12/4/2002 The Ground Conundrum - Class 20 Assignment: Find and research papers on this subject, be prepared to defend research

2 12/4/2002The Ground Conundrum 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

3 12/4/2002The Ground Conundrum 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.

4 12/4/2002The Ground Conundrum 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 I1 -I1 I2 -I2 I1 -I1 Lets simplify a  -strip to simple resistors Signal path Return path

5 12/4/2002The Ground Conundrum The lossless return path  Calculate voltage at the load as a reference

6 12/4/2002The Ground Conundrum 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.

7 12/4/2002The Ground Conundrum The reduced ground circuit  Thevinize the ground resistor into the signal path.  Both lossy ground circuit and the reduced ground circuit produce volts at the loads. One ground node

8 12/4/2002The Ground Conundrum 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. I2 -I2 I1 -I1 I3 -I3 I4 -I4 Tline model: 2 lines plus return in 2 lines plus return out This seems OK so far but consider –I1, -I2, -I3, and -I4 are combined

9 12/4/2002The Ground Conundrum Take a closer look at where the current are and the voltages are developed Z1Z2 Z1gZ2g Z12g Z12 There’s can be a voltage drop between these two nodes Current in plane It is possible to collapse both ground nodes into one node but that creates issues

10 12/4/2002The Ground Conundrum Circuit Simulation Ground Rules – Transmission Line Rules 1. Ground reference transmission lines 2. Include return currents in the transmission path. 3. Do not use transmission line reference node for return path analysis

11 12/4/2002The Ground Conundrum 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. Green is assigned for ground pins  Cascading transmission lines is accurate if reduced grounds are used.

12 12/4/2002The Ground Conundrum Matching up Connector and T-line signals  The connector has 3 grounds  The line model has 1 ground  How do we connect ground? ? ? ? ? ? ?

13 12/4/2002The Ground Conundrum 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.

14 12/4/2002The Ground Conundrum Connector Interface on PWB Transmission line ends here. So signal reference is defined here. Connector ground pin starts here. 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 Impedance between ground pins

15 12/4/2002The Ground Conundrum Circuit Simulation Ground Rules – Between T and Connector Rule 3. Short grounds at connector if impedance between pins < 0.001*z0 for relevant frequencies. 4. Use circuit to model return path if impedance between pins > 0.001*z0 for relevant frequencies. 1. Or evaluate need

16 12/4/2002The Ground Conundrum Example of Reduce Ground Connector Ground reduced coupled model produces same results

17 12/4/2002The Ground Conundrum 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

18 12/4/2002The Ground Conundrum Circuit Simulation Ground Rules – Connector and packages 5. Ground reduce connector and package models

19 12/4/2002The Ground Conundrum Reducing Ground: 3 Inductor Connector  Start with a 3 pin connector  1nH self inductance  0.2nH mutual between any leg GND L22 L11 L12 L21L31 L32 L23L13L33 I1 I2 -I1-I2 PIN 1 PIN 2

20 12/4/2002The Ground Conundrum Create Current loop matrix

21 12/4/2002The Ground Conundrum Use the return current definition

22 12/4/2002The Ground Conundrum Equate to the voltage across the connector

23 12/4/2002The Ground Conundrum 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

24 12/4/2002The Ground Conundrum Do the same for V2-V3

25 12/4/2002The Ground Conundrum Apply values to the connector example

26 12/4/2002The Ground Conundrum 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

27 12/4/2002The Ground Conundrum 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.

28 12/4/2002The Ground Conundrum Use 400 ps UI to exaggerate effects Printed Wiring Board Buffers package Data generator Printed Wiring Board Buffers package Data generator

29 12/4/2002The Ground Conundrum Main programs example

30 12/4/2002The Ground Conundrum No measure voltage difference

31 12/4/2002The Ground Conundrum Now look at vss and signal nodes individually These spikes can cause simulator instabilities. In some circuits, these spikes can reach thousands of volts.

32 12/4/2002The Ground Conundrum 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

33 12/4/2002The Ground Conundrum Look at voltage measurements relative to row k

34 12/4/2002The Ground Conundrum Lets look a elements required for a 3 pin resistor model

35 12/4/2002The Ground Conundrum Create the return path matrix equation

36 12/4/2002The Ground Conundrum Develop difference voltages

37 12/4/2002The Ground Conundrum Implement resistor matrix spice

38 12/4/2002The Ground Conundrum Part 2 Anatomy of 3D modeling

39 12/4/2002The Ground Conundrum 3-D Aspects of Ground I2 -I2 I1 -I1 I3 -I3 I4 -I4 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 Terminals Voltage drop may exist across reference nodes

40 12/4/2002The Ground Conundrum For TEM, Cutting Up the Geometry is OK Ground Reduction still works OK An Interesting thought” “Where is ground?”

41 12/4/2002The Ground Conundrum Multi-conductor T-Lines are Ground Reduced Ground referenced model A reasonably involved process that Ansoft and other 2 D solvers can do.

42 12/4/2002The Ground Conundrum For Non TEM, Cascading Models Introduces Errors TEM Non TEM Wave propagation E-H Field vectors Cascading elements does not account for the non transverse components correctly

43 12/4/2002The Ground Conundrum Circuit Simulation Ground Rules - Cascading Rule 6. Cut models on TEM or Quasi TEM boundaries Left and right half TEM need to match

44 12/4/2002The Ground Conundrum Assignment: Use Ansoft to create a ground reduced spice model Er= mils Pins are 25 mil diameter Cu centered posts Green is ground, purple is signal Board connect is on the bottom layer Components insert on top layer of PWB 50 mils Mated Connector