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Noise Immunity
Transients and ESD
Practical Design Considerations
prepared by Prof. George Slack (EE)
Copyright © 2006 Rochester Institute of Technology
All rights reserved.
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noise immunity
• 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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When is Noise Immunity design
needed?
• Senior Design I
– Paper Design wks 1-8
• Senior Design II
– Prototype device testing wks 1- 3.
– Pre-integration testing with lab equipment DUT
(device under test). Wks 3- 6
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Where does noise get into
electronics?
•
6.4.1 Sources *
•
•
•
•
through ground connections and loops
through power supply connections
through signal inputs
through inadvertent ESD
– (human touch, lightning)
• through Inductive devices (motors)
•
Electronic Instrumentation Design, Kim R. Fowler
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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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ESD and unwanted signals
• Fatal to Electronics:
• Inadvertent user misuse.
• Extreme cases of user 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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Bare exposure to a CMOS gate
Reduce Transients
Protect from?
• Misuse during debug and testing.
• ESD and reverse polarity
– Solution: Diode circuits to protect against
• reduce transients
– Component Specification Sheets
– Solution: Low pass filter
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Polarity Protection
(+)
+
Schottky diode 1N5822 or
1N5817
–
(–)
1N4001
Input Port
+
Input Port
–
see 7.4.1 page 238, MOSFET solution
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Over-voltage Protection
Fuse
(+)
+
input port
–
(–)
Zener diode
1N5339 (5.6V for a 5.0V input)
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Over-voltage Protection for
Digital Inputs
Vdd
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Minimize Bandwith
One solution: Lowpass Filter
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Knowing Your Design Specification
1. If considering various output devices, review various
manufactures specification sheets .
2. Current (i.e. CMOS versus TTL ).
3. Control Voltage (Vdd vary from 1.5 VDC to 12 VDC
typically)
4. Needed minimum Signal Transient Response.
5. RLC loads (dangerous current and voltage spikes)
6. Apply to Thevenin Equivalent Circuits
– Source electronics
– Device being driven
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Harness -Twisted Pair
When purchasing harness cable:
• Unshielded Twisted Pair (UTP)
• Shielded Twisted Pair (STP)
Study the manufacturer’s Specification to
match to your needs.
Practical Design Considerations
http://www.cirris.com/testing/twisted_pair/twist.html
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Harness Signal Response
Design Considerations: data rate, distance, noise,
parasitic capacitance, reflections.
1. Frequency Response/ Rise Time needs
–
–
–
–
–
–
DC to 100 khz Open Wire
DC to 40 MB/s Ribbon Cable (less than 3’)
•
•
SCSI, SPI-3 applications
http://www.csee.umbc.edu/~plusquel/650/slides/ribbon_cables.pdf
DC to 300 mhz Twisted Pair - unshielded, ribbon
DC to 100/1000 MB/s 10/ 100/ 1000 BaseT
ethernet Cat 5 minimum spec. RJ45 connector.
Coax, VHF 3000 megahz (scope probe, cable TV)
attenuation, reflectance, Cable TV, Rf
•
http://www.netspec.com/helpdesk/wiredoc.html
DC to 4 gigahz Fiber Optics
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Harness -Twisted Pair
What is does: Cancels out crosstalk from neighboring wires and
electromagnetic interference from external sources
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CMOS characteristics Print out the
specification sheets.
• 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 for
well protected electronics but susceptible to
misuse when connecting to the outside world.
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Isolating Noise
• Analog and Digital Optocoupler /Optoisolators Somewhat
expensive ($1/ channel) but good isolation.
– an electronic device that uses optics to transfer a signal while
keeping the receiving and transmitting circuits electrically isolated
• http://www.analog.com (Analog Devices)
• http://www.optoinc.com/
• http://www.optoinc.com/optocouplers1.html
• Power connection.
• What does not work?
– Fuses tipically are ineffective since a fuse is too slow to stop ESD
current but good for harness shortcircuit.
• Shielding
• Fiber Optics
– When higher speed data is needed.
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Isolators
•
Analog Devices, Isolators
http://www.analog.com/en/subCat/0,2879,767%255F827%255F0%255F%255F
0%255F,00.html
•
Isolators:
http://www.analog.com/en/prod/0,2877,ADuM2401,00.html
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components and spec sheets
http://www.electronicstalk.com/guides/short-circuitprotection.html
http://www.maximic.com/appnotes.cfm/appnote_number/38
http://www.onsemi.com/PowerSolutions/taxonomy.do?id=29
8 EMI RFI
http://www.edn.com/article/CA326921.html
http://www.bussmann.com/library/techspec/TechSpec19_s
up.pdf#search=%22short%20circuit%20protection%22
http://www.elecdesign.com/Articles/Index.cfm?AD=1&Artic
leID=5195 UL 489 and UL1077 standards
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