electronics prototyping presentation

8/20/2019 Electronics Prototyping Presentation

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Electronics Protot in

Prototyping :: Slide 1 of 20David Rye :: MTRX 3700


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Groundin and Power Su l



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• “Ground”, +0V and resistance

• Single-point (Star) grounding and common powersupply returns

•

• Induced voltage due to changing magnetic field

dBV kNA

dt

where B = Flux density (Gauss), N = number of turns,

A = cross section of loop (m2

) (Faraday’s law of induction)



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• Solving

• Star point

• Find the current – make it flow where you want it

• Low resistance metal cable ducts

• Close to metal chassis• rea e pa

• Transformer coupling

•

• Filter

•



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• Connection between the equipment chassis or

frame and the ground at zero potential• Purpose

• Avoid a potentially lethal potential on the equipment in

case of a fault• Trigger protection systems placed upstream to isolate

the equipment



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• Big capacitor 10uF – 47uF and 1–2uF in parallel at entry

po n• (Large capacitor has low self-resonance frequency)

• ec ro y c or an a um



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• “Ground bounce” and power supply bypass (decoupling)

n

dV i C

dt

• where C n

is ‘node’ capacitance, ≈ 50pF

• d dt ≈ 1V/ns for ALS

• i ≈ 50mA per gate (400mA for 8 gates!)

• One 10-100pF (0.01-0.1μF) ceramic or tantalum per 2 logic ICs

• Close to power pins (15mm)



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• Connect power supply and grounds first

• Check them

•

• On first power-up:

•

• Monitor current



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Wires



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• = ,

• Cable = multiple conductors

• e.g. 1/0.6 rated at 3.0A @ 25ºC, 64Ω/km

• . . .• e.g. 7/0.2 rated at 2.0A @ 25ºC, 88Ω/km

• Colour code• Strictl : Black = +0V; Red = +5.0V

• Other colours to help tracing

• Put colours (and pin numbers) on circuit diagrams


• Keep schematics up to date


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• es s ance s propor ona o e eng o e con uc or,l (m) and inversely proportional to the cross section of the

2,

• Resistivity cu = 1.72×10-8 Ωm is the constant of

lcu

A



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-

• Cross-talk introduced by mutual inductance and/or

capacitance• How to minimise

• Keep conductors bundled

• Twisted pairs reduce magnetic and capacitive interference• Shield signal lines

• Maintain low impedance terminations

• Filter connectors• Ferrite beads

• Grounding shields – which end?

• Farada Ca ePrototyping :: Slide 12 of 20David Rye :: MTRX 3700


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• Physically separate analog and digital circuits

• Connect analog & digital grounds at one point only



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• Control analog tracks – minimise loop

area to minimise induced voltage

AdB

V kN

•

possible on analog wiring

•

minimise differential pickup• Minimises loo area to reduce

magnetically induced noise

• Shields from electrically induced noise



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• Keep lengths of individual bus conductors the same to

reduce “skew”• λ ≈ 300m @ 1MHz (c = f.λ, c ≈ 300 x 106 m/s)

• Actually, v ≈ 0.8c in real media

• Consider transmission line effects ifshortest rise time < 3 x propagation time along conductor



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Anything with mechanical bits

1. Connectors

2. Switches

Trim Pots

• A trim pot is a 2-terminal device! (falling off the end)

• Set pots to mid-range before powering-up…



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” .

before crimping well crimped over-crimped

in housing



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• Cumulative! Expensive to repair in field

• Static sensitive when

• Not in a circuit• Unpowered

• In lab• Store in anti-static foam and/or bag before use

• Don’t touch circuit conductors or chi ins

• Earth your hand to discharge static before handling• In roduction

• Also use anti-static mat and wrist strap



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Lo ic Gates



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• There is no such thing as digital electronics!

• Bypass power pins generously

• Unused gate inputs

– min,

• CMOS – tie to +5V or +0V



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• Horowitz & Hill. The Art of Electronics. 2ed., CUP, 1989.

• Williams. The Circuit Designer’s Companion. Newnes,2ed., 2005.


electronics prototyping presentation

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