30 January 2012

App Note 86

A standards lab grade 20-bit DAC with 0.1ppm/°C drift: The dedicated art of digitizing one part per million. 52(?) pages.

The main text of this app note is only four pages long, and there are 48 glorious pages of appendices.

Well, "glorious" is the wrong word. Unfortunately, one of the appendices contains TWENTY-TWO pages of source code (Appendix D). Really? Couldn't Linear Tech have used a smaller font? Printed the code in two landscape columns? Or simply not printed it at all? (Do you think anyone, even one single person, typed in this code and used it? Really?) If we include the source code, this app note is the fourth longest one. However, if we ignore the code, then this app note is only 30 pages long, a little longer than average. It still results in a large appendix-to-main-text ratio, but it's not the less-than-ten-percent ratio we initially suspected...

This app note discusses a 20-bit digital-to-analog converter. The DAC circuit includes an interesting "table-turning" topology: DACs are often used in the feedback path to create a good ADC (for example, as in the successive-approximator topology); in this circuit, a high-quality analog-to-digital converter (the LTC2400) is used in the feedback path to implement a high-quality DAC.

The construction of the 20-bit "slave" DAC is interesting. Jim says, "The sole DAC requirement is that it be monotonic. No other components in the loop need to be stable." The circuit is shown in Figure 2, which using two 16-bit DACs, with eight bits of overlap, and four bits of sub-LSB twiddle.

The results are (briefly) shown on pages 3 and 4, which are heavily footnoted (referring the reader to the numerous appendices). For example, the following quote appears on page 3, with footnote,
Figure 3 is a plot of linearity vs output code. The data shows linearity is within 1ppm over all codes (Establishing and maintaining confidence in a 1ppm linearity measurement is uncomfortably close to the state of the art.).

Appendix A talks a little bit about the history of digital-to-analog conversion and includes a glamor shot of some items from his collection (we saw this photo before in App Note 74). Appendix B lists some of the specifications of the LTC2400 ADC.

Appendix C discusses the operation and use of a Kelvin-Varley divider to verify the linearity of the 20-bit DAC. "The actual construction of a 0.1ppm KVD is more artistry and witchcraft than science." The individual components must be selected (see the table in Figure C4 for the LTC1152 chopper-stabilized op amp) to obtain the necessary performance. Figure C7 shows the complete schematic for the voltage source. The construction of this circuit is a work of art (read the text carefully). "Adjust for 5.000000V at A." That's a lot of zeros! The best quote appears in the footnote on page 11,
The author, wholly unenthralled by web surfing, has spent many delightful hours "surfing the Kelvin." This activity consists of dialing various Kelvin-Varley divider settings and noting monitoring A-to-D agreement within 1ppm. This is astonishingly nerdy behavior, but thrills certain types.

Appendix D contains the source code for the digital comparator. Jim once commented to me, "The summing junction of this circuit is in software. It took a while for me to get my head around it, but it really does work."

Appendix E discusses linearity correction for the LTC2400, and Appendix F discusses improved output-buffer stages. Appendix G shows a gain-of-2000 settling-time measurement circuit, modified from App Note 74.

Appendix H discusses microvolt-level noise measurement. A high-gain preamplifier is necessary, as well as an oscilloscope with a plug-in capable of low-level measurements (2mV/division is not good enough). Figure H3 shows the instrumentation setup, using the same cookie-tin as the noise measurement in App Note 83 Figure 6 (he really did like those cookies!).

Appendix I discusses voltage references for this application, with the LM199A and the LTZ1000A receiving highest marks. Both of these parts use a temperature-control loop to maintain constant temperature on the Zener diode, thus improving the temperature drift to sub-ppm. Figure I2 shows an example circuit for the roll-your-own LTZ1000A. (Bob Dobkin designed these parts. The LM199A is good, but (I suspect) that the loop gain isn't high enough to reject ambient changes in temperature. He got it right in the LTZ1000A with the external loop.)

Appendix J discusses parasitic thermocouples and other construction pitfalls, a topic that he previously discussed in App Note 9 and App Note 28.
Readers finding [Figure J4's] information seemingly academic should be awakened by Figure J5. This chart lists thermoelectric potentials for commonly employed laboratory connectors. Thermocouple activity of some types is more than 20 times greater than others. Be careful!

This app note does end in a cartoon, but I can't reproduce it here. The cartoon demonstrates "one part per million" by printing one million dots. A JPEG on your computer screen doesn't do it justice. Go download the PDF file, print out the last page (on a good printer), grab a magnifying glass, and ponder the real-world difficulty of 20 bits.

29 January 2012

Scope Sunday 25

When I visited the Computer History Museum three weeks ago, I also stopped at two electronic surplus shops, WeirdStuff and HSC (I also visited these stores back in October). Unfortunately, the pickings were slim.

At WeirdStuff, there was a Tektronix 502 oscilloscope. Unfortunately, it was priced at $160.

The Tektronix 7904 that I saw last time was still on the back wall, now with a sticker that says "no power", but still with a price tag of $250. (This scope has been there for at least six months.)

I think both of these scopes are priced five times too high. Fifty dollars is a fair price for a broken 7904.

I saw similar overpricing at HSC. They seem to have a lot of untested gear in the $45 to $75 range. I buy a lot of electronic surplus junk, but my what-the-hell price (as in, "what the hell, I'll just buy it and see") for a random untested, unidentified box is a lot less than $45.

However, I have bought things at HSC in the past, and I will buy things at HSC in the future. For one thing, I appreciate and support any business that does this:

This picture shows part of the databook library at HSC. This collection of databooks puts the collection of databooks in the M.I.T. Library to shame. I have referenced these books in the past, and I hope to be able to reference them in the future. Not everything is available on the web (as my commentary in the "Bibliography of Jim Williams" attests). Having real, printed databooks and handbooks available for perusal is a valuable resource and a public service. Thank you, HSC.

23 January 2012

App Note 85

"Low noise varactor biasing with switching regulators: Vanquishing villainous vitiators vis-à-vis vital varactors." 24 pages.

This app note discusses low-noise bias-voltage generators for varactor diodes. The main application, as shown in Figure 1, is VCO tuning for phase-locked loops. This application is extremely sensitive to power-supply ripple, as any corruption of the bias voltage will cause spurs in the VCO output. In the circuit shown in Figure 6, careful power-supply design and appropriate filtering produce the varactor-bias voltage shown in Figure 11, which exhibits only 20 microvolts of ripple and noise.

However, the major emphasis of this app note isn't the circuitry, but the instrumentation (this topic is well-worn territory for Jim, of course). Measuring 20 microvolts of ripple and noise is HARD, and Figures 12 through 17 show several ways that improper measurement technique ruin the measurement. Figures 19 through 24 show frequency-domain measurements of the VCO output using a spectrum analyzer. Again, it is shown that improper measurement technique, or careless construction and layout, degrade performance significantly.

Appendix A is a primer on varactor diodes written by Neil Chadderton of Zetex.

Appendix B discusses amplifier and oscilloscope selection in order to facilitate the 20-microvolt sensitivity needed for Figure 11. This appendix is borrowed from Appendix B in App Note 70. I can't resist quoting the footnote again:
In our work we have found Tektronix types 454, 454A, 547 and 556 excellent choices. Their pristine trace presentation is ideal for discerning small signals of interest against a noise floor limited background.
Appendix C is copied from App Note 70 Appendix C (as he explains in the footnote).

The app note ends with a varactor cartoon. Probably the only varactor cartoon in the world. "I never had it so good."

22 January 2012

Scope Sunday 24

In response to my post last week about the lack of analog instrumentation the Computer History Museum, one person asked, "Haven't they ever heard of ANALOG computers?" The answer is "sort of"...

The museum does have a small number of analog computers in the main exhibit hall, including one of my personal favorites, an EAI PACE TR-48.

There is also a small display case that discusses the history of operational amplifiers, including a copy of Henry Paynter's "Palimpsest on the Electronic Analog Art", a Philbrick K2-W op amp, a Fairchild uA709 op amp, and a National LM10 op amp.

Over in the "Digital Logic" section of the museum, there are a few analog integrated circuits in the corner, along with a picture of Bob Widlar.

But really, that's about the extent of the analog content (being something of an analog bigot, I'd prefer to see much more).

One thing that I note with disappointment is an item that was formerly on display. When the museum had its "Visible Storage" exhibit hall, they had a beautiful analog computer by the GPS Instrument Company on display. This computer was a lovely, monstrous beast that showed the scale, and the artistry, of "big iron" analog machines. The museum does have a large picture of the machine hanging in the lobby (because, really, it is a work of art), but they no longer have the machine itself on display.

This machine is also close to my heart for two other reasons: 1. It was built in Massachusetts, not far from my house, and 2. Before the museum knew what they had, I found a reference that described the machine and the history of the GPS Instrument Company (the paper was Per A. Holst, "Sam Giser and the GPS Instrument Company: Pioneering compressed-time scale (high-speed) analog computing", in John McLeod, editor, Pioneers and Peers, San Diego: The Society for Computer Simulation International, 1988.) Finding this paper is why my name appears on this page of credits.

UPDATE: In response to this post, I received a very nice email from Alex Bochannek, who is a Curator at the Computer History Museum. He gave me permission to share his comments here.
In developing the exhibit, I made the conscious decision to focus on general purpose, indirect analogs, i.e., differential analyzers and electronic analog computers. The goal is to show a historical arc which begins with the work done under Vannevar Bush at MIT and ends with hybrid digital/analog systems and DDAs. The continuity in this changing technology story lies with the users’ needs for rapid, interactive calculation for (primarily) engineering problems.

The operational amplifier is displayed as an example of changing implementation technology and as a contrast to the integrator in the DA story. The latter also is a nod to mathematical instruments; slide rules and sectors you will find in the Calculators gallery. The op-amp text is slightly more technical in nature as indicated by the words Tech Talk on the panel. As you noted, you will find Bob Widlar repeated in the Digital Logic gallery.

Analog computing techniques in special purpose applications are represented by many of the devices in the Real-Time Computing gallery, for which I also was responsible. Direct analogs (e.g., network analyzers, electrolytic tanks, etc.) were excluded from the exhibit for lack of artifacts and available exhibit space.

I intentionally limited the content of the Analog Computing gallery to general purpose systems to highlight the broad applicability of analog computers. As you can imagine, most people in the computing field are not only unfamiliar with analog computers but tend to write them off as a pre-digital curiosity which surely must have disappeared many decades ago. By displaying unexpected and beautiful artifacts that represent a vibrant and innovative computing community, I hope to raise our visitors’ awareness of this important story in computing history.

Here is my response:

I truly enjoyed the exhibits. As I commented in "Scope Sunday 23" (the one about the oscilloscopes), I am impressed with the stunning collection of artifacts that the Museum has on display. Many computing professionals don't appreciate the importance of analog computers. I am reminded (and constantly humbled by) a quote from Ted Nelson:
There are two kinds of computers: analog and digital. Analog computers are so unimportant compared to digital computers that we can polish them off in a couple of paragraphs. (Nelson 1974)
Of course, Ted Nelson is a smart guy (coined the word "hypertext", inspired the creation of the Web) and many people seem to agree with him, but he doesn't (or didn't) know his history. I appreciate that the Museum is working to correct this view (and I hope Ted Nelson no longer feels this way!)

I also agree with your exclusion of direct analogs from the museum exhibits. Some people think that direct analogs are a "under-appreciated story" in the history of computing, but I do not. For example, William Aspray wrote a paper on Edwin Harder and the Westinghouse Anacom, which is little known compared to Bush's Analyzers. In his paper, Aspray laments
What accounts for the difference in recognition accorded these machines? One may be that Bush’s machines were built earlier that the Anacom… Perhaps more important was that Bush’s machines had a high profile at MIT, where they were used by many industries, as well as by many students who went on to important computing careers… It may also be an artifact of historical scholarship. Bush’s prominent role in organizing science for the Second World War and his plan for postwar government support of science has attracted scholars to examine other aspects of his career, including his calculating machinery. MIT as an institution had received considerable attention from historians of science and technology, whereas Westinghouse has been surprisingly neglected given its long history of contributions to electrical technology. (Aspray 1993)
I disagree with Aspray, here. The key issue is that the Differential Analyzers were true general-purpose computers, while things like the Anacom, network analyzers, and electrolytic tanks were really just scale models. While the work performed on these machines was of profound importance to the research teams using them, they were not analog computers. Furthermore, these "non-computing analogies" did not contribute significantly to the "culture of computing" that arose in the middle of the 20th century.

As you say, it is this culture of computing, grown on analog computers, that the Museum celebrates.

20 January 2012

App Note 83

"Performance verification of low noise, low dropout regulators: Silence of the amps." 20 pages.

This app note discusses testing LDO regulators, primarily the measurement of output noise. In effect, this app note is the linear-regulator version of App Note 70 (which discussed the measurement of output noise for low-noise switching regulators).

The schematic of the measurement chain is shown in Figure 2. Note that single-pole highpass filters on use on the input and output to remove any DC components from the signals. This arrangement leads to a non-optimal low-frequency cutoff. (See the frequency response in Figure 3, which shows a nice high-frequency roll-off, but a considerably "rounder" low-frequency roll-off... Butterworth is better. He could have done better by moving the second-order Butterworth poles to the third-order locations, since he already had a pole (or two) on the axis.)

Nevertheless, the plots on pages 8 and 9 are very nice, and are clearly the product of painstaking work.

The best quote (from page 3, with footnote): "The metal can encloses the regulator under test and its internal battery power supply (the cookies were excellent, particularly the thin ones with sugar on top)."

Appendix A discusses the internal architecture of low-dropout regulators. This verbiage is an updated and expanded version of Appendix A from App Note 32. Note that Figures A3 and A4 show the transient response for a load step (from 10mA to 100mA); he will return to this topic in a later app note.

Appendix B discusses capacitor selection, and warns of the voltage-dependent and temperature-dependent horrors of some ceramic types.

Appendix C is an in-depth discussion of RMS voltmeters. He has discusses RMS voltmeters before (most notably in Appendix C of App Note 65), but this discussion is the most detailed, and includes comparisons of voltmeter types (Figures C1, C2, and C3), as well as comparisons of specific models (Figure C5). Of course, the HP3400A and HP3403C work very well, as does his special baby, the LT1088-based circuit in Figure C6. Thermal great, logarithmic good, rectify and average bad.

The app note ends with a cartoon. "Speak softly and carry a big PNP."

17 January 2012

App Note 81

"Ultracompact LCD backlight inverters: A svelte beast cuts high voltage down to size" 24 pages.

This short app note (seven pages of main text) is the sad conclusion to the grand saga of cold-cathode fluorescent lamps (CCFLs). Just as a reminder:
This conclusion is sad, because this app note contains a great idea (using piezoelectric transformers) that utterly failed to achieve traction in the marketplace. Jim gave a talk on this subject at M.I.T., and he seemed genuinely disappointed that this technology wasn't more widely accepted. (If I recall correctly, the issue was in assembly. Mounting the piezoelectric transformer on a circuit board requires specialized assembly equipment, and none of the usual-suspect laptop vendors wanted to invest in the technology.)

Piezoelectric transformers are not new to Jim. In fact, he discussed them in National Application Note 285, "An acoustic transformer powered super-high isolation amplifier", back in 1981. He also used one in Figure 51 of LTC App Note 29.

The enabling technology here is shown in Figure 7: the transformer terminal labeled "resonance feedback" allows for simplified drive circuitry, ensuring start-up and oscillation at resonance. The complete circuit is shown in Figure 9.

Appendices A and B were written by his coauthor at CTS Wireless Components. Appendix A is a short introduction to piezoelectric transformers, and Appendix B is a longer theoretical treatise.

Appendix C provides the best quote
Veterans of feedback loop compensation battles will exercise immediate caution when confronted with a pure and lengthy delay in a loop. Neophyte designers will gain a lesson they will not easily forget.
Again, I think he's mocking me in footnote 2. I must admit that "glop comp" is not a terminology that I am familiar with (although I do use "dominant-pole compensation" in everyday conversation).

The app note ends with a cartoon. "I can't believe I've been replaced by that skinny nothing."

15 January 2012

Scope Sunday 23

Last week, I spent a few hours at the Computer History Museum in Mountain View, California. I had previously toured the "Visual Storage" exhibit hall several years ago, but they have a new, beautiful display hall called "Revolution" that I hadn't seen yet. I spent the afternoon wandering around the exhibits and taking pictures. The Museum has a stunning collection of computer-related artifacts. Simply breathtaking.

However, (since it is "Scope Sunday") I have to report that they only have two pieces of Tektronix gear on display in their collection (other than the equipment currently on Jim Williams's desk). In the "PDP-1 Restoration" exhibit hall, there is an original PDP-1 that has been restored and is occasionally demonstrated to the public. In the back of the exhibit hall is a Tektronix 535A, which is probably the "historically accurate" instrumentation for working on a PDP-1!

In the "Revolution" exhibit hall, in the "Minicomputers" section, there is a PDP-8/E that is part of some custom hospital equipment that mapped the brain's response to stimuli during surgery. This "brain surgery station" includes several pieces of Tektronix gear, including a RM561 oscilloscope (with 2A60 and 72 plug-ins), a 161 pulse generator, and a 162 waveform generator.

However, if you look closely at the other exhibits and photographs, there is much evidence that Tektronix oscilloscopes are important to the development of computers. In many of the historic photographs, there is Tektronix equipment plainly visible in the background (and foreground). It's like a subliminal Tektronix exhibit. Here's a list of the Tektronix gear that I found:

  1. MADDIDA Customer Demonstration
    What do your customers want to see? A Tek 511A front and center.
  2. UNIVAC System brochure
    This sales brochure has several pictures of a UNIVAC installation that include a Tektronix oscilloscope. Check out pages 2, 11, 13, and 15. I especially like how the drawing on page 2 includes an oscilloscope (looks like a Tek 535 or 545). Of course you would want one!
  3. IBM 701 assembly floor
    Two oscilloscopes are seen on the assembly floor (looks like a Tek 511 in front and a Tek 512 in back).
  4. NTDS Combat Information Center training
    Two oscilloscopes are seen in the training center (one front and one back). The one facing forwards looks like a Tek 535/545.
  5. Assembly of IBM 1401 computers
    Look very closely at this photo. I count a dozen Tektronix scopes! The one in front looks like a Tek 535A or 545A.
  6. Building the ORACLE computer CPU
    Is that a Tek scope behind the 19" rack?
  7. RAMAC Assembly
    I see three Tektronix scopes in this picture, but there must be more.
  8. Los Alamos MANIAC computer
    One scope in the very back of the lab, looks like a Tek 511.
  9. Operators at the ILLIAC IV at NASA Ames Research Center
    The lone 400-series scope in these pictures. Perhaps a Tek 465?
  10. Max Palevsky and Robert Beck, in the lab
    Another laboratory picture with a Tek 535A/545A scope front and center.
  11. PDP-1 computer
    A Tek scope on a cart, with a rack-mount scope above it.
  12. Fairchild co-founders in discussion
    Tek 575 curve tracers, as far as the eye can see! There are six on the nearest bench (three facing forwards and three facing backwards).
  13. The Beast at Johns Hopkins Applied Physics Lab
    Robot development at APL, with a Tek 551 dual-beam scope.
  14. Sven Wahlstrom and Nils Nilsson with Shakey
    In the very back of the lab, there appears to be the back of a Tek scope on a cart.
  15. Spiral scanner at Lawrence Berkeley National Laboratory
    In the back, a Tek 535A/545A scope on a cart.

With that many pictures of oscilloscopes in the museum, shouldn't Tektronix scopes have an exhibit of their own?

(Sorry for the giant list of links. My cell-phone photographs of the photographs didn't turn out very well, so I thought that it was better to link directly to the source.)

Postscript: I am aware of the Vintage TEK Museum in Portland, Oregon (I haven't had a chance to visit yet, but it's on my 2012 list). However, I think that classic Tektronix oscilloscopes belong in all museums, including computer history museums, science museums, fine art museums, etc... Don't you?

13 January 2012

App Note 79

"30 nanosecond settling time measurement for a precision wideband amplifier: Quantifying prompt certainty" 32 pages.

While technically the long-awaited update to App Note 10, to be honest, this app note is an abridged and modified version of App Note 74. Here the application is the measurement of amplifier settling time instead of DAC settling time, but the techniques (and the text) are significantly similar. The measurement problem tackled here is actually easier; amplifier settling is determined at the 0.1% point, while 16-bit DAC settling requires measurement of the 0.0015% point. (The amount of laboratory work involved was no less, however; Appendix E is proof of that fact.)

The settling-time measurement circuit is shown in Figure 6 (compare to App Note 74 Figure 6). The modifications for this application include a change to the input drive (to drive a voltage step to the op amp, instead of a digital command to the DAC), removal of the output amplifier, and removal of the temperature-control loop on the diodes (both because 0.0015% accuracy is no longer needed). Also, the sample delay and window generator is now implemented with LT1720 comparators instead of 74HC123 TTL logic.

Also, instead of three alternative measurement methods for comparison (as in App Note 74), he provides just one: the classical sampling oscilloscope. The "essentially identical" results of these two measurement are shown in Figures 18 and 19. Perhaps he felt he has less to prove this time?

The best quote is the app note's conclusion: "Examination of the photographs shows nearly identical settling times and settling waveform signatures. The shape of the settling waveform is essentially identical in both photos. This kind of agreement provides a high degree of credibility to the measured results."

Many of the appendices previously appeared in App Note 74. Appendix A is the same as App Note 74 Appendix B. Appendices C and D are similar to App Note 74 Appendices C and D (with light modifications for the new topology of the settling-time measurement circuit, and for op-amp settling instead of DAC-output-amp settling, respectively).

Appendix B is new, discussing one of his favorite topics, subnanosecond pulse generators. He complains about the prices of current production units ($10,000 to $30,000), discusses his favorite vintage units (HP-8082A, HP-215A, Tek 109, and Tek 111), and then shows his own design in Figure B1. This circuit, as he would likely say, "is the beneficiary of considerable attention over a protracted period of time." It is now loaded with features, including a fully adjustable pulse amplitude, an external input to determine repetition rate, and an output trigger pulse that is settable from before-to-after the main pulse. Figure B4 shows the high-speed pulse in all its glory, measured with a Tek 547 with 1S2 sampling plug-in.

Appendix E discusses breadboard construction (like App Note 74 Appendix G) and includes another photo essay on the construction of the settling-time measurement circuit (Figures E1 to E6), proving that the lab work was exhaustive.

The app note ends with a cartoon, of course. Thirty nanoseconds is hard!

11 January 2012

App Note 75 part 2

The waveform sampler in Figure 19 is another great circuit. This "track-and-not-hold" topology is basically the core of his settling-time measurement circuit from App Note 74.

Several random instrumentation circuits are included in this app note. Figure 24 is a extremely low-power chopped amplifier. Figure 25 is a thermocouple amplifier for detecting the pilot-light flame in a furnace or hot-water heater (some home improvement during his sabbatical, perhaps?). Figure 26 is a tip detector for shipping containers.

Several oscillator circuits are also included. Figure 27 is the simple 32,768-hertz crystal-oscillator circuit that is used in Figure 10. Figures 29 and 31 are 10-MHz complementary-output oscillators, the former with 50% duty cycle and the latter with nonoverlapping outputs.

A CCFL power-supply circuit returns in Figure 33. I can't believe he isn't completely burned-out on CCFL circuits. Interestingly, this circuit uses the Royer topology, but does not use one of Linear Technology's specialty Royer control chips. Perhaps this circuit is for a low-cost application? A single footnote refers the reader to App Note 65.

The applications section of the app note concludes with more three power supplies. Figures 36 to 39 were contributed by Jeff Witt. Figure 40 is a low-noise off-line power supply using the LT1533 (see App Note 70). The drive on the cascode transistors and the power-limit/current-limit circuits are very interesting.

Best quote (discussing Figure 44): "This data was taken with no input filtering LC components and a nominally nonoptimal layout."

Second-best quote (footnote 12):
Veterans of LTC Application Notes, a weary brigade, may recognize this reference as the object of Application Note 70's (Footnote 14) champagne prize offer. The mystery solved, the messenger was compensated as specified (Veuve Clicquot Ponsardin).
This paper (Reference 24) is the earliest use of the word "cascode" in the literature.

The appendices are updated versions of "Box Sections" A and C from App Note 23. Appendix A discusses low-power techniques and the design evolution of the zoo circuit. Figures A1, A2, and A3 are from App Note 23. Figure A4 is the original Zoo Circuit, and Figure A5 is from App Note 45 Figure 21. Figures 1 and 4, as discussed last time, are the latest circuits in this evolution ("utilizing contemporary components").

Appendix B discusses the effects of test equipment on micropower circuits (in short, avoid loading, or worse, powering, your micropower circuits with your test equipment). Figure B3 is a new addition, borrowed from App Note 47 Appendix E.

The app note concludes with a cartoon that expresses his dog's point of view.


09 January 2012

App Note 75 part 1

"Circuitry for Signal Conditioning and Power Conversion: Designs From a Once Lazy Sabbatical." 32 pages.

This app note is another collection of circuits, like App Note 45 (June 1991) and App Note 61 (August 1994). He seems to do one of these collections every four or five years. This one was inspired by his sabbatical and the acquisition of a HP 215A pulse generator. "I took it home, repaired it, and used it to characterize a fast coincidence detector... I had previously abandoned. This exercise proved fatally catalytic."

The first two circuits are improved voltage-to-frequency converters, loosely based on "The Zoo Circuit" from App Note 23. Figure 1 shares much in common with the original Zoo Circuit, exploiting some component improvements (using an LTC1441 in place of the original LT1017) and consuming only 20 microamps at full scale. Figure 4, using a reworked reference chain, is even lower power (less than 9 uA at full scale). These are impressive results. As he says, "these voltage-to-frequency circuits are the beneficiaries of considerable attention over a protracted period of time."

Figures 6 and 10 show low-power single-slope analog-to-digital converters. One interesting feature of Figure 6 is his use of diode-connected transistors instead of diodes, because "Q2, lacking gold doping, temperature tracks the LM334 more closely than a small signal diode would." (The 1N914 is doped with gold to increase its speed, but I had never thought about the effect on temperature.)

Figure 11 shows another RMS-to-DC converter using the LT1088. This circuit is very similar to the circuit in App Note 61 Figure 22, with the addition of a differential front-end, using the LT1207 dual power op amps.

The best circuit, the inspiration for this app note (as explained on page 1), is the coincidence detector in Figure 14. The circuit is relatively simple, but the construction and instrumentation are certainly not. "Evaluating circuit performance requires a sub-nanosecond rise-time pulse generator and a very fast oscilloscope."

I'll cover the rest of the circuits next time.

The best quote is the footnote on page 3: "Okay all you SPICE types out there, start your computers and model the charge pump drift and the reference compensation mechanism." Had someone been needling him about the supposed superiority of simulation?


08 January 2012

Scope Sunday 22

Last week, I was able to spend a few hours at the Computer History Museum. Of course, Jim's lab bench from Linear Tech is still on display, but the main purpose of my visit was to explore the exhibits (which were closed when I attended the unveiling of Jim's bench). I will have several upcoming blog posts (about oscilloscopes!) when I finish sorting through my photographs, but I thought I'd start with a quick post today.

In App Note 55, Jim showed the collection of equipment that he titled "Typical Efficiency Measurement Instrumentation", including RMS voltmeters, a clip-on ammeter, and a input DVM. He also included a "hand calculator" (lower right) for computing the circuit efficiency.

The "calculator" that he included is a rare and beautiful artifact, the hand-cranked Curta calculator, with an amazingly intricate mechanism. The Computer History Museum has several on display.

The Museum also has a webpage with some more photos of the calculator.

06 January 2012

App Note 74 part 2

This app note contains eight appendices (more than half of all the pages), and, as always, the appendices are great stuff.

Appendix A talks a little bit about the history of digital-to-analog conversion and includes a glamour shot of some items from his collection. The Kelvin-Varley divider is very nice. "What Lord Kelvin would have given for a credit card and LTC's phone number."

Appendix B is an updated discussion of oscilloscope-overdrive performance (see App Note 72 Figures 32 to 37 and App Note 47 Figures 45 to 50). Here he has added Figure B1, which compares the topology of various oscilloscopes, and he sings the praises of vintage analog instruments. "Unfortunately, classical sampling oscilloscopes are no longer manufactured, so if you have one, take care of it!" Indeed.

Several of the appendices follow-up on and expand on some issues from the main text. Appendix C discusses calibration of the amplifier delay in the settling-time measurement circuit. Appendix D discusses amplifier compensation (in his usual, seat-of-the-pants way), and the moral of the story seems to be "build it and see." Appendix F shows the circuitry necessary to interface DACs with serial-data interfaces to the settling-time measurement circuit.

Appendix E discusses the special case of using a chopper-stabilized amplifiers and the possible dangers involved therein. The scope trace in Figure E3 is especially frightening, although he admits,
This is admittedly worst case. It can only happen if the DAC slewing interval coincides with the amplifier's internal clock cycle, but it can happen. (Footnote: Readers are invited to speculate on the instrumentation requirements for obtaining Figure E3's photo.)
Such a tease! It'd be nice if he occasionally exposed the trickery behind some of these displays.

Appendix G discusses breadboard construction, and in particular talks about proper steering of the ground currents. Wise advice! As the footnote says, "I do not wax pendantic here. My abuse of this postulate runs deep." This appendix also includes a very nice photo essay on the construction of the settling-time measurement circuit (Figures G1 to G10). I'd love to see some high-resolution color versions of these pictures.

Finally, Appendix H shows some power-gain stages, some of them borrowed from App Note 18 and App Note 47 Appendix C.

The app note concludes with a picture of his work bench instead of a hand-drawn cartoon.

On the left is his Tek 661 sampling scope, with a Tek 454 on top. Next to that is his Tek 547, and in front of the 547 is the General Radio 1422-CL variable capacitor, propped up by a data book. Classic stuff.

(Yes, I really want a 661.)


04 January 2012

App Note 74 part 1

"Component and measurement advances ensure 16-Bit DAC settling time: The art of timely accuracy." 48 pages.

This app note discusses the settling time of digital-to-analog converters. The measurement of settling time is a topic that Jim has discussed before (see Appendix B in App Note 47), but this app note is the most exhaustive treatment so far. This app note is also very, very dense.

There is a plethora of good advice here, included here for several reasons. First of all, the problem is hard. As Jim explains on the first page
In particular, the settling time of the DAC and its output amplifier is extraordinarily difficult to determine to 16-bit resolution... Measuring anything at any speed to 16 bits (0.0015%) is hard.
Secondly, and most importantly, he solves the problem FOUR times. Not content to simply update the settling-time test circuit from App Note 47 (compare App Note 47 Figure B2 with App Note 74 Figure 6), but he also verifies the measurement three more times. One, a bootstrapped clamp in shown in Figure 19. Two, a direct interface to a classical sampling oscilloscope is shown in Figure 26. And three, a unique differential amplifier is employed in Figure 28. These four methods are summarized on page 15.

These results are the fruits of a monumental effort. These four similar scope traces probably represent months of work, and it is an amazing testament to his laboratory skill that they all match so well.

Out of these four measurements, I think the two most interesting ones are the settling-time test circuit and the direct interface to a sampling oscilloscope. The settling-time test circuit in Figure 6 is an update to the circuit discussed in App Note 47. He has made a few improvements to the circuits and replaced a few of the amplifiers with better components. The big improvement is in the choice of the diode bridge; instead of four Schottky diodes, he uses a monolithic array of vanilla diodes with a temperature-control loop. (The temperature control is similar in concept to his temperature-stabilized transistor array in National App Note 299.)

The other interesting measurement uses his Tektronix 661 sampling scope in Figure 26. As he says on page 3, "The only oscilloscope technology that offers inherent overdrive immunity is the classical sampling 'scope." In the footnote, he comments about Appendix B and some of the references and, in particular, "Reference 15 is noteworthy; it is the most clearly written, concise explanation of classical sampling instruments the author is aware of. A 12 page jewel." This reference can still be found on the Tektronix website.

Two more great quotes are worthy of mention. On page 7, he discusses clamp diodes that protect the diode array from damage. In the footnote, he confesses,
This can and did happen. The author was unfit for human companionship upon discovering this mishap. Replacing the sampling bridge was a lengthy and highly emotionally charged task.
On page 17, he discusses the General Radio model 1422-CL precision variable air capacitor. Again, the great quote is the description in the footnote,
A thing of transcendent beauty. It is worth owning this instrument just to look at it. It is difficult to believe humanity could fashion anything so perfectly gorgeous.
There is a similar model capacitor currently listed on eBay for $3,000!

I'll cover the appendices next time.


02 January 2012

Top App Notes (so far)

With all of the great application notes that Jim wrote, which ones are best? App Note 47 is one of my favorites, and App Note 65 represents a significant and lengthy development effort, but which other ones are worthy of the title "Best App Notes"?

To help settle the question (or at least kick off the conversation), I've started a list: Jim Williams's Best App Notes (so far).

For now, the list includes App Notes 47, 65, 70, 43, and 25. As I continue reading through the rest of his app notes, the list will change as new app notes get added, the order is rearranged, and old app notes drop off the bottom of the list.

(Eventually, this list will probably be a "Top Ten" list, but for now, it's "Top Five". There are some great app notes coming up in the next few weeks.)

Which app notes are you favorites?

01 January 2012

Top Five Posts 2011

Since all the cool kids are doing it, I thought I'd jump on the bandwagon. I started this blog in July, so I really only have six months of traffic, but that doesn't stop me from declaring:

The Top Five Most Popular Posts in 2011
  1. Scope Sunday 3
  2. My airline adventure with a Tektronix 453, along with a letter from Jim.
  3. App Note 65 part 2
  4. Commentary about CCFL power supplies, and the "Royer" pin on the LT1183. This post generated a lot of traffic and spawned a discussion and the dedicated post on "IC pins named after persons".
  5. Scope Sunday 13
  6. Photos from my trip to the Computer History Museum to see the opening night of Jim's Linear Technology laboratory bench on display (with my business card in the mess).
  7. Introduction
  8. The entry point to the blog, often posted on other web sites (and listed here under Jim's picture).
  9. App Note 35 part 1
  10. Jim's amazing simultaneous time-domain and frequency-domain measurement (using a Tek 556 scope from the 1960s), that puts Tektronix's new MDO4000 to shame.

Of course, listing these links gives these posts a head start in the race for "Most Popular Posts in 2012" (it's an interesting positive-feedback loop, isn't it?), but I hope that I will produce even more interesting posts in the coming months!

Which post was your favorite?

Postscript: Coincidentally, this post is my one-hundredth post to this blog. Yikes!