The myth about ideal ground

Zhiping Yang, Ph.D.Google

Quiz

Which package model is the best at high frequency?

L_PWR

L_GND

L_sig1

L_sig2

L_PWR

L_GND

L_sig1

L_sig2

L_PWR

L_sig1

L_sig2

M12M12 M23

M13

M23

M13

M24

M14M34

A B C

Content

● What are the myths about ground

● History of ground in electrical design

● Terms related to ground

● A package example with signal and ground nets only

● A package example with ground, signal and power nets

● S-parameter and W-element models in high-speed modeling and simulation

● Conclusions

What are myths about ground

● Myth #1: Does ideal ground really exist? If not, why do we use it in modeling and simulation?

● Myth #2: Should we put partial inductance and resistance on the ground net? (i.e. is ground bounce real?)

● Myth #3: Should I have a different reference node for each port of W-element or S-parameter model?

History of ground in electrical design

● Ground was first used by telegraph system to reduce wiring cost. (Discovered by German scientist Carl August Steinheil in 1836-1837)

●

History of ground in electrical design

● Later, power engineers used the ground as return path in Single Wire Earth Return (SWER) electrical distribution system

● The ground is connected to protective earth (PE) or Neutral (N) conductor of modern power systems for prevention of electrical shock (Safety) and fault detection and protection

● The ground of circuit boards and IC chips are typically connected to the metal box and ground of AC power systems

History of ground in electrical design (continued)

● The ideal ground concept (zero voltage and infinite current sink capability) was introduced to analog and digital circuits to simplify circuit schematics and analysis.

● The ideal ground node was used in SPICE-type simulators to solve KCL and KVL more effectively.

● At high frequency, the concept of local ground/reference was used to get more precisely and meaningful results

One of the unwanted misimpressions from ideal ground node: The voltage signal can travel from driver to receiver on a single wire.

A true story about ground

Daddy, what are they for?

This is…, so you'll be grounded...

Cisco Lab

Grounded! GROUNDED? What did

I do wrong?!!

Terms related to ground

● Global Ground, Ideal Ground and Virtual Ground

● Local Ground or Local reference

● Self Loop Inductance, Mutual Loop Inductance, Partial Self Inductance, Partial Mutual Inductance, Partial Loop Self Inductance, and Partial Loop Mutual inductance.

● Ground Bounce, Power Bounce and Voltage Bounce.

“Ground is a place where potatoes and carrots thrive”—by Bruce R. Archambeault, PCB Design for Real-World EMC Control

A package example with signal and ground nets only

Die side

PCB side

i2

i1

L1

W1

a1b1

b2

d1c1

a2

c2 d2Cutout on ground plane

W1

H1T2

S1

W2

A valid circuit model at low frequency

Good things about this model:1. It is valid at very low frequency at which current is evenly distributed

across the ground plane and c1 & c2 ( d1&d2) have the same voltage potential.

2. You can actually probe the voltage across any two points in this circuit and it is meaningful and could match the measured data very well, so ground bounce (voltage difference between c1/c2 and d1/d2) is well defined.

Die side PCB side

R1a1 b1

R2a2 b2

i2

c1 d1i1+i2

c2 d2Rgnd

L2

L1

Lgnd

M1

M2

M3

i1

What could change at high frequency?

Die side

PCB side

i2

i1

L1

W1

a1b1

b2

d1c1

a2

c2 d2Cutout on ground plane

W1

W2

H1T2

S1

Is previous low-frequency model still valid at high frequency?

Die side PCB side

R1a1 b1

R2a2 b2

i2

c1 d1i1+i2

c2 d2Rgnd

L2

L1

Lgnd

M1

M2

M3

i1

What could be wrong with this model?1. It is clearly shown that the return current for conductor 1 and 2 are not the same.

How can Rgnd and Lgnd be well defined?2. How to separate the L1, Lgnd and M1 from the physical loop inductance? No

unique definition for L1, Lgnd and M1.3. At high-frequency, when wavelength is close to the structure size, (such as L in the

previous figure), the voltage is not well defined between die ground and PCB ground. Local reference voltage definition is the only meaningful voltage at high frequency.

A working circuit model at high frequency

Compared with the previous model, the above is a much better and accurate model. The only drawback is too many components (especially partial and mutual inductances) and these inductances are not well defined. Since we only care about the voltage with local reference, this model can be further simplified by model reduction.

Die side PCB side

R1a1 b1

R2a2 b2i2

c1 d1

i1

c2 d2

Rgnd1

L2

L1Lgnd1 M1

M2Rgnd2

Lgnd2 M3

i2

M4R12

A Simplified and better model

Die side PCB side

R1+Rgnd1

a1 b1

R2+Rgnd2

a2 b2

i2

c1 d1i1

c2 d2

L2+Lgnd2-2*M2

L1+Lgnd1-2*M2

i1

M3+M4

i2

R12

This model is much simpler and it is accurate from DC to very high-frequency. All components in this circuit are well defined and have unique physical meanings.

The ground nodes c1/c2/d1/d2 could be shorted together (by doing so, it loses the mapping between physical locations and circuit nodes.

A package example with power, signal and ground nets

Die side

PCB side

i2

i1

L1

W1

a1b1

b2

d1c1 q2

c2 d2

Cutout on ground plane

q1

p1 a2 p2

W2

H1T2

S1Ground

VDD

VDDQ

Which package model is better?

Die side PCB side

i_s1

R1’a1 b1

R2’a2 b2

i_s2

c1c2

p2i_vdd

q1 q2

L2’

L1’

Lp’

M1’

M2’

Rq’Lq’

M3’

M4’

Rp’p1

d1d2

i_vddqM5’

M6’

i_gnd

Die side PCB side

R1

a1 b1

R2

a2 b2i_s2

c1

c2

p2i_vdd

q1 q2

Rgnd

L2

L1

Lp

M1

M2

i_s1

Rq

LqM3

M4

Rpp1

d1d2

Lgnd

i_vddq

M5

M8

M6

M7M9

M10

i_gnd

a. Package model with ground parasitic

b. Package model without ground parasitic

R12

R23

R34

R13

R24R14

S-parameter and W-element model in high-speed modeling and simulation

i2

i1

L1

W1

a1b1

b2

d1c1

a2

c2 d2Cutout on ground plane

Port #1

Port #2

Port #3

Port #4

S4P S-parameter file

Ref

a1

a2

b1

b2

a. S-parameter file connection case #1

S4P S-parameter file

a1c1a2c2

b1d1

b2d2

b. S-parameter file connection case #2

More complicated port settings

i2

i1

L1

W1

a1b1

b2

d1c1

a2

c2 d2Cutout on ground plane

Port #1

Port #2

Port #3 is a differential-mode portPort #4 is a common-mod port

S4P S-parameter file

Ref

a1

a2

dif

com

a. S-parameter file connection case #1

S4P S-parameter file

a1c1a2c2

difref1mmcomref2

b. S-parameter file connection case #2

Suggestions for the application of S-parameter model

● Do not add circuit components between positive port terminals if their negative terminals are not defined on the same surface and conductor(s).

● It is better to have common reference conductor(s) for all ports located on the same surface.

Quiz

Which package model is the best at high frequency?

L_PWR

L_GND

L_sig1

L_sig2

L_PWR

L_GND

L_sig1

L_sig2

L_PWR

L_sig1

L_sig2

M12M12 M23

M13

M23

M13

M24

M14M34

A B C

Conclusions● Ground bounce and power bounce are only valid terms

at low frequency.● Voltage bounce is more precise and should be used at

high frequency.● Partial loop inductance and partial mutual inductance

are well defined and have unique values.● Package model without parasitic in ground net is not

only valid, but also more computationally effective and accurate.

● S-parameter based model should be connected with other circuit elements in ways which are consistent with the port definition when it was extracted.

Q&A

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