The Ground Conundrum - Class 20

1. The Ground Conundrum - Class 20 Assignment: Find and research papers on this subject, be prepared to defend research

2. Grounding Thoughts • Ground is only a convenience for a voltage measurement reference • The ground paradox • Ground is 0 volts • Ground is relative • There is not an absolute ground • Ground can be anywhere • Any sources referenced to ground returns power to ground • All real sources have a reference to ground The Ground Conundrum

3. Reduced Ground Definition • The reason for the reduced ground concept is related to the modeling of transmission lines in any spice-like simulator. • We will develop the concept of reduced ground first and subsequently illustrate why it is required. • The reduced ground: • Collapses return path circuits to a single reference node. • Signal measurement accuracy of a network is then preserved by incorporating the return path effects as elements added into the signal path. • Measurement of ground bounce is hidden. • Return path may be power or ground. • We will only concern ourselves with ground for now. • Reduce Power is a good research and development topic. The Ground Conundrum

4. First Order View - Resistor Model • This Simple model will be sufficient to illustrate the ground reduction concept and issues of creating multi-line reduced models I2 -I2 I1 -I1 -I1 I1 Lets simplify a m-strip to simple resistors Signal path Return path The Ground Conundrum

5. The lossless return path • Calculate voltage at the load as a reference The Ground Conundrum

6. The lossy return path • Calculate the voltage at the load • Notice this is less that the voltage on the previous slide because the return path is considered here. • Now lets use this voltage a reference • The goal is to create a circuit that produces the same load voltage and current but only has one ground node. The Ground Conundrum

7. The reduced ground circuit • Thevinize the ground resistor into the signal path. • Both lossy ground circuit and the reduced ground circuit produce 0.9615 volts at the loads. One ground node The Ground Conundrum

8. Add another line! • Lets keep with a resistor model and a reduced ground path • Spice only allows a single node for return path for multi conductor transmission line element. • Tline model: • 2 lines plus return in • 2 lines plus return out I2 I4 -I2 -I4 This seems OK so far but consider –I1, -I2, -I3, and -I4 are combined I1 I3 -I1 -I3 The Ground Conundrum

9. Take a closer look at where the current are and the voltages are developed It is possible to collapse both ground nodes into one node but that creates issues There’s can be a voltage drop between these two nodes Z12 Z1 Z2 Current in plane Z12g Z1g Z2g The Ground Conundrum

10. Circuit Simulation Ground Rules – Transmission Line Rules • Ground reference transmission lines • Include return currents in the transmission path. • Do not use transmission line reference node for return path analysis The Ground Conundrum

11. Connectors and Transmission Lines • Cascading a connecter (or package) is a different story. • Like the T line, start with a simple resistor model for a connector. • Cascading transmission lines is accurate if reduced grounds are used. Green is assigned forground pins The Ground Conundrum

12. Matching up Connector and T-line signals • The connector has 3 grounds • The line model has 1 ground • How do we connect ground? ? ? ? ? ? ? The Ground Conundrum

13. Connector Model Usage • Is this the model usage for the connector? • Short all ground pins together? • Or is this the model usage? • Connect to ground with a circuit. The Ground Conundrum

14. Impedance between ground pins Connector Interface on PWB • Case 1 is when: • Impedance between ground pins and transmission line pins are very small. Less than 0.1% of line impedance across frequency range. • Case 2 is when: • Impedance between ground pins is significant. Greater than 0.1% of line impedance across frequency range Transmission line ends here. So signal reference is defined here. Connector ground pin starts here. The Ground Conundrum

15. Circuit Simulation Ground Rules – Between T and Connector Rule • Short grounds at connector if impedance between pins < 0.001*z0 for relevant frequencies. • Use circuit to model return path if impedance between pins > 0.001*z0 for relevant frequencies. • Or evaluate need The Ground Conundrum

16. Example of Reduce Ground Connector Ground reduced coupled model produces same results The Ground Conundrum

17. Reduced model connects to T line • The connector’s 3 grounds have been folded into the circuit. • The T line model has 1 ground • The following preserves crosstalk • Most 2 D modelers can produce ground reduced models for transmission lines The Ground Conundrum

18. Circuit Simulation Ground Rules – Connector and packages • Ground reduce connector and package models The Ground Conundrum

19. Reducing Ground: 3 Inductor Connector • Start with a 3 pin connector • 1nH self inductance • 0.2nH mutual between any leg PIN 1 I1 L11 L21 L31 I2 PIN 2 L12 L22 L32 -I1-I2 GND L13 L23 L33 The Ground Conundrum

20. Create Current loop matrix The Ground Conundrum

21. Use the return current definition The Ground Conundrum

22. Equate to the voltage across the connector The Ground Conundrum

23. The equivalent voltage at pin 1 • Remove s for now because its only common factor • Convert to columns with the matrix transpose operation (T) and so we can use the column function to extract V1 or column 0 minus V3 or column 2 The Ground Conundrum

24. Do the same for V2-V3 The Ground Conundrum

25. Apply values to the connector example The Ground Conundrum

26. Evaluate methods with spice • Use testckt.sp as starting point and create return_path_reduction.sp • Insert the previous 3 pin connector example for the package model • Replace the single node vss with two node for vss in and out The Ground Conundrum

27. Use library replacement • Compare difference between received voltage for the 3 pin model and the 2 pin return path reduced model. • For the three pin case vss will only be tied ground at the transmitter. The Ground Conundrum

28. Printed WiringBoard Printed WiringBoard Data generator Data generator package package package package Buffers Buffers Use 400 ps UI to exaggerate effects The Ground Conundrum

29. Main programs example The Ground Conundrum

30. No measure voltage difference The Ground Conundrum

31. Now look at vss and signal nodes individually These spikes can cause simulator instabilities. In some circuits, these spikes can reach thousands of volts. The Ground Conundrum

32. Generalized Return Path Reductions • Many 3D modelers have this operation as a feature • Start with s parameters • This can be acquired from a modeling tool or measurements The Ground Conundrum

33. Look at voltage measurements relative to row k The Ground Conundrum

34. Lets look a elements required for a 3 pin resistor model The Ground Conundrum

35. Create the return path matrix equation The Ground Conundrum

36. Develop difference voltages The Ground Conundrum

37. Implement resistor matrix spice The Ground Conundrum

38. Part 2 Anatomy of 3D modeling The Ground Conundrum

39. 3-D Aspects of Ground • Current distribution in ground plane is not at a point • 3-D modeling accounts for distribution. • Defining a terminal port is “point” assumption • This point has the potential to create circuit concatenation issues I2 I4 -I2 -I4 I1 I3 -I1 -I3 Terminals Voltage drop may exist across reference nodes The Ground Conundrum

40. For TEM, Cutting Up the Geometry is OK An Interesting thought” “Where is ground?” Ground Reduction still works OK The Ground Conundrum

41. Multi-conductor T-Lines are Ground Reduced A reasonably involved process that Ansoft and other 2 D solvers can do. Ground referenced model The Ground Conundrum

42. For Non TEM, Cascading Models Introduces Errors Wave propagation TEM Wave propagation Non TEM E-H Field vectors Cascading elements does not account for the non transverse components correctly The Ground Conundrum

43. Circuit Simulation Ground Rules - Cascading Rule • Cut models on TEM or Quasi TEM boundaries • Left and right half TEM need to match The Ground Conundrum

44. Assignment: Use Ansoft to create a ground reduced spice model 100 mils Er=3.8 100 mils 100 mils Mated Connector 50 mils • Green is ground, purple is signal • Board connect is on the bottom layer • Components insert on top layer of PWB Pins are 25 mil diameter Cu centered posts The Ground Conundrum