PRACTICAL DESIGN CONSIDERATIONS

Prof. George SlackLecturer, Electrical and Microelectronic Engineering

Blding 9, Office 09-3189(585) 475-5105

Rochester Institute of Technology79 Lomb Memorial Drive

Rochester, NY 14623-5603Copyright © 2012 Rochester Institute of TechnologyAll rights reserved.

input and output electronics

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SCHEMATIC - CHECKPOINTS

Step 1. Schematic Bullet List Plan

Step 2. Input Output Device Advice

a. Interface Specifications

b. Spec Sheets & App Notes

Step 3. Input Devices

Step 4. Controller and protocols

Step 5. Noise immunity

Step 6. Harnesses

Step 7. Switches/Connectors/ Pins/TPs/ LEDs

Step 8. Custom PCB versus universal prototype board.

Appendix

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Typical Example list: Microcontroller, Software,

OS needs. Data sample rate?

Frequency response?

Integrate / discrete components / hybrid

Warning: Don’t stop at the board’s edge! Remove the magic of Black Boxes Why? Neither you, your team nor faculty can help with a series of black boxes.

Step 1. Schematic Bullet List Plan

Circuit Board?• test points –LEDs, JTag, USB, RS232, (may not need the form factor)

accessory power connectors Power On switch(s), reset and safety switches, function select switch 3

STEP 2: INPUT OUTPUT DEVICE ADVICEa: Interface Specifications

Bandwidth, frequency response; rf to dc Data rate, Sample rate Protocol: RS232, I2C, CAN, Bluetooth, Zigbee, …. Analog versus digital (you should know by now)

• Need vs. Available: Resolution 8, 12, 16 bit

Circuit protection Power/ Current Voltage Levels: Vdd/Vcc, multiple? Level shifting needed? Devices (Vdd vary from 1.5 VDC to 18 VDC) Microprocessor I/O - GPIO output: Sink or source? Microprocessor I/O - Analog; ADC, DAC Data Controls: enables, select lines, etc. EMI Noise Heat

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Step 2. Input Output Device Adviceb. Spec Sheets & App Notes

The Devil is in the Details”Save Hours of design time! Exploit manufacture’s Design Notes!

1. Specification Sheets have application circuits that may apply directly to your design! Use them!

“Fail Early, Fail Fast”Good engineers don’t start from scratch!

2. Purchase device evaluation/ demo kits (Microcontroller, Motor controllers, sensors)

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3. INPUT DEVICES

Temp Probes, Thermocouples, Thermistors

Sonars

Hall Effect

Encoders

IR

RFID

Strain Gauge, Load Cells

Level Shifter

Pressure Transducer

IMU

H Bridge

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TEMPERATURE PROBES, THERMOCOUPLES, THERMISTOR

Thermo monitoring:

• One and 70 microvolts per degree Celsius (µV/°C) for standard metal combinations, air, gas surface.

• Low current, high impedance circuit needed.

• Tip types beaded, probe clip

• Review manufacture’s specifications

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SONAR(SOUND NAVIGATION AND RANGING)

Acoustic Device for measuring gases (i.e. air) and water.

Robotics; typically used to measure distance from a few inches to several meters.

Overview: Periodic send are receive cycles. Starts with a transmitted ultrasound ping, then waits for its echo. Calculates sound propagation. It’s that simple!

• Most sonars have internal electronics that translates the time propagation into a representative voltage level.

• Electrical Interface: This may be an analog value or digital via a serial interface.

• Environmental concerns: Whale activists accuse US navy submarines

to beach whales during practice events…

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HALL EFFECT SENSOR

Rotation Movement

• Uses internal magnetic fields.

• External rotation blocks and unblocks a metallic slotted plate which changes the magnetic field and creates high and low voltage levels.

• Like optical sensors, these are non-contact sensors. Both are accurate.

• Hall effect is better in extreme environmental conditions. Optical sensors need to stay clean. Hall effect relies on a magnetic field so oils, water, dirt, fumes, heat have little or no effect.

• Micro-amp power – may be passive (non-powered)

• CMOS IC subminiature sized for ease of mounting, inexpensive.

• Used in automotive, robotics, biomedical, industrial.

• Rotational or touch, touchpads.

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ANGULAR PLACEMENT: ENCODERS

• Optical , hall effect or magnetic, potentiometers

• Can measure rotation (i.e. cw, ccw)

• Can measure home position.

• Detailed theory of encoders:

http://www.automationdirect.com/static/manuals/d4hsc/appxa.pdf

http://www.lynxmotion.com/p-653-gear-head-motor-12vdc-301-200rpm-6mm-shaft.aspx

http://www.pololu.com/catalog/product/1442

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(Continued) ANGULAR PLACEMENT: ENCODERS,

Color Function

Black motor power

Red motor power

BlueHall sensor Vcc (3.5 –20 V)

Green Hall sensor GND

Yellow Hall sensor A output

White Hall sensor B output

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ANGULAR PLACEMENT POSITIONING: HALL SENSOR A AND B

This will tell the RPM speed. Often 400 pulses/ rev• A and B pulses are offset by 90°. This offers rotational direction of motor. That is, “A” lead edge is leading “B”, thus indicates CW direction. When “A” leads “B”, then CCW.

• Some encoders (not this one) have another sensor output (i.e. C) labeled “Home Position” which offers one pulse per revolution.

• Some encoders have serial interface and calculates RPM and periodically sends various parameters. 12

RFID –RADIO FREQUENCY IDENTIFICATION

Wireless Short Range Locator

How it works: RF unit transmits to listening devices (RFIDs) within its range. Once within range, this listening device will chirp back its ID.

Popular frequencies: 13 MHz, 2.4 Ghz. FCC compliant: < 80dBuA @ 13 MHz; 4W @ 2.4 GHz.

Uses: In pets, people, cards, badges, cloths, marathon runner shoe, UPS tracking, voter ID.

Readily available and inexpensive. SOC makes it reasonably simple.

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STRAIN GAUGE, LOAD CELLS

As the gauge bends or flexes, resistance changes. That simple!Output voltage range is typically in millivolts. Supply voltage is typically in Vcc range. Good linear response.

Metallic – shown to the right. Range from 30 to 3000 Ω.Wheatstone bridge circuit may be used for better precision and calibration.

Semiconductor – more reliable, more predictable

Piezoresistive – alternative. 14

LEVEL SHIFTER

• Needed for multiple voltage devices. That is, when mixing CMOS, TTL and other IC devices, the threshold voltage levels will be different.

• Don’t make your own, buy an IC to get the correct level thresholds.

• Independent of VCC and VDD.

• Offers precise switching thresholds.

• Check for mono or bi-directional needs.

• Some have fixed Vcc values some are not.

• Typical example - Single-Supply Voltage Translator

• 1.8 V to 3.3 V (at VCC = 3.3 V)

• 2.5 V to 3.3 V (at VCC = 3.3 V)

• 1.8 V to 2.5 V (at VCC = 2.5 V)

• 3.3 V to 2.5 V (at VCC = 2.5 V)

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PRESSURE TRANSDUCER

• Wide range of forces available.

• As low as finger touch, to load cell in tons.

• Measure liquid, gas, touch force.

• Piezoelectric materials is common.

• Recommendation: Know your environment, range & precision and go shopping! Circuits are simple and readily available.

• There are numerous manufactures. Start with Mouser, DigiKey, McMaster-Carr.

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IMUs AND ACCELEROMETERS

Inertial Measurement Unit

Measure acceleration and gyroscope

• serve as orientation sensors in the human field of motion.

• At the core of Segways, Airplanes, Ships, some robots.

• Supplies data to guidance controller.

• IMUs are “blind” and relies on previous calculations to know their current location. Periodic “zeroing” or dead reckoning is needed.

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H BRIDGE

H Bridge has two functions:

• Boost the power to rotate a motor

• Control direction DC motor

• Schematic offers forward and reverse motor direction.

• H-bridge got its name from its schematic shape. That is, it forms an H. Four switching elements at the "corners" of the H and the motor forms the cross bar.

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H BRIDGE

How H Bridges work

The switches are turned on in pairs, eitherhigh left and lower right, or lower left andhigh right, but never both switches on thesame "side" of the bridge. If both switcheson one side of a bridge are turned on itcreates a short circuit between thebattery plus and battery minus terminals.

Anything that can carry a current willwork, from four SPST switches, one DPDTswitch, relays, transistors, MOSFETs.

Brake mode. Upper or lower switchesclosed.

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H BRIDGESUPPRESSION DIODES AND ARCING

Why? Protect the four transistors.

How do Suppression Diodes help? First, note these diodes arereversed from normal power supply so no current will flow whenthe motor is off or when the motor is on.

The issue is when a motor switches off. Since internal electricalcircuitry of a motor is a coil of wire (i.e. inductor), current in aninductor needs to continue to flow. When the diagonaltransistor pair switches off and thus causing IC go to zero, themotor coils (i.e. inductors) are still charged with current andmust flow through the internal inductors in the motor. This iscalled Back EMF. Without suppression diodes, the current mustflow through the transistors which are switched off. Since thetransistors are off, the Back EMF will go in 100’s of voltages topush the current through the motor’s inductor circuitry. ThisBack EMG voltage is high enough to damage transistors. Sothe current path when the transistor pair switches off is throughthe motor (Back EMF), through the suppression diode to V+,through the power supply, through the Gnd, through the lowersuppression diode and then back into the motor. This currentsurge will end without damaging the electronics. Okay whencurrent flows into the positive lead of the power supply, thevoltage level will dip or perhaps even go negative for a shortperiod of time. You can trigger your scope to see this.Electronics such as a microcontroller can easily reset itself whenthis happens. Some times at random.

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H BRIDGEBUY OR BUILD H BRIDGE?

• Your project’s Needs!

• Cost?

• Reliability?

• Features?

• If you buy an H Bridge, there are 100’s to pick from! For your DC motor, know your voltage and current specifications. As an example of a small H bridge,

http://www.ti.com/lit/ds/symlink/drv8837.pdf

• This “small power” IC drives a 11V at 1.8A DC motor.

• Has a brake function, arc suppression (they all do….)

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STEP 4. CONTROLLERS AND PROTOCOLS

• Refer to Dr. Gomez’s Lecture.• WARNING: Arduinos – Good for component tryouts but

typically lacks data rate performance for projects. Good for quick circuit tryouts from motors to IMU’s but once integrated often lacks needed performance. • Not a resume builder at Career Fair. Viewed as a hobby toy. • Vast libraries and examples make it appealing.

• WARNING: Serial Interfaces - I2C, SPI, CAN may be difficult to implement. • Unless you have first hand experience or plan to purchase closely

integrated interface solutions. • If a serial interface is needed and the project data rate is “slow”, then

the old and trusted RS232 protocol is trustworthy.• Self defined protocols are lacking in needed data sync’ing. • For digital wireless select proven Bluetooth, Zigbee protocols. • Borrow a Protocol Data Analyzer – See Prof. Slack

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STEP 5. NOISE IMMUNITY

Know the project’s planned environment

Circuit noise immunity is the ability of a device or component to operate in the presence of noise disturbance .

Electro Static Discharge is the sudden discharge (i.e. transients, surge). To the circuit, this is a rapid high voltage, low current situation.

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NOISE IMMUNITY

lab misuse, ESD, unwanted signals

Fatal to Electronics:

• Senior Design Lab misuse, abuse.

• KEEP A CLEAN BENCHTOP…… Fasten boards and circuits to the bench! STOP and clean up! Get some sleep!

• End-user or consumer abuse and solutions:

• Bridge to block reverse polarity,

• Schottky diode: very fast switching times and low forward voltage drop. As low as 0.15 volts for low ma applications.

• Zener diode across the input.

• Circuit breaker – GFI

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NOISE IMMUNITY

optoisolator, optocoupler – input signal

The LED input circuit is isolated from the photo transistor, as shown.Slower transient pulses (> 0.1 ms) may still power the LED and thus pass a signal to the transistor side.

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NOISE IMMUNITY

optoisolator – driver side

Acrobat Document

Isolating noisy output

Typical Specifications:

Controlling or switching DC loads of 5-60 VDC. 4000 volts (transient) of optical isolation between the field devices and the control logic.

Typical uses and applications for DC output modules include switching the following loads:

DC relays DC solenoids DC lamps or indicators PLC logic

NOISE IMMUNITY

optoisolator, optocoupler – on-line signal & drive

Analog and Digital Optocoupler /OptoisolatorsAnalog Devices:

• http://www.analog.com (Analog Devices)

• High speed couplers - Analog Deviceshttp://www.analog.com/en/interface-isolation/digital-isolators/products/optocoupler_alternative_icoupler/fca.html

Low voltage 1.8 V isolators for bus or multiple I/O lines

• http://videos.analog.com/video/1553875079001/ADuM348x-First-Isolator-to-Interface-to-18V-IO/

• http://www.analog.com/en/prod/0,2877,ADuM2401,00.html

• http://www.vishay.com/optocouplers/

Solid State Relays

• http://www.vishay.com/solid-state-relays/ ;

• http://www.vishay.com/docs/83820/lh1521ba.pdf

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NOISE IMMUNITY

POLARITY PROTECTION

+

Input Port–

(+)

(–)

+

Input Port

–

1N5822 or

1N5817

1N4001

or

Schottky

Schottky diode

Schottky Low forward voltage drop

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NOISE IMMUNITY

OVER-VOLTAGE PROTECTION

Fuse

1N5339 (5.6V for a 5.0V input)

+

input port–

(+)

(–)

Zener diode

(also MOV)

Helps against transients and over voltage protection. Unfortunately fuses may not be sufficiently fast to protect your circuit. Understand the incoming environmental conditions.

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NOISE IMMUNITY

TRANSIENT PROTECTION FOR DIGITAL INPUTS

Many digital devices including microcontrollers include diodes on their GPIO to protect from transients. Check the manufacturer’s spec sheet.

Add if transients may be an issue to protect CMOS inputs (or any input circuit).

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NOISE IMMUNITY

A SOLUTION: LOWPASS FILTER

Remove high frequency noise. Simple RC low pass filter (left) or active circuit (right) for improved isolation, gain and current driving capability. May want to add software sampling to remove glitches or switch bounce.

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NOISE IMMUNITY

CMOS AND NOISE

• 1 uA input? Not sure? Get the spec sheet!...

• Draw the equivalent circuit for each internal pin. That is, CMOS versus TTL input impedance, output is pull-up or pull-down circuit, current limiting resistor value.

• CMOS inputs have very high input impedance which is good for low power consumption but susceptible to sudden discharge or misuse when connecting to the outside world.

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NOISE IMMUNITY

DECOUPLING CAPACITORS

• Know your performance response needs.

• Try to eliminate frequencies outside your performance needs (i.e. filtering, smooth out spikes in DC power of IC’s).

• Digital Isolation Caps:

• Common practice is to isolate digital IC’s with isolation capacitors. See manufacturer’s Spec Sheet for capacitor values. Be sure to add to your schematic prior to Design Review.

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STEP 6 HARNESSESANALYSIS HARNESS SIGNAL RESPONSE

Research you max data rate, distance, noise, parasitic capacitance, reflections, distortion spec, etc.

Frequency Response for both: Digital rise time to support data rate and if analog max frequency and distortion needs.

• DC to 100 khz Open Wire • DC to 40 MB/s Ribbon Cable (less than 3’)

• SCSI, SPI-3 applications• DC to 300 mhz Twisted Pair - unshielded, ribbon• DC to 100/1000 MB/s ethernet Cat 5 minimum

spec. RJ45 connector.• Coax, VHF 3000 megahz (scope probe, cable

TV) attenuation, reflectance, Cable TV, Rf• DC to 4 gigahz Fiber Optics

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STEP 6 HARNESSESHARNESS -TWISTED PAIR

When purchasing harness cable:

• Unshielded Twisted Pair (UTP)

• Shielded Twisted Pair (STP) • Purchase or make your own! To make your own, start with 10’ to 50’ of wire and wrap with an electric drill to get the desired wrap.

Study the manufacturer’s Specification to match to your needs.

Practical Design Considerations – What are twisted pairs? http://www.cirris.com/testing/twisted_pair/twist.html

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TWISTED PAIR

What is does: Cancels out crosstalk from neighboring wires and electromagnetic interference from external sources

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PRACTICAL CONSIDERATIONS

1. Current, Voltage Needs• Gauge of wire, insulation

2. Customer’s Use and Abuse • Connect/ disconnect needs • Routing, protecting• Mounting, vibration and stability• Strained or solid• Gold versus Tin; environment, corrosion, current rating• Instrumentation quality for long life

3. Color Coding for ease in Debug and future use.As an example;1. Red Vcc/Vdd2. Black Grd/ Vss3. Orange, color by major node.

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STEP 7 CONNECTORS/ PINS/ TP’S/ LEDS

Add all needed I/O connectors to your schematic(s).1. Connect/ disconnect needs – screw terminators,

push type 2. Major Manufactures: Amp, Molex3. Location

• between assemblies• interface to other projects (collaborate with other teams) • instrumentation

4. Get crimp compatibility to pin manufacturers (style and gauge).

5. Pins: crimp versus solder6. Current rating rule of thumb is 10x. i.e. 100 ma

purchase 1 amp pins7. Types; ribbon, D shell, PCB mount

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• Start your schematic(s), if not already. • Sketch a drawing or visualization your project including the

following:

• Define I/O Design Needs

• Specification Sheets & Application Notes

• Analysis I/O

• Design Output Devices

• Design Input Devices

• Consider Noise immunity

• Harness; Signal Response

• Connectors/ Pins

Next Steps:

• Define your control electronics and software modeling, subsystem lab tryouts and code development

Summary:

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ELECTRONICS DESIGN BOOKS(NOT CLASSROOM TEXT BOOKS)

Electronic Instrumentation Design – Kim R. Fowler, Oxford, ISBN 0-19-508371-7

• Given your project budget, you may want to consider purchasing this text. Less than $10 used on Amazon.

A Baker’s Dozen – Real Analog Solutions for Digital Designers. Bonnie Baker. Newnes, ISBN: 0-7506-7819-4. $70 on Amazon.

• Best integration of analog and digital electronics. Approaches design from an engineer’s “problem to be solved” rather than text book “solve this equation”. Presents the problem and then goes into the needed electronics and mathematics to solve your design issue. Engineering “humor” throughout….

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APPENDIX

More resources and notes

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Step 1. Schematic Bullet List Planbest practices, fine print…

Some example areas: Develop Engineering Specifications from your general specifications table

Start at the interfaces, then drill down.

List Interface Devices

List ALL inputs and outputs

Research dynamics for electronic devices

I2C, RS232, 24 VDC motor @ 20 amps, MSP430.

Can you sketch the internal device electronics at the interfaces to complete a circuit diagram?

Detailed Design Review readiness: don’t stop at the board’s edge! Scan or copy key pages from the

manufacturer’s specifications sheets.

Don’t forget the “simple” stuff!

Power On/Off switch?, reset and safety switches, function select switch

Add Connectors, cables, harnesses!

test points – you will thank me later!

LEDs (more is better)

JTag, USB, RS232, (may not need the form factor)

accessory power connectors

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Step 2. Input Output Devices

Where to start?1. Google search

2. Wallace RIT Library http://library.rit.edu/

3. RIT IEEE http://ieeexplore.ieee.org/Xplore/home.jsp

4. Manufacturer’s: TI, Freescale, Analog Devices, ….

5. Suppliers: DigiKey, Mouser and Allied offer good on-line spec sheets

6. http://www.findchips.com/

If the spec sheet document is large, send me the link and I will print for you. Some may be 400 pages but DO IT! You will thank me later….

Step 2. Input Output DeviceTypical Output Devices

Matching (Voltage, Impedance, Current)

Discrete Components

• Bipolar Transistor: Single stage and Darlington.

• FET

• JFET

• MOSFET

• IGBT DC to AC inverters (motors)

DC to DC converters

• voltage boasting/ ± polarity

• current boasting

• buck/ boost44

Step 2. Input Output Device Advice

1. Impedance matching to your control electronics

Outputs• GPIO Logic (VOH and IOH) converted to either a lower or higher power range

• DAC (Vcc) active range to either a lower or higher power range

Inputs• GPIO Logic (CMOS gate impedance and protective devices)

• ADC (Vcc) active range

2. The basics! Apply to Thevenin Equivalent Circuits• Source electronics – Input interface• Device being driven – Output interface• R, RL, RC, RLC loads (& potential for unwanted current and voltage

spikes)

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STEP 5. NOISE IMMUNITYHOW DOES NOISE GET INTO ELECTRONICS?

• ground connections and loops

• power supply connections

• high impedance signal inputs

• inadvertent ESD

• (human touch, lightning)

• Reverse current surges via inductive devices (motors, solenoids, inductive devices)

• rf emissions

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STEP 5. NOISE IMMUNITY CONTINUED

HOW DOES NOISE GET INTO ELECTRONICS?

Energy Coupling

(Conductive, Inductive, Capacitive)

• EMI - Current surges

(ElectroMagnetic Interference) An electrical disturbance in a system due to natural phenomena, low-frequency waves from electromechanical devices or high-frequency waves (RFI) from chips and other electronic devices. Allowable limits are governed by the FCC.

• RFI – high impedance devices requiring very limited current.

(Radio Frequency Interference) High-frequency electromagnetic waves that emanate from electronic devices such as chips.

If the source is sufficiently strong this can enter your circuit.

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STEP 5. NOISE IMMUNITYdesign solutions

Protect from?

• ESD

• Solution: Proper grounding, optical isolators, capacitors

• Transients• Solution: Low pass filter, optical isolators, software sampling

• Reverse polarity

• Solution: Diode circuits to protect against, fuse.

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