
Post by raymyl on Feb 15^th, 2004, 5:14pm

The substrate has to be connected to the same ground node as the source if, say, the NMOS/PMOS is used in current mirror. On the other hand, if the internal ground bounce of a digital part is severe that a separate and more silent substrate ground will reduce the injection of noise into the substrate.