Electronic Design

Bob's Mailbox

Hey Bob: This motorcycle road-racing engineer can tell you that the whole nitrogen-fill thing is usually a load of you know what (see "Bob's Mailbox," March 16, p. 20). The first thing they do when demounting/mounting your tire is to spray the entire bead area with copious amounts of soapy water. Repeatedly. So all the dry nitrogen in the world isn't going to amount to a hill of beans when the environment inside the tire is quite moist. To make it worth anything you'd need to use a commercial non-water-based tire mounting lubricant. So for those who need the very best, they should make sure that they get the right lube with that nitrogen!

  • Scott Traurig (via e-mail)
  • Pease: Hey, Scott, I forgot about that!! Maybe if they ran dry nitrogen through the tire (in and out?) after it was mounted, they could eventually dry it out. Inflate/deflate, nine times? Absurd!!! Maybe if they used some solvent that was not water-based, they could avoid the moisture. Maybe racers do that. I mean, I understand why they do want dry air—to minimize changes in inflation. Would they use some kind of rubber cement? Next time I buy a tire, I'll stand there and watch them mount it. Then if they wanted to sell me dry nitrogen, I'd ask why they wet it down first. But nobody has ever asked me if I wanted nitrogen. Not yet. Thanks for being so observant. I don't buy tires very often or have them remounted. I usually bring in a flat and tell them I'll be back tomorrow for a new tire....

Hiya Rob/Bob: I'm currently doing an undergraduate thesis on reference circuits at the University of New South Wales, Australia. My search on bandgap references brought me to your page and the page about Bob Widlar. I just wanted you to know that it is rather refreshing to see engineers with personality and flair, and hopefully I will be able to enjoy what I do when I finish my degree as much as you guys. All the best.

  • Min Sun (via e-mail)
  • Pease: Hello, Min. When I came to NSC, slightly over 30 years ago, Bob Widlar was just preparing to bring out the LM10. That was one hell of an op amp with 85 transistors. I am still lecturing about the latest and greatest version of the LM10 op amp this week (as I write this) and next week. And about its reference. When you graduate, if you think you got "personality and flair" and circuit talent, the analog engineering world needs you!

Dear Bob: Just read your piece on dielectric absorption (DA) at www.national.com/rap/Application/0,1570,28,00.html. Very useful information there. However, I still have a question. How does one know the DA of conventional surface-mount caps? (It's easy. Charge it up to +10 V, for perhaps a second. Then short it out for a few milliseconds with a switch. Watch its VOUT rise up, using a high-impedance voltmeter, or a good op amp with IB less than 1 pA. If it rises a few mV, it's NPO. If it rises dozens of mV, that's some inferior ceramic. /rap)

Our application uses an LF398M sample and hold, with a 0.01-µF cap for an acquisition time of about 20 µs and hold step of 1 mV. But there are no curves in the Typical Performance Characteristics graph for surface-mount caps. Any idea on that, or is it time to build up a test rig?

(The test rig is very simple. Set up the LF398 with +10-V input, and sample for a second or two. Then go to hold. Then set the VIN to zero, and put in a short sample pulse, perhaps a few milliseconds. Watch its VOUT. There will be a big difference. The LF398's leakage of perhaps 50 pA will cause a leak rate of 5 mV per second. That is good enough. Note: If you do not charge the X7R cap up to 10 V, then the lurch after it is shorted will not be bad, and it would be hard to tell an X7R from a COG. /rap)

The input signal is coming directly off an AD622 instrumentation amp connected to a photodetector. And the signal level is about 10 mV. (The LF398 would do a better job if there were a preamp ahead of it to bring that 10 mV up to 100 or 200 or 500 mV. /rap)

  • Dexter Francis (via e-mail)
  • Pease: Hello, Dexter. Most 0.01-µF ceramics are not NPOs, and they have lousy DA (soakage). Ask your purchasing people what they bought/are buying—most likely, X7R types. NPO ceramics that are 0.01 µF are found in 1206 packages (3.2 mm by 1.6 mm by 1.2 mm tall). You can look it up in Digikey— and they cost $0.40 to $0.90 in quantities of 100! Most 0.01s are in a size 0204 or 0306 and cost a dime or less. They would be X7R or Y5V. And they have lousy DA.

Comments invited! [email protected] —or: Mail Stop D2597A, National Semiconductor P.O. Box 58090, Santa Clara, CA 95052-8090

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