chassis grounding

chassis grounding

Grounding

Demystified

3 - 1

Importance Of Grounding Techniques

45

40

35

30

25

Grounding 42%

Case Shield 22%

Cable Shielding 18%

20

15

10

5

0

Filter

12%

PCB 6%

Filtering 12%

PCB Layout 6%

Percent Used

• Categories of solutions applied for compliance.

3 - 2

Ground Systems

• Grounding concepts take more time to understand than any other EMI concept because,

• Ground systems have diverse requirements and sometimes they appear conflicting.

3 - 3

Examples:

1.

Can we connect signal ground to chassis?

2.

Should you connect the cable shield to chassis?

3.

You must avoid a Ground Loop.

3 - 4

What Is Grounding?

• Connecting all grounds in the system in a manner such that all of the objectives are met.

• What is ground?

• Ground wire

• Zero volts

• Ground plane

• Signal ground

• Chassis ground

• Conductive paint

• A trace on PCB connecting chassis

3 - 5

Two Approaches to Limit the Noise

1.

Stop the noise escaping the system

2.

Stop at the circuit

• Often you use combination of the two.

3 - 6

Noise Generation In a Digital System

I/O cable

Power Cable

Noise source

• In a digital system noise is generated by circuits.

3 - 7

Faraday Cage

• Faraday cage is the quiet RF reference.

• The chassis can be the Faraday cage if-

1.

It encloses the electronics.

2.

It is several times thicker than the skin depth.

3.

No conductor violates the Faraday cage.

4.

Large openings are avoided.

3 - 8

Chassis Ground

Capacitor

Printed Wiring Board

Mounting Hole

• A direct low impedance connection to chassis is important.

• A low cost approach is to use the mother board mounting pads.

• Capacitive connection can be made to the signal ground at several points if single point ground is to be maintained.

• The effective capacitor leads must be short.

3 - 9

Connection To Chassis

Card edge guides

Connector bonded to system enclosure

Motherboard

• Another economical way to make chassis connection is by means of connector body- such as D connectors.

• DIN connectors are available with shield and spring contacts for easy and reliable contact to the chassis.

• Power line filter body should also be used for chassis connection.

3 - 10

Reflected wave

Wave Propagation Through A Shield

B1 B2

• E

0

= Incident field strength

E

1

E

2

Transmitted wave

E

3

• R

1

= Reflection loss at B

= 20 log (E

0

/E

1

)

1

• A = Absorption loss

= 20 log (E

1

/E

2

)

• R

2

= Reflection loss at B

2

= 20 log (E

2

/E

3

)

• Total shielding effectiveness

Eo = Incident wave

• = E

= R

1

0

/E

3 x A x R

2 x C m

3 - 11

Absorption Loss

• Absorption loss occurs due to induced currents

• The field decays with distance (d) traveled

• The decay is exponential, and is dependent on skin depth δ

• Skin depth depends on

• µ= Permeability

σ= conductivity

• ω= Angular frequency of the wave

• δ= 2/(µωσ)

3 - 12

Grounding Considerations

• System performance: system must perform reliably.

• Safety of personnel: minimize electrical shock hazard.

• AF noise emissions and susceptibility.

• RF noise emissions and susceptibility.

• ESD immunity.

• Generally, the noise emission and noise susceptibility approaches are similar.

3 - 13

Grounding For Personnel Safety

• The main concern is that the metal enclosure remain at “safe” potentials.

• So it should be connected to the green wire ground of the power cord by “reliable” means.

3 - 14

Grounding For Safety

CHASSIS

115 VAC

Equipment Circuit

NEUTRAL

GROUND

• Safety ground wire is connected to the chassis.

• EMI filter capacitors are limited by leakage current

-UL 1950 or IEC 950 - 3.5 mA

-Medical devices - in micro-amps.

3 - 15

Ground Definitions

--- Based on Purpose

:

• General - Equipotential reference surface.

• EMC - Low effective impedance path for the return.

• ESD - Surface that can source or sink large amount of charge without changing its potential.

• Safety - Conductor providing a path for currents to flow during a circuit fault.

3 - 16

Ground Design Objectives For EMC

• Minimize Cross- talk.

• Minimize Emissions.

• Minimize Susceptibility.

• One must consider signal characteristics as well as allowable noise levels when designing a grounding scheme.

3 - 17

Ground System Considerations

• There are four important circuit characteristics to be considered during the design of ground system:

1.

Frequency of signal: Digital signal is broadband.

2.

Effective Impedance of path: not the resistance.

3.

Current Amplitude: The voltage drop is proportional to the signal current.

4.

Noise voltage threshold: The noise level that a circuit can withstand or generate.

3 - 18

Avoid a Ground Loop

A B

• If a ground connects point A to B, it should not have an alternate path.

3 - 19

Ground Loop

I n

= Induced noise current

V n

=Noise voltage

V n

= I n

X R s

V

S

R

S

I n

+

V

IN

-

• Definition : A ground circuit allowing ground currents to flow in a loop causing two problems.

1.

Induced noise voltage: magnetic coupling causes induced current resulting noise voltage.

2.

The return current may take a path further away from the signal current and create a radiating loop.

3 - 20

Low Frequency Grounds

-Separated According to Circuit Noise Levels

Chassis

Ground

High

Noise

Ground

Low Noise

Analog

Ground

Digital

Ground

Single Point

• Chassis ground normally carries no current.

• This arrangement avoids ground loops.

• Noise coupling by conduction is avoided.

• Chassis is connected to power ground for safety.

• It carries current only in fault condition.

3 - 21

Typical Single Point Grounding

-for Low Frequency

System Chassis

Relays/Solenoids

Motor Drivers

Digital Logic

115 Volts

Analog Circuits

Power

Supply

Neutral

Ground

Digital Logic

• This grounding is inadequate for RF signals between the boards.

3 - 22

Ground Systems For Signal Currents

• Single point ground

• Series or Parallel ground connection

• Multi- Point Ground

• When signal spectrum contains high frequency energy.

• Multi- Point AC Ground

• When low frequency and high frequency is present.

• These ground systems are selected based on the frequency of signal and noise.

3 - 23

Series Ground Connection

Circuit

1

Circuit

2

Circuit

3

SPG

Z1

I

1

+ I

2

+ I

3

Z2

Z3

I

3

I

2

+ I

3

• Question :

• when do you connect ground in this manner?

3 - 24

Parallel Ground Connection

Circuit

1

Circuit

2

Circuit

3

Z1

Z2

I

1

Z3

I

2

SPG

I

3

• Q: When do you connect ground in your system in this manner?

3 - 25

Multi-point Ground Connection

Circuit 1 Circuit 2 Circuit 3 Circuit 4

Low impedance ground reference

• Definition: circuits are connected to a reference ground plane at several different points by low impedance connections.

• The low impedance, single reference ground replaces the SPG, when we add a ground plane on the PWB.

3 - 26

A

D

Problem With SPG

B

E

C

F

It ignores signal connections!

G

• With the SPG, the signal circuit has magnetic loop coupling:

• These are formed by signal conductors and all ground paths returning through SPG. The coupling increases with frequency.

3 - 27

A

D

Solution

B

E

C

F

G

• Provide ground paths close to the signal connections.

• This parallel path can be: (a) Twisted conductor with each signal

(b) coaxial cable shield or (c) a conductor in the ribbon cable.

• Should you worry about the ground loop? Not for RF designs.

3 - 28

What Is The Return Current Path?

Multi-layer board with ground plane

• Choices:

• Return current takes path of the lowest resistance.

• Return current is distributed inversely proportional to the resistance of each path.

• Return current takes path of the lowest impedance.

3 - 29

Return Current Division

• Current is divided : (1) shield and (2) ground plane.

I

L

I

L

I

SHIELD

R

L

I

GROUND

Equivalent Circuit- Assuming ground resistance = 0

L

SHIELD

M

R

SHIELD

I

SHIELD

I

L

-I

SHIELD

R

L

I

L

3 - 30

Single Point Or RF Grounding

• Grounding scheme is chosen according to requirements. The RF and AF requirements are not contradictory.

• When low (audio) frequency and high (RF) frequency protection is required, use multi- point AC ground with only one DC connection.

• Separate grounds according to signal levels - since induced noise can affect signal only if ground loop is part of the signal circuit.

3 - 31

Read

Head

Ground and Signal Go Together

V

TH

Shift Register

AMP AMP

COMP

Digital circuit

• Keep ground with the signal when connecting different circuits.

• Ground is the return path for the signal and power current.

• This rule is very important - when we are breaking ground loop.

3 - 32

L

L

Transmission Line

L L

C

L

C

L

C

• Distributed parameters, and characteristic Impedance.

• Reflections can be controlled by controlling the impedance.

• The transmission lines used in practice are not ideal.

For example, the distributed parameters include resistors attenuating the propagating signal.

3 - 33

Layer Stacks For Four Layer PCB

or

Signal

Ground

Power

Signal

Ground

Signal/Power

Signal

Ground

• Would it help to put the ground layers on the outside surface?

• How useful are high frequency signals embedded into the ground and power planes?

3 - 34

Mother board

Large Loops In Signal Return

Paths

Daughter

Board

• Even with a ground plane in the PWB, a large loop in the signal path can exist.

• A return pin far away from signal pin will cause a loop.

• Large loops in signal return paths can be avoided by using distributed grounds.

3 - 35

Layout Near Board Edge

Field lines Signal traces

Ground plane

• Fringing near edge changes the characteristic impedance of the signal.

• This can result in ringing and additional radiation for high frequency signals.

• The advantages of the ground plane may be lost completely, if traces are laid outside the ground plane boundary.

3 - 36

Six Layer Board

Layer 1 = signal

Layer 2

Layer 3 = signal

Layer 4 = signal

Layer 5

Layer 6 = signal

Ground Plane

Power Plane

• The ground layer is two and power plane is five.

• The distance between signal layers and the reference planes should be maintained constant, say X.

• The distance between layer three and four > 3X.

3 - 37

Summary

• Chassis ground is important for RF.

• Consider Signal loop more important than ground loop – look at ground as return path.

• Transmission line is your goal when you add ground and power planes on PCB.

3 - 38

Was this manual useful for you? yes no
Thank you for your participation!

* Your assessment is very important for improving the work of artificial intelligence, which forms the content of this project

Download PDF

advertisement