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Nwell used for substrate shielding connected to gnd instead of Vdd

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sthota04

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I see the following figure in Kenneth Martin textbook.
Capture.JPG


How does nwell underneath a digital interconnect shield the substrate? And, the nwell is connected to ground line instead of vdd line. I'm really confused about this. Shouldn't the nwell be connected to vdd instead of gnd? Please help me understand this.


Thanks.
 

sthota04 said:
How does nwell underneath a digital interconnect shield the substrate?
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Even if there's no reverse voltage between the n-well and p-substrate, there will be a small space charge region at the junction (mainly into the p-substrate). This reduces the coupling capacitance between the interconnect and the substrate, because it's in series to the interconnect-nwell capacitance.

sthota04 said:
And, the nwell is connected to ground line instead of vdd line... Shouldn't the nwell be connected to vdd instead of gnd?
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Usually - if you have active devices in the n-well - yes, you're right. But in this case the n-well is just for shielding, so a GND connection is better if the signals to be shielded are GND referenced, because VDD might be more noisy (from switching circuitry).

For VDD referenced signals, however, the usual n-well connection to VDD would be better.
 

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