I just finished reading the article on Analog Verification in a System on Chip ( in IEEE Solid State Circuits Magazine Fall 2009 ) and found the approach described very interesting. I think the authors missed one aspect of mixed signal simulation in their approach.

As I understand from the example on the Equalizer, you write the Verilog AMS code for the equalizer and interface it with the Analog Circuit at the Model Level only ( not the schematic transistor level ).

Now for high speed digital logic, the transitions from the digital logic will couple to the analog portions of the circuit through the parasitic capacitance of the transistor, interconnect and the substrate. This will corrupt the analog output and may cause digitizing errors in subsequent blocks. Only a combined transient simulation ( of analog and digital combined ) will capture this effect. So a transient simulation at the schematic level for the analog and digital sections interfaced will still be necessary. If the noise margins for the subsequent blocks are known beforehand, interfacing between different blocks may be avoided by calculating the glitches in the output of the analog blocks.

I've not (yet) read the article.. but as a practitioner in this area, I'll say that you're correct. Parasistic coupling effects, like gate delays, are not part of the domain of functional verification, and you do need OTHER analyses to find these issues. The point of functional verification is to make sure to catch the stupid connection issues that would prevent you from running the first part on the test bench to discover that you might have a parasitic issue.
Functional verification is NOT going to find this issue, but it will catch most of the REST of the design bugs so that when you do run that transistor extracted simulation, you can let it run to the point where the subtle issues can be identified.  Its really hard to do that, if there are 20 design bugs to find and fix in a 10day simulation, unless you have a great foosball table, and six months of funding to burn. (imagining that once in a while you'll be able to fix two bugs before the next run.)


 

Hi Yawaei. As I understand, the described methodology uses behavioral models for core analog circuits and then combines them with Verilog AMS based models for the digital circuits to the verification.

For PLLs and ADCs which have an inherent analog functionality, you will have to do a transient simulation to capture all the charge sharing / pumping and coupling effects. Once all specs have been checked for, you may try to build a behavioral model which can then be coupled with Verilog AMS based models for the subsequent parts in the chain. The accuracy of system simulation will be determined by how close your behavioral models are to the actual transient simulation.