Printed Circuit Design & Fab - April 2008 - (Page 30) SI OPTIMIZATION nineteen possible variables are reduced to eight. Using the circuit topology of Figure 4 and the parametric values discussed above, optimization is performed with a goal of maximizing the eye opening at all four loads. The results of the first optimization step indicates that the circuit performed best when the sourceseries resistor at the driver was 1 Ohm, which is at the bottom limit. In addition, the intermediate series terminators along the primary transmission path provide best performance when they have low values. Accordingly, we can effectively remove these circuit elements by replacing them with direct shorts. Similarly, the shunt resistor values at the driver and along the primary signal path are sufficiently high that we consider them to be opens, and thus remove them from the circuit. The resulting circuit is shown in FIGURE 5, which is now much more practical to implement. Using the revised circuit, optimization is performed again to fine-tune the remaining parasitic values. We then FIGURE 6. Final circuit topology and associated waveforms for multi-drop bus. round off the results to practical values, and the resulting circuit performance is shown in FIGURE 6. The original non-optimized circuit 30 PRINTED CIRCUIT DESIGN & FAB APRIL 2008 http://www.midwestaccurate.com http://www.midwestaccurate.com
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