29 July 2011

App Note 13 part 3

The appendices of App Note 13 include a wealth of practical information. Appendix A talks about bypass capacitors and includes five scope traces that warn of potential troubles. Figure A7 is particularly horrifying. (I wish he named some names here; I'd like to know what specific combination of capacitors caused that shameful ringing. I guess I'll have to experiment myself... Personally, I've been using a combination of tantalum and X7R for bypassing. I really should check it out, as Jim suggests.)

Appendix B further discusses probes and oscilloscopes. Again, he doesn't name any specific makes and models of oscilloscopes, but we can guess what he's using (a Tek 547 and a Tek 556). It's funny how he suggests that the oscilloscope should have 150 MHz of bandwidth, after admitting that "90% of the development work was done with a 50MHz oscilloscope." More space is devoted to discussing probes, FET probes, current probes, and (of course) grounding. I think that I will steal the test circuit in Figure B1 to use at the basis of a lecture demo and/or lab project. It is simple, yet instructive. The picture in Figure B5 shows a wide variety of probe types ("Note the ground strap on the third finger."). 

Appendix C discusses some suggestions for ground planes. In short, use them and love them. 

All three of the above appendices will appear again (in one form or another) in App Note 47.

Appendix D shows an interesting and strange circuit for producing very fast pulses. First comment: the LM301A is only specified for a maximum voltage of 36V. The military-grade version, the LM101A, is specified to 44V. I wonder why he didn't suggest the LM101A? Second comment: the circuit uses a TD-263B tunnel diode! That's cool (it's the right tool for the job), but I don't think that Germanium Power Devices even makes tunnel diodes any more. Does anyone? In Figure D2 and the accompanying caption, we learn that the heretical HP scope that we occasionally see is a 275-MHz unit. (I don't know my HP scopes very well. Can anyone identify this model? Is it an HP 1725A?)

Appendix E discusses high-speed level shifters. Figure E2 shows a TTL-inspired level shift with a 15-volt output. I like figure E3 with the speed-up capacitor and the Baker clamp. I really do have a soft spot in my heart for old logic-circuit topologies. I'm curious about what application requires that power FET switching one amp(!) in 9 nanoseconds in Figure E4.

Best quote (page AN13-27): "Probes are the most overlooked cause of oscilloscope mismeasurement." Yep.



Conrad Hoffman said...

That bypass info is incredibly useful and it's amazing that nobody else has given it such a practical treatment.

The trick of using the comp pins on the LM318 probably goes back to a phono preamp that National did where they used the LM394 multi-transistor super matched pair as the front end. It worked quite well.

Doctor Analog said...

Thank you for recalling that. Now that you mention it, I do remember that phono preamp, and I was able to find it: it appears in App Note 299, which was written by Jim while he was at National.

Anonymous said...

For the beginner, to find an app note of Jim's (see the list of the ones he authored, below his photo on this page) at the Linear Technology web site, go to the URL http://www.linear.com/doclist/?dt=2&au=Jim+Williams. They are all (or most of them) listed there. Or at that page you can just enter the app note title (use the AN prefix, ie, "AN13".

Doctor Analog said...

@Anonymous: Or you could just click the link that says "You can find them on the Linear Tech web site" below his photo.