The Ground Conundrum

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 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
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

I2
I1
-I2
-I1
-I1
I1
Lets simplify a m-strip to simple resistors
Signal path
Return path
5
The lossless return path

• Calculate voltage at the load as a reference

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.

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

11
Connectors and Transmission Lines

• Cascading transmission lines is accurate if
  reduced grounds are used.

• 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
12
Matching up Connector and T-line signals

• The connector has 3 grounds
• The line model has 1 ground
• How 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.

14
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.
15
Circuit Simulation Ground Rules Between T and
Connector Rule

• Short grounds at connector if impedance between
  pins lt 0.001z0 for relevant frequencies.
• Use circuit to model return path if impedance
  between pins gt 0.001z0 for relevant frequencies.
• Or evaluate need

16
Example of Reduce Ground Connector
Ground reduced coupled model produces same results
17
Reduced model connects to T line

• The connectors 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
Circuit Simulation Ground Rules Connector and
packages

• Ground reduce connector and package models

19
Reducing Ground 3 Inductor Connector
PIN 1
I1
L11
L21
L31
I2
PIN 2
L22
L12
L32
GND
-I1-I2
L23
L13
L33

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

20
Create Current loop matrix
21
Use the return current definition
22
Equate to the voltage across the connector
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

24
Do the same for V2-V3
25
Apply values to the connector example
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
29
Main programs example
30
No measure voltage difference
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 parameters
• This can be acquired from a modeling tool or
  measurements

33
Look at voltage measurements relative to row k
34
Lets look a elements required for a 3 pin
resistor model
35
Create the return path matrix equation
36
Develop difference voltages
37
Implement resistor matrix spice
38
Part 2 Anatomy of 3D modeling
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
40
For TEM, Cutting Up the Geometry is OK
An Interesting thought Where is ground?
Ground Reduction still works OK
41
Multi-conductor T-Lines are Ground Reduced
A reasonably involved process that Ansoft and
other 2 D solvers can do.
Ground referenced model
42
For Non TEM, Cascading Models Introduces Errors
Wave propagation
TEM
Non TEM
Wave propagation
E-H Field vectors
Cascading elements does not account for the non
transverse components correctly
43
Circuit Simulation Ground Rules - Cascading Rule

• 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
100 mils
Er3.8
100 mils
100 mils
Mated Connector
50 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