1 the ground conundrum assignment: find and research papers on this subject, be prepared to defend...

1

The Ground Conundrum

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 paradoxGround is 0 voltsGround 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

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

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 pathReturn path


5

The lossless return path

Calculate voltage at the load as a reference

R_source=50 R_tline=2 R_load=502V


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.


Rg_tline=2


7

The reduced ground circuitThevinize the ground resistor into

the signal path.Both lossy ground circuit and the

reduced ground circuit produce 0.9615 volts at the loads.

One groundnode


8



Rg_tline=2

Rg_tline=2

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

9

Take a closer look at where the current are and the voltages are developed

Z1 Z2

Z1g

Z2g

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

Circuit Simulation Ground Rules – Transmission Line Rules1. Ground reference transmission

lines2. Include return currents in the

transmission path.3. Do not use transmission line

reference node for return path analysis


11Connectors 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 forground pins

Cascading transmission lines is accurate if reduced grounds are used.


12Matching up Connector and T-line signals

The connector has 3 groundsThe line model has 1 groundHow do we connect ground?

?

?

?

?

?

?


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.

2V

2V


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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


15Circuit 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


16Example of Reduce Ground Connector

1.007 .03247 0.975

Ground reduced coupled model produces same results


17

Reduced model connects to T line

The connector’s 3 grounds have been folded into the circuit.

The T line model has 1 groundThe following preserves crosstalk

Most 2 D modelers can produce ground reduced models for transmission lines


18

Circuit Simulation Ground Rules – Connector and packages

5. Ground reduce connector and package models


19Reducing Ground: 3 Inductor Connector

Start with a 3 pin connector 1nH self inductance 0.2nH mutual between any leg

GND

L22

L11

L12

L21 L31

L32

L23L13 L33

I1

I2

-I1-I2

PIN 1

PIN 2


20

Create Current loop matrix

Create a 3x3 inductance matrix and a current matrix

L

L1 1

L1 2

L1 3

L2 1

L2 2

L2 3

L3 1

L3 2

L3 3

I

I1

I2

I3

Create current equations

L1 1

L1 2

L1 3

L2 1

L2 2

L2 3

L3 1

L3 2

L3 3

I1

I2

I3

s

L1 1 I

1 L

2 1 I2

L3 1 I

3 s

L1 2 I

1 L

2 2 I2

L3 2 I

3 s

L1 3 I

1 L

2 3 I2

L3 3 I

3 s


21

Use the return current definition

The current I3 is there return path for I1 and I2

I3

I1

I2

SubstituteL1 1

L1 2

L1 3

L2 1

L2 2

L2 3

L3 1

L3 2

L3 3

I1

I2

I1

I2

s

L1 1 I

1s L

2 1 I2s L

3 1 I1

I2

s

L1 2 I

1s L

2 2 I2s L

3 2 I1

I2

s

L1 3 I

1s L

2 3 I2s L

3 3 I1

I2

s


22Equate to the voltage across the connector

Set up the new voltage relationships

L1 1 I

1 L

2 1 I2

L3 1 I

1 I

2

L1 2 I

1 L

2 2 I2

L3 2 I

1 I

2

L1 3 I

1 L

2 3 I2

L3 3 I

1 I

2

s

V1

V2

V3


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

L1 1 I

1 L

2 1 I2

L3 1 I

1 I

2

L1 2 I

1 L

2 2 I2

L3 2 I

1 I

2

L1 3 I

1 L

2 3 I2

L3 3 I

1 I

2

T

0

0

L1 1 I

1 L

2 1 I2

L3 1 I

1 I

2

L1 2 I

1 L

2 2 I2

L3 2 I

1 I

2

L1 3 I

1 L

2 3 I2

L3 3 I

1 I

2

T

2

L1 1 I

1 L

2 1 I2

L3 1 I

1 I

2 L

1 3 I1

L2 3 I

2 L

3 3 I1

I2


24

Do the same for V2-V3

L1 1 I

1 L

2 1 I2

L3 1 I

1 I

2

L1 2 I

1 L

2 2 I2

L3 2 I

1 I

2

L1 3 I

1 L

2 3 I2

L3 3 I

1 I

2

T

1

0

L1 1 I

1 L

2 1 I2

L3 1 I

1 I

2

L1 2 I

1 L

2 2 I2

L3 2 I

1 I

2

L1 3 I

1 L

2 3 I2

L3 3 I

1 I

2

T

2

L1 2 I

1 L

2 2 I2

L3 2 I

1 I

2 L

1 3 I1

L2 3 I

2 L

3 3 I1

I2


25Apply values to the connector example

Example with values

KL21

L1 L2K13 K12 K23 .2

L1 1

L1 2

L1 3

L2 1

L2 2

L2 3

L3 1

L3 2

L3 3

1nH

.2nH

.2nH

.2nH

1nH

.2nH

.2nH

.2nH

1nH

Find reduced inductance matrixL1 1 L

3 1 L1 3 L

3 3 L1 2 L

3 2 L1 3 L

3 3 L2 1 L

3 1 L2 3 L

3 3 L2 2 L

3 2 L2 3 L

3 3

1.6

0.8

0.8

1.6

nH

Find new coupling factor for spice

Kp.8

1.62

Kp 0.5


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


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.


28

Use 400 ps UI to exaggerate effects

Printed WiringBoardBuffer

s

packa

ge

packa

ge

Data

gen

era

tor

Printed WiringBoardBuffer

s

packa

ge

packa

ge

Data

gen

era

tor


29

Main programs example


30

No measure voltage difference


31

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.


32

Generalized Return Path Reductions

Many 3D modelers have this operation as a feature

Start with s parametersThis can be acquired from a modeling tool or measurements

Z Zo I

S

1 I

S

Convert s to Z


33

33

Look at voltage measurements relative to row k

Determine reduction row k (ground)

ZT i ZT k

Convert Z back to s

S I

ZZo

1 I

ZZo

1


34

34

Lets look a elements required for a 3 pin resistor model

R1

R2

Rg


35

35

Create the return path matrix equation

R1

0

0

0

R1g

0

0

0

R2

I1

I2

I2 I1

V1

V2

Vg


36

Develop difference voltages

R1

0

0

0

R2

0

0

0

Rg

T

0 R1

0

0

0

R2

0

0

0

Rg

T

2

I1

I2

I2 I1

R1 I1

Rg I2 I1( )

R1

0

0

0

R2

0

0

0

Rg

T

1 R1

0

0

0

R2

0

0

0

Rg

T

2

I1

I2

I2 I1

R2 I2 Rg I2 I1( )


37

Implement resistor matrix spice

R1 Rg

Rg

Rg

R2 Rg

I1

I2

R1

R2

Rg

Rg

Rg*I2

Rg*I1

- 0V +

- 0V +


38

Part 2 Anatomy of 3D modeling


39

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


40For TEM, Cutting Up the Geometry is OK

Ground Reduction still works OK

An Interesting thought”

“Where is ground?”


41

Multi-conductor T-Lines are Ground Reduced

Ground referenced

model

A reasonably involved process that Ansoft and other 2 D solvers can do.


42

For Non TEM, Cascading Models Introduces Errors

TEM

Non TEM

Wave propagation

Wave propagation

E-H Field vectors

Cascading elements does not account for the non transverse components correctly


43

Circuit Simulation Ground Rules - Cascading Rule

6. Cut models on TEM or Quasi TEM boundaries

Left and right half TEM need to match


44

Assignment: Use Ansoft to create a ground reduced spice model

Er=3.8

100 mils

100 mils100 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

1 the ground conundrum assignment: find and research papers on this subject, be prepared to defend...

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