The File - March 16 , 2009 - (Page 8)

In Focus | Analogue-mixed signal design Eliminate substrate coupling in MS SoCs continued from page and conductivity of substrate material, which acts as a coupling medium between different parts of IC. This unwanted coupling can result in malfunctioning of certain IC portions. In summary, substrate coupling, or crosstalk, is a 3D problem which is dependent on process technology. Manual calculation of this effect will be based on trial and error method and is surely not suitable for sub-nm designs. Substrate model A simplistic model of non-ideal (ideal case being substrate at equipotential) device parasitics is shown in figure 1. There are two types of substrates: lightly doped and heavily doped. Lightly doped substrate faces displacement current problem and heavily doped substrate faces eddy current problem. Most of the RF applications are fabricated using lightly doped substrate. Displacement currents therefore dominate RF designs. Figure 2 shows the relationship between effective propagation constant, frequency of operation and Si doping. It is clearly visible that RF designs fall in slow wave mode region where effective permitivity is much greater than silicon permitivity. Figure 4: Displacement current modelling. Figure 5: Isolation technique in heavily doped substrate. Substrate effects Figures 3 and 4 illustrate eddy current and displacement current respectively. Eddy current is induced due to the magnetic field while displacement current is induced due to the electric current. Substrate isolation There are multiple ways through which substrate noise can be reduced, if one knows the origin and cause. The following are isolation techniques that you can use: • Avoid switching of large capacitive nodes • Direct coupling into substrate and supply bounce Figure 6: Isolation technique in lightly doped substrate. • • • • • • can be reduced. • Shielding reduces current injection into substrate. Turn off function not in use Minimise the instantaneous current Slow rise and fall time Lower the voltage swing Package inductance value as low as possible. Use of multiple power supplies. Figures 5 and 6 show examples of isolation technique in heavily doped substrate (using multiple pins) and lightly doped substrate (using multiple power supplies). All the methods mentioned above would have other side effects like increased die size, pessimistic design specification and delayed schedules, etc. The best way to eliminate substrate coupling is to use an application specific tool. This tool should allow design analysis in order to find out areas of concern. It should also provide information that will help in figuring out the best method of isolation, which meets both the design specification and schedule. Read the full article to learn about the CAD solution for overcoming the substrate noise problem. ■ Do more on EE Times India Ask the author Share article Say more • Comment on this article • Discuss: Suggestions for substrate modelling tools Read related articles • Simulate RFIC designs with substrate parasitics • Analogue, mixed-signal connectivity IP at 65nm, below • Mixed-signal grounding method provides signal integrity EE Times-India | March 16-31, 2009 | www.eetindia.com http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/65nm.HTM?TheFile_090316 http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/substrate.HTM?TheFile_090316 http://www.eetindia.co.in/SEARCH/SUMMARY/technical-articles/substrate.HTM?TheFile_090316 http://www.eetindia.co.in/article/sendInquiry.do?articleId=8800565280&catId=1800000?TheFile090316 http://www.eetindia.co.in/ART_8800565280_1800000_TA_4a5cf933.HTM?TheFile_090316#cnt http://forum.eetindia.co.in/FORUM_POST_1000039193_1200109670_0.HTM?TheFile_090316 http://www.eetindia.co.in/ART_8800545847_1800000_TA_a19ec223.HTM?TheFile_090316 http://www.eetindia.co.in/ART_8800545552_1800000_TA_a7b2c69b.HTM?TheFile_090316 http://www.eetindia.co.in/ART_8800565280_1800000_TA_4a5cf933.HTM?TheFile_090316 http://www.eetindia.co.in/ART_8800535047_1800002_TA_71eb858e.HTM?TheFile_090316 http://www.eetindia.co.in/STATIC/REDIRECT/Newsletter_090316_EETI02.htm

Table of Contents for the Digital Edition of The File - March 16 , 2009

The File - March 16 , 2009