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Analog Angle

When Engineers Are a Little Too Smart for Themselves

Bill Schweber
zeeglen
zeeglen
11/18/2015 10:43:06 AM
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Artist
Capacitors that don't
When purchasing demanded a 2nd source for a capacitor used in a resonant LC tank circuit I grabbed a part number of one that looked good - NP0, low loss - but never actually ordered the part to test it in the circuit.  Of course production did use the 2nd source and it did not work.  A closer look at the small print in the data sheet revealed that the tan loss angle was spec'd at 1KHz instead of 1MHz.

For another engineering blooper see http : / / www . edn . com/electronics-blogs/tales-from-the-cube/4327332/The-case-of-the-stolen-capacitor  (remove spaces)

Then there was the purchasing guy who told us our choice of VCXO (voltage controlled crystal osclillator) was way too expensive and he could source one for a lot cheaper that met the same spec.  Good thing we looked at his choice - ours had voltage controlled frequency for use in a PLL, his was voltage controlled all right - HI for ON, LO for tristate.

 

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Brad Albing
Brad Albing
3/28/2013 4:41:39 PM
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Blogger
Re: "Off Data Sheet" uses
We bought enough of them tho' to get some additional screening done by the manufacturer.

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Brad Albing
Brad Albing
2/12/2013 4:36:28 PM
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Blogger
Re: "Off Data Sheet" uses
We had a similar problem once with some N-channel JFETs we were using in a circuit that detected the presence (or lack) of a gas flame by using "flame rectification." The circuit would work quite well, but every once-in-a-while, we'd start getting bad FETs. Eventually decided it was pro'ly due to ESD in the manufacturing area causing damage to the FETs' gate-source junction. No good specs on the data sheet....

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eafpres1
eafpres1
2/8/2013 11:17:59 PM
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Blogger
"Off Data Sheet" uses
Great reminder, Bill.  I can recall a couple of instances where an engineering designed in a part for a function it wasn't specified for, and was counting on some related property never changing, even though it wasn't on the data sheet.  

In one case a clever RF/Antenna engineer figured out how to make a 0402 wire-wound inductor radiate, thereby having an SMT (surface mount technology) antenna before they were commonplace.  Unfortunately the whole thing was a really high Q design, and inductors that met spec would not work in some cases.  Thankfully it never went to production, or yield would have been terrible in test.

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JAYARAMAN KIRUTHI VASAN
JAYARAMAN KIRUTHI VASAN
2/8/2013 7:50:09 AM
User Rank
Master
Parasitic voltage
BS,

Nice post.

We were designing a load cell based measurement module and used a 24 bit ADC. We laid the PCB as per the advise of the manufacturer. Soldering was good and the code was working perfectly. We had oscillations, used all averaging techniques and still the reading was oscillating. 

Our circuit had decoupling capacitors on the AVCC , one 4.7 MFD and 0.1 MFD. This combination was normally used in many of our digital circuits and in some of the stepper motor controller chip circuits. Our engineer faithfully reproduced this part and used it at many places  in the circuit. After discussing with one of the EMC consultants, it turned out that due to the internal composition and the values of these parallel capacitors, a parasitic voltage is created and this was disturbing the measurement. The oscillation disappeared once the 4.7 MFD was removed.

As some of them here are EMC and analog experts, may be , this could be discussed further.

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