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No Bugs!
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Copyright (c) 2009 Design/Analysis Consultants, Inc.
DESIGN/ANALYSIS NEWSLETTER
1st & 2nd Qtr
2009
Dedicated to Design Excellence
Feedback: newsletter@daci-wca.com
OFF TOPIC
The Stimulus and Governmental Malpractice
Applying engineering thinking to life
The Obama administration is suggesting that we can borrow money to "stimulate" the economy. But engineering thinking --- the kind of thinking that we engineers routinely apply to providing the most reliable and cost-effective products in the world --- can quickly identify the absurdity of this notion by translating it into plain English:

President Obama: We need to invest in infrastructure and other government-directed activities to create jobs.

Engineer: Hmmm. President Obama proposes that we borrow money from ourselves (or the Chinese, or other countries) to give ourselves a job that we presently don't have. Assuming that we borrow enough to give ourselves a job for one year, what happens at the end of the year?

Engineer: At the end of a year we are out of that job, plus we owe someone for that year's worth of salary, plus interest. Therefore all we have done is postpone the fact that we need to obtain a real job, and we have enormously increased our debt.

Engineering conclusion: dumb idea.

As most of us have had to endure and survive the severe scrutiny of design reviews, where emotion-based or theoretically fanciful ideas are quickly punctured by rational thinking, we know how to arrive --- albeit sometimes painfully --- at the truth. The U..S. Congress is presently populated mainly by political "science" graduates and by lawyers, but by zero engineers. We think that is the root cause of today's grossly ineffective government.

The solution is simple: Elect more former engineers and other science-based professionals, those who know how to think critically and rationally.

Are you ready to serve?
TECH TIP
SIGHTINGS: The Case of the Defective Detectors
A manufacturer of optical components was sued by Company X, who used one of the manufacturer's detectors in a camera. Company X claimed that the detectors were defective, which resulted in the failure of their camera business.

The optical component manufacturer asked Charlie the consultant to review the matter. Charlie agreed, and it quickly became apparent that Company X seemed oblivious to an important detail; namely that Company X was selling their camera for extended outdoor use.

Charlie reviewed the camera design and found no provision for humidity control. This meant that the camera would be unreliable because the high-impedance circuitry could easily be rendered unusable by condensation. Lens fogging was also likely for the camera's intended overnight use, particularly when exposed to the morning's dew. This, of course, had nothing to do with the detector. But to be certain, Charlie requested and received full test data on detectors that had been retrieved from the alleged failed cameras. As expected, all the detectors met their specifications. Also, inspection of some returned cameras contained evidence of interior moisture contamination.

"You detectors are fine," Charlie told his client. "You're being sued by someone who doesn't understand proper electronics design from a systems level."

Charlie provided his client with his analysis and conclusions, which were presented in court. The lawsuit was dismissed.
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Decoupling Capacitors: How Much Is Enough?
The days are long gone when decoupling was achieved by simply adding a 0.1uF capacitor across each IC. With today's high-density and high-power chips, a single capacitor may be woefully inadequate. So, how much decoupling does the diligent engineer need to add across those larger power-hungry ICs? Here's a handy formula:
Caps = ESRmax * 2 * Imax / (Vmin * Ripple/100)
SpokesmanReview.com, April 21 2009
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where
ESRmax = capacitor maximum equivalent series resistance in ohms at the ripple frequencies of interest
Imax = maximum IC current, A
Vmin = minimum IC supply volts
Rippple = desired maximum ripple voltage on IC supply terminal, percent
Example: Assume you have a microprocessor chip that draws 2.3 amps max on its 3.3V 10% supply, you wish to maintain the ripple voltage at 1% of the supply voltage, and you will be using 0.1uF ceramic decoupling capacitors that have an ESRmax of 0.056 ohms from 100MHz to 1GHz.
Caps = ESRmax * 2 * Imax / (Vmin * Ripple/100) = 0.056 * 2 * 2.3 / (0.9*3.3 * 1/100) = 8.7 capacitors; e.g. use nine 0.1 uF caps.
HINT: To help avoid potential resonances, for large capacitor arrays it's a good idea to include some different values to obtain the total capacitance; e.g. instead of using all 0.1uF caps, also mix in a few 0.01uF and 0.001uF values.
Sightings is a collection of true experiences as reported by credible sources