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  1. #1
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    Deep nwell connection

    Please help me with few questions related to deep nwell.

    I have 3 inverters with 3 different power and ground. Power anyways will be isolated by nwell. To isolate 3 different grounds i need to use deep nwell. In this case can i use a single deep nwell for all 3 different grounds.?

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  2. #2
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    Re: Deep nwell connection

    Provided that you reverse bias the deep nwell to the highest of the three supplies, you can share the deep nwell. Just make sure you follow any violations/considerations implemented by the DRC rule deck/PDK. The three different grounds would each have their own ground biased pwells within the deep nwell. Make sure you have a lot of well contacts too.



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    Re: Deep nwell connection

    In my opinion, it is better if you use 3 dnwell separately. To prevent noise from each pwell to others.



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    Re: Deep nwell connection

    Quote Originally Posted by quyleanh View Post
    In my opinion, it is better if you use 3 dnwell separately. To prevent noise from each pwell to others.


    I agree. Unless you have hundreds of wells, you’re not going to save a lot of space by combining just three. There’s less risk with keeping them separate.



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    Re: Deep nwell connection

    Why could he share the deep n well for the three different positive voltages? Wouldn't each of the inverter contact the deep n-well with their own n-well? Or is this feature technology dependand?



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    Re: Deep nwell connection

    Deep nwell just provides a well to put p-implant. And the most important thing to consider is that the p well to dn well never forward biases. That is usually done by reverse biasing to the highest supply. The PMOS devices in the n-well usually are not in the deep nwell. If they are, then you’re right, you can’t connect them all together because they’ll create an ERC violation/short. The only time I’ve seen pmos devices in deep nwell is when they’re trying to increase the stand-off voltage. When I made my initial comment, I assumed only the NFETs would be in the deep nwell and you’d have a separate region for the pfets, there are standard cell guard rings in some PDKs that create this type of structure.



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  7. #7
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    Re: Deep nwell connection

    I have only worked with 2 technologies yet, but in both the deep n-wells were used isolate whole circuit blocks (at my current design even a square milimiter big block) from the substrate. For our optoelectronic designs the substrate will be biased to around -25V so any circuit needs to be isolated completly!



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