12/4/2002 the ground conundrum - class 20 assignment: find and research papers on this subject, be...
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12/4/2002
The Ground Conundrum - Class 20
Assignment: Find and research papers on this subject, be prepared to defend research
12/4/2002The Ground Conundrum
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
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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.
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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
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The lossless return path
Calculate voltage at the load as a reference
R_source=50 R_tline=2 R_load=502V
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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.
R_source=50 R_tline=2 R_load=502V
Rg_tline=2
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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
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R_source=50 R_tline=2 R_load=502V
R_source=50 R_tline=2 R_load=502V
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
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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
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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
12/4/2002The Ground Conundrum
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.
12/4/2002The Ground Conundrum
12Matching up Connector and T-line signals
The connector has 3 groundsThe line model has 1 groundHow do we connect ground?
?
?
?
?
?
?
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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
12/4/2002The Ground Conundrum
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
12/4/2002The Ground Conundrum
16Example of Reduce Ground Connector
1.007 .03247 0.975
Ground reduced coupled model produces same results
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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
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Circuit Simulation Ground Rules – Connector and packages
5. Ground reduce connector and package models
12/4/2002The Ground Conundrum
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
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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
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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
12/4/2002The Ground Conundrum
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
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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
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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
12/4/2002The Ground Conundrum
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
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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
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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.
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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
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Main programs example
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No measure voltage difference
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Now look at vss and signal nodes individually
These spikes can cause simulator instabilities. In some circuits, these spikes can reach thousands of volts.
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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
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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
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Lets look a elements required for a 3 pin resistor model
R1
R2
Rg
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Create the return path matrix equation
R1
0
0
0
R1g
0
0
0
R2
I1
I2
I2 I1
V1
V2
Vg
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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( )
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Implement resistor matrix spice
R1 Rg
Rg
Rg
R2 Rg
I1
I2
R1
R2
Rg
Rg
Rg*I2
Rg*I1
- 0V +
- 0V +
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Part 2 Anatomy of 3D modeling
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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
12/4/2002The Ground Conundrum
40For TEM, Cutting Up the Geometry is OK
Ground Reduction still works OK
An Interesting thought”
“Where is ground?”
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Multi-conductor T-Lines are Ground Reduced
Ground referenced
model
A reasonably involved process that Ansoft and other 2 D solvers can do.
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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
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Circuit Simulation Ground Rules - Cascading Rule
6. Cut models on TEM or Quasi TEM boundaries
Left and right half TEM need to match
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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