Noise and Other Considerations in InterfacingConsiderations in Interfacing

to Sensor Devices

CSE 5243/4342

Embedded SystemsEmbedded Systems

• Most real-time embedded systems areMost real time embedded systems are required to interface one or more sensors to microcomputer systems.p y

• Improper connections of these sensors can render the data useless or at best difficult to interpret by the software.

• Good physical as well as proper grounding p y p p g gare essential for both safety and signal integrity

CSE 5343/4342

Connection StandardsConnection Standards

• Even for connections that look deceptivelyEven for connections that look deceptively simple improper connections can introduce excessive noise lower reliabilityintroduce excessive noise, lower reliability, and serious hazards.

• Standards exist for some important types• Standards exist for some important types of interconnections, particularly to insure safely and reliablysafely and reliably.

CSE 5343/4342

Signal ConnectionSignal Connection

• Standards also exist for minimizing theStandards also exist for minimizing the effect of noise, but in some cases, proper grounding and shielding is an art and onlygrounding and shielding is an art and only properly accomplished with experienced technicians or engineerstechnicians or engineers.

• Interconnections are often a major source of noise in a computer systemof noise in a computer system.

CSE 5343/4342

Ground LoopsGround Loops

• Ground Loops occur when there are two or more ground paths between two modules

signal

ground

B

0 volts –iground

CSE 5343/4342

main ground

Proper Chassis GroundingProper Chassis Grounding

• Consider the signal ground in two chassisConsider the signal ground in two chassis connected to the electrical ground of the AC system through the ground prong ofAC system through the ground prong of the AC connector. If the chassis are located in the same cabinet or are in closelocated in the same cabinet or are in close physical proximity, then it is possible to make a safe electrical connection betweenmake a safe electrical connection between the chassis ground points on the two chassischassis.

CSE 5343/4342

Grounds between ChassisGrounds between Chassis

CSE 5343/4342

Connect Chassis Grounds only if S V l P i lat Same Voltage Potential.

• Grounds between chassis can be tied together gfor close physical proximity where the chassis are essentially at the same potential ( no current flows through the conductor connecting toflows through the conductor connecting to ground points).

• For large physical distances The ground pointsFor large physical distances. The ground points are from different AC connectors. Even when the AC connectors are in the same room, they can b t ll d b diff t i it b k thbe controlled by different circuit breakers or the ground conductors may be separate and current could flow between chassis.cou d o bet ee c ass s

CSE 5343/4342

Grounds Between BuildingsGrounds Between Buildings

• For cases where the two chassis lie in twoFor cases where the two chassis lie in two different buildings, and the grounds are not connected directly together by a y g yconductor, but rather are each individually connected to an earth connection in their

ti b ildi Th th idrespective buildings. The earth provides a path for electrical flow between the grounds In this instance the groundgrounds. In this instance the ground potentials to which the chassis are attached may differ by several voltsattached may differ by several volts.

CSE 5343/4342

Grounds Between Buildings –CCont.

• Ground loops can exist running from the grounding point f Ch i A th h Ch i A t Ch i B th tfor Chassis A, through Chassis A to Chassis B, then to B's grounding point, and from there back to the ground point for A.

• For cables with alternating current in this loop, causes a changing field that induces a potential across the ends of the loop (between the two grounding points) and thus athe loop (between the two grounding points), and thus a current will flow in the loop.

• Since the impedance of the loop is low, a few volts potential difference can produce several amperes ofpotential difference can produce several amperes of current, which could cause damage to electrical components.

CSE 5343/4342

Grounds Between Buildings –CCont.

The close physical proximity between b and a in p y p yslide 7 insures that the ground loop is not threaded by large numbers of power cables or possible lightning strikes However it can bepossible lightning strikes. However it can be threaded by the equipment power cords and signal conductors in the interface cabling, each g g,of which induces a small residual current flow around the ground loop. This flow can create a small voltage difference between the grounds insmall voltage difference between the grounds in the two chassis that can result in noise voltage thus affecting signal voltage.

CSE 5343/4342

Grounds Between Buildings –CCont.

• Do not arbitrarily connect equipment grounds y q p gtogether. As ground points may not be at the same potential. (**FOLLOW ALL ELECTICAL CODES**))

• Even Potential differences between grounds on printed-circuit boards within a chassis can result in instantaneously exceeding logic thresholdsin instantaneously exceeding logic thresholds. From chassis to chassis, improper cabling can lead to ground potential differences that have the shape of short signal pulses They can resultthe shape of short signal pulses. They can result in false triggering of logic gates, or false signal oscillations.

CSE 5343/4342

Effects of Noise on ComponentsEffects of Noise on Components

• Computer components are susceptible toComputer components are susceptible to electronic interference on interconnecting cables AC power lines or from noisecables, AC power lines, or from noise sources in the interior of equipment. There are basic rules for shielding groundingare basic rules for shielding, grounding, and cabling for minimizing the noise couplingcoupling.

CSE 5343/4342

Grounding Components in a g pModule

CSE 5343/4342

Shielding a Circuit Prevents Potential Differences within the Shield due toDifferences within the Shield due to

Outside Noise

CSE 5343/4342

Rule 1: Connect the shield of electronic i it t th i l dcircuit to the signal ground.

CSE 5343/4342

When Shield Grounds are DiffDifferent

Wrong Way –g yGround Loop On Signal Ground

Correct Way for Short Distances

Rule 2: The shield should be connected to earth ground at the zero-voltage reference point for the input signal.

Improper transmission of signals over l di tlarge distances

Can use Fiber optic link, optical isolator or balanced (doubled ended) electrical connection

CSE 5343/4342

electrical connection

Balanced Connections for R d i N iReducing Noise

CSE 5343/4342

Using Balanced Transmissions with T i d P iTwisted Pair

CSE 5343/4342

Other Design Considerations

CSE 5343/4342

Improper Grounding Can result in False Signals

In next slide, the signals being sent to a bus are enabled via a tri-state driver enableare enabled via a tri-state driver enable, where each driver can sink x mils. When gated on, and say all sink currents occur at the g , ysame time (0 output), the ground line carries a transient current that might move above ground causing the tri-state enable control to fall below its enable threshold. The signals can oscillate until the control returns to its

CSE 5343/4342

can oscillate until the control returns to its proper level.

Improper Grounding Can result in F l Si lFalse Signals

CSE 5343/4342

Preventing Cross-TalkPreventing Cross Talk

CSE 5343/4342

T i i LiTransmission Line Considerations – ReflectionConsiderations Reflection

Problems Can Begin to Occur at Si l 10 Mh d hi hSignals 10 Mhz and higher

CSE 5343/4342

Design Considerations for Transmission of Higher

Frequency signals (Point toFrequency signals (Point to Point)

Z0 is the wave impedance of the line and a functionof the line and a function of L and C ( values per unit length)

Line ImpedanceLine Impedance

CSE 5343/4342

Reflection CoefficientReflection Coefficient

VR/V0 = Reflection Coefficient =VR/V0 = Reflection Coefficient = (ZL-Z0) / (ZL + Z0)

Matching Line Impedance

Note: When load impedance, ZL =Note: When load impedance, ZL wave impedance, Z0 Reflection

Coefficient = 0CSE 5343/4342

Coefficient = 0

Voltage Wave Form when Line gterminated with matched

impedance at Source Endimpedance at Source End

CSE 5343/4342

Voltage Wave Form when Line terminated with matched impedance at Far Endimpedance at Far End

CSE 5343/4342

Voltage Wave Form on Line When Neither Source nor Far

End Impedance MatchingEnd Impedance Matching

CSE 5343/4342

When several Taps on the data, waveform becomes complicated

CSE 5343/4342

SummarySummary

• Avoid ground loops• Following Rule1 proper grounding of shield in modules to prevent feedback and Rule 2in modules to prevent feedback, and Rule 2 shield connected to earth (or main ground) at zero-voltage reference points.zero voltage reference points. • Use Balanced Transmissions, optical isolators, or fiber optics when sending signals long distances.• For higher clock and signal speeds beware of reflections

CSE 5343/4342

of reflections.