Main
Mixed-signal circuit design has historically been a challenge
for several reasons. Parasitic interactions between analog
and digital systems on a single die are one such challenge. Switching
transients induced by digital circuits inject noise into the
common substrate. Analog circuits lack the large noise
margins of digital circuits, thus making them susceptible to
substrate voltage variations. This problem is exacerbated
at higher frequencies as the effectiveness of standard isolation
technique diminishes considerably.
The effect of substrate noise on the circuits within an IC is
typically observed during the testing phase only after the chip
has been fabricated. Determination of the substrate noise coupling
during the design phase would be extremely beneficial to circuit
designers who can see the effect of the coupling on the circuits
and re-design accordingly before fabrication. This would reduce
the turn around time for circuits and increase the yield of working
chips.
We developed a substrate noise analysis tool (SNAT) that can
be used at any point in the design flow. The tool requires information
on the circuit as well as the technology. The circuit information
can be as descriptive as the circuit netlist complete with extracted
parasitics or as coarse as a verilog netlist. The technology
information can be as descriptive as a full substrate doping
profile with layout or as coarse as knowing only the substrate
resistivity and die size.
The tool generates equivalent noise macromodels to describe
the digital system. These macromodels are then coupled
with a model for the substrate to yield noise information such
as the time domain profile or spectrum at different points on
the substrate. The noise as a result of shaping by different
isolation techniques can also be determined. The resulting substrate
noise data can then be used to simulate its effect on various
analog circuits.
We have verified the results of SNAT with measurements on a
digital PLL designed in TI’s 90 nm technology. SNAT
yields 12% error in the RMS voltage of the substrate noise when
compared to measurements.
SNAT
SNAT is a substrate noise analysis CAD tool to generate substrate
noise profiles of large digital systems.
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