App Note 93

Instrumentation applications for a monolithic oscillator: A clock for all reasons. 20 pages.

This app note discusses applications of the LTC1799 oscillator chip. The approach here is classic Jim: Linear Technology produces a simple oscillator chip, and Jim's first thought is "temperature measurement!" The frequency of the LTC1799 is set with a single resistor, so Jim immediately replaces it with a platinum RTD (Figure 5) or a thermistor (Figures 7 and 9), in a variation of his normal voltage-to-frequency-converter theme. Figure 9 also includes galvanic isolation. Other unique sensors also appear. Figures 11, 13, and 15 use a fragile capacitive humidity sensor (which we've seen before), using the LTC1799 as either a reference oscillator or a clock source.

Figures 17 and 19 continue the instrumentation theme with a chopper-stabilized amplifiers. Figure 17 uses a bipolar-op-amp input, while Figure 19 uses JFETs for lower bias current, but slightly higher noise. He also includes two sine-wave generators, Figure 21 uses a resonator loop and Figure 23 is a memory-based DDS (see also Appendix E in App Note 35).

A few almost-digital circuits wrap up the note. A clock-tunable notch filter is shown in Figure 28 and a clock-tunable interval generator in shown in Figure 30. The last circuit (Figure 32) is a single-slope analog-to-digital converter that shares some topology with the interval generator.

The two appendices discuss the LTC1799 in a little more detail. Appendix A discusses the internal topology and the master oscillator, and Appendix B discusses the care and feeding of the RSET pin (and potential bootstrapping).

The best quote is the pedantry in the first footnote,

Strictly speaking, an oscillator (from the Latin verb, "oscillo," to swing) produces sinusoids; a clock has rectangular or square wave output. The terms have come to be used interchangably and this publication bends to that convention.

The cartoon extols the simplicity of the LTC1799, "Everything should be as simple as possible, but not too simple."

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?

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Related:

• App Note 75 part 2

App Note 72 part 2

The applications section beings on page 21.

Again, as in App Note 13, he shows a wide variety of applications for this high-speed comparator. He starts with a crystal oscillator (Figure 47), also shown with switchable outputs (Figure 48), temperature compensation (varactor-biasing in Figure 49), and voltage control (more varactor-biasing in Figure 51). Figure 53 shows a voltage-control clock-skew generator with plus-and-minus 10-nanosecond skew. Voltage-to-frequency converters make an appearance in Figure 55 and 57. Figure 55 is a simple V-to-F, while Figure 57 is a precision topology, offering seven decades of frequency output (1 Hz to 10 MHz).

Several instrumentation applications are also included. Some of these circuits are related to the instrumentation circuits that he has used in recent (and will use in future) app notes. Trigger circuits with variable threshold (Figure 60) and adaptive threshold (Figure 63) are shown (we saw the adaptive-trigger circuit in Appendix C of App Note 70). A technique for increasing the comparator gain (using the venerable 733 amplifier, achieving 500-microvolt sensitivity) is shown in Figure 65.

A variable-controlled delay, up to 300 ns, is shown in Figure 69. Two circuits that use this delay are shown next: A high-speed sample-and-hold for repetitive signals (like Figure 29 in App Note 13) is shown in Figure 73. Figure 69's delay is also used in Figure 75 to add a programmable-delay trigger to his favorite pulse-generator topology (I think this circuit is the best circuit in the app note; I really need to build one for myself). Figure 77 shows these fast pulses using a 3.9-GHz sampling scope (a Tektronix 661 with a 4S2 plug-in).

Figure 78 shows a high-speed pulse stretcher that can trigger on pulses as small as 2-ns width input. The final circuit is a overload-protection circuit breaker, shown in Figure 83.

The app note ends with two appendices. The first one, "About level shifts", is from App Note 13. The second one, "Measuring probe-oscilloscope response", is a slightly modified reprint of Appendix D from App Note 47. This version includes Figure B2, taken with the 12GHz sampling oscilloscope that was alluded to in the footnote on page 93 of App Note 47 (seven years after writing that footnote, he finally gets to print the scope trace!).

The app note concludes with a cartoon that compares the LT1394 with other comparators, such as the LM311, the LM360/361, and the AD790.

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Related:

• App Note 72 part 1

App Note 61 part 2

Continuing on, Figure 26 shows an improved high-speed pulse generator. Compared to Appendix D in App Note 47, this pulse generator has a distinct advantage in that it is triggered. "This feature permits synchronization to a clock or other event." The 2N2369 is biased just below avalanche breakdown, and the incoming trigger input causes breakdown (due to the difference between Vceo and Vces). Note the comment about the Tektronix 661 with the 4S2 sampling plug-in ("I'm sorry, but 3.9GHz is the fastest 'scope in my house"). Nice upgrade from the 1-GHz 1S2 sampler that he has used previously. "Ground plane type construction with high speed layout, connection and termination techniques are essential for good results from this circuit." Yep. (I think this circuit is the best circuit of the app note.)

Figure 29 shows a special voltage regulator for flash-memory programming. Unfortunately, he does not compare and contrast this design with the designs in App Note 31. Is this circuit just as good? (It probably is, but why?). What are the features of the LT1109 that make it a good fit for this (touchy and sensitive) application?

Figure 31 shows a low-voltage voltage-to-frequency converter (operating from a 3.3-volt supply). The next two circuits are broadband noise sources. Figure 33 uses a Noisecom NC201 noise diode with a selectable filter to produce the noise, and a RMS AGC loop to set the amplitude. Figure 37 uses a standard zener diode as the noise source, but requires a trim to set the initial noise level. Figure 38 is a switchable-output crystal oscillator, which uses diodes to select which crystal is active in the feedback path of the comparator. Cute.

Appendix A includes significant reprints from the manual of an HP3400A True RMS Voltmeter. Jim compares the approach used here to his approach in Figure 22. The instrument he discusses here is a classic, with an impressively creative solutions to several design challenges. The input buffer in Figure A1 uses Nuvistors (because JFETs weren't good enough in 1965). Figure A2 shows a photograph of the input-buffer circuit board. The "video amplifier" in Figures A3 and A4 is an impressive design with DC and AC feedback loops and clever bootstrapping. The chopper amplifier in Figures A5 and A6 uses neon bulbs and photocells for the chopping action! (As Jim says, "Hewlett-Packard has a long and successful history of using lamps for unintended purposes.") Figure A7 shows the circuit board for Jim's RMS-to-DC converter from Figure 22.

The topic of RMS-to-DC conversion was near and dear to Jim's heart. He covered these circuits in detail in App Note 22, and he used them in his CCFL explorations. He helped to design the LT1088 IC and was its main evangelist. In the footnote on page 28, Jim tries to explain the context: "We are all constantly harangued about the advances made in computers since the days of the IBM360. This section gives analog aficionados a stage for their own bragging rights. Of course, an HP3400A was much more interesting than an IBM360 in 1965. Similarly, Figure 22's capabilities are more impressive than any contemporary computer I'm aware of."

Do I detect some frustration in Jim's voice here? Compare all of the circuitry of the HP3400A with the circuit in Figure 22. In effect, I think Jim is saying, "Look how easy I'm making it for you! Why aren't you buying more LT1088 chips?"

Best quote (page AN61-38): "Incidentally, what were you doing in 1965?"

The app note concludes with a great cartoon (perhaps the best cartoon so far).

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Related:

• App Note 61 part 1

App Note 47 part 4

The final two application sections (pages AN47-51 to AN47-66) cover data-conversion circuits and some miscellaneous things. As footnote 15 on page AN47-51 says, "Seasoned readers of LTC literature, a hardened corps, may recognize this and other circuits in this publication as updated versions of previous LTC applications. The partial repetition is justified based on improved specifications and/or simplification of the original circuit." Jim undersells the application circuits here; some of the improvements (utilizing these high-speed op amps) are a big deal.

Figure 118 shows a sine-wave VCO, again using a AD639 as a triangle-wave-to-sine-wave converter (see Figure 21 in App Note 13). As I've said before, the AD639 was a Barrie Gilbert's brilliant Universal Trigonometric Function Converter, and using it as a triangle-to-sinusoid converter is like using a Lamborghini to go get your groceries.

Figure 121 is a 1Hz-to-10MHz V-to-F converter. Ten megahertz is pretty fast, but remember that App Note 14 includes the 100-MHz "King Kong" V-to-F converter. Of course, the advantage here is that with the high-speed op amps, there's no more need for the scope-trigger circuits and exotic 10H ECL parts. I especially like the caption of Figure 123 "(Whoosh!)".

Figure 124 is a high-speed (100 ns!) sample and hold, using a four-diode gate with transformer drive. This topology is an elegant solution for a high-speed S&H. Compare to the discrete 200-ns sample-and-hold circuit in Figure 23 of App Note 13.

Figure 131 is a trigger circuit with adaptive threshold, basically the adaptive subcircuit from Figure 97 (I wonder why he didn't present these two circuits in the opposite order?).

Figure 132 is a simple pulse-width-to-voltage converter, using a current source charging up a capacitor, like the single-slope converter in Figure 33 of App Note 13. Despite (or, perhaps, due to) being simple, the performance is very fast: able to resolve 1% accuracy on 250-ns pulses. The drive circuitry for Q3, including the Baker clamp and the speed-up capacitor, is especially instructive.

Figure 137 shows another application circuit for his LT1088 RMS-to-DC converter. This circuit is the same as Figure 8 in App Note 22, except the LT1223 is used instead of the discrete buffer suggested in App Note 22. Figure 139A shows a RF-leveling loop (from App Note 22 Figure 27), using the RMS-to-DC converter from Figure 137. (Figure 139B show a much simpler RF-leveling loop.)

Figure 140 shows a voltage-controlled current source (basically, it's Figure 8 from App Note 45 with much faster op amps). Figure 142 shows a higher-power version of the current source, using a discrete output stage.

Figure 144 shows a high-speed (18 ns!) circuit breaker (compare to the 12-ns version in Figure 40 of App Note 13, which required a floating load).

The best circuit in these sections is a toss up between the elegant sample-and-hold circuit in Figure 124 or the high-speed pulse measurement in Figure 132. (I still think the best circuit in the whole app note is the AM radio station in Figure 116.)

Best quote (page AN47-58): "Digital methods of achieving similar results dictate clock speeds of 1GHz, which is cumbersome." Understatement?

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Related:

• App Note 47 part 1
• App Note 47 part 2
• App Note 47 part 3
• App Note 47 part 5
• App Note 47 part 6

App Note 45 part 2

Again, we have a collection of circuits here that captured Jim's imagination in some way or are improvements of circuits from previous app notes.

Figure 18 shows a quartz-stabilized oscillator, which is a different approach from the Hewlett-Packard-inspired oscillators in App Note 43. This circuit achieves 9 ppm distortion (Figure 48 in App Note 43 achieved 3 ppm distortion), but it requires a 4-kHz J-cut crystal.

Figure 19 is a single-cell-powered temperature-compensated crystal oscillator, similar to App Note 15, Figure 9. A boost converter is used to drive the varactor diode with bias voltages up to 4V.

Figure 21 appears to be an improved version of the "Zoo Circuit" V-to-F converter (App Note 23 Figure 16) with even lower power consumption (maximum 90 microamps). Figure 24 is another V-to-F converter, this one with a bipolar input (and a start-up circuit adapted from the Tek 547 trigger circuit).

Figure 27 is a 350-ps rise-time pulse generator. This circuit will be very useful in App Note 47 and other upcoming app notes (and it is much better than the 1-ns pulse generator in App Note 13 Figure D1). The pulse in Figure 28 is very clean, shown on his Tek 556 with the 1S1 sampling plugin. "I'm sorry, but 1GHz is the fastest scope in my house." (See Reference 7.)

Figure 30 is a low-dropout regulator using the LT1123 and the specially design (and now unavailable?) MJE1123 transistor. A germanium 2N4276 is explored as a replacement (but is no easier to obtain!).

Figure 36 is a power supply for a cold-cathode fluorescent lamp. Look at all the bottles! I count 48 of them. Yikes. Although Jim may not know it yet, this application is the beginning of a long-term obsession (or was it an assignment?). More praise for the Tektronix 556 and 547 on the bottom of page AN45-22.

Best quote (from Figure 36, a harbinger of future difficulties): "Do not substitute components."

More than half of the references on page AN45-23 (References 9 to 17) have to do with fetal heart monitoring (as shown in Figure 1). I feel a little sorry for Jim's wife and unborn son at this point. The App Note concludes with a great picture of Michael, perched atop a Tektronix 556.

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Related:

• App Note 45 part 1

Book 1 Chapter 23

Chapter 23: "The zoo circuit: History, mistakes, and some monkeys design a circuit."

This chapter discusses the development of the "zoo circuit", a low-cost low-power voltage-to-frequency converter, which originally appeared in App Note 23. Powered off of a 9-volt battery, the circuit works from 0 to 10 kHz with good linearity, and consumes only 200 microamps.

Quite a few of the figures are drawn from previously published app notes.

• Figures 23-3 to 23-6 come from App Note 14, Figures B2 to B5.
• Figure 23-7 is reproduced from the front page of the LT1055/LT1056 data sheet. (Did it also appear in another App Note? I can't seem to find it.)
• Figures 23-9, 23-11, and 23-13 come from App Note 23, Figures A1, A2, and A3.
• And, of course, the end result in Figures 23-19 to 23-22 come from App Note 23, Figures 16 to 19 (although there's a small error in Figure 16)

The text of this chapter is a good study in methodical circuit design. (This chapter is as good of a polemic against the promises of "effortless" computer-aided design as yesterday's chapter is.) Jim starts the development of his circuit with a version of Bob Pease's Teledyne-Philbrick 4701, shown in Figure 23-7. In the following development, he carefully investigates the sources of power consumption, nonlinearity, and temperature drift, and attacks them in turn. After some careful iteration, and a flash of insight at the zoo, he arrives at the final circuit in Figure 23-19.

Finally, here's the the eponymous quote (first the first page): "Most of the ideas came from history, making mistakes, and the best source of help was some monkeys at the San Francisco Zoo."

A bibliographic note: this chapter also appears as Chapter 18 in Bob Pease's book, "Analog Circuits: World Class Designs". By the way, "Zoo Circuit" is one of the search terms that seems to bring people to this blog. For those searching for such information, don't miss the updated versions of this circuit, which consume as little as 8 microamps, in App Note 75 (see Appendix A and pages 1 to 4).

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Related:

• Book 1 Introduction
• Book 1 Chapter 4
• Book 1 Chapter 7
• Book 1 Chapter 13
• App Note 14
• App Note 23

App Note 23

"Micropower circuits for signal conditioning." 24 pages.

This app note discusses techniques for designing extremely low-power circuits.  As Jim says, "Although micropower ICs are available, the interconnection of these devices to form a functioning micropower circuit requires care."

The first two circuits are low-power temperature sensors (the first with a thermistor and the second with a thermocouple).  The next two circuits are strain-gauge amplifiers, which use sampling techniques to achieve micropower performance, exploiting low duty cycle to give low power consumption.  (Many more bridge circuits are coming up in App Note 43.)  The next three circuits are temperature-monitoring applications for the LTC1040, LTC1041, and LTC1042 family of micropower comparator circuits.  Figure 6 is a clever 4mA-to-20mA current-loop thermistor amplifier that uses the current loop signal as the power source.

Figure 9 is a micropower SAR analog-to-digital converter.  Successive approximation is a good technique for low-power A-to-D, but unfortunately, now that SA registers (like the 74C905 used here) have been discontinued, it's harder to implement in discrete form.  Figure 11 shows a micropower single-slope A-to-D converter, which only consumes 100 microamps (the recently discontinued 74C906 could easily be replaced by another low-offset CMOS switch).  Figure 14 is a micropower sample and hold (SAR A-to-D converters require a S&H front end). This circuit cleverly uses the programming pin on the LT1006 to turn down the power consumption in the hold mode.

The best circuit is Figure 16, a micropower 10-kHz voltage-to-frequency converter.  This circuit is also known as "The Zoo Circuit" and Jim wrote a chapter in his first book dedicated to it.  Note the quote on page AN23-13: "A nice day at the San Francisco Zoo…, instrumental in arriving at the final configuration, is happily acknowledged."  Rather than discuss the circuit here, I'll wait until I review his first book.  (However, I will note here that there is an error in the schematic: the base of Q4 should be connected to its collector.)  Figure 20 is a higher-speed V-to-F converter, reaching 1 MHz.

The final circuits are power regulators.  Figure 22 is a switching regulator (using a discontinued 74C907 as the switch, we've got the whole family now).  Figure 25 is a switching regulator that maintains a constant voltage drop across the LT1020 regulator, using the integral comparator.  I like the start-up circuitry here.  Figures 28 through 31 show off other tricks using the LT1020.

The box sections at the end (why not appendices?) cover a number of topics.  Box Section A discusses low-power techniques and the design evolution of the zoo circuit.  Box Section B discusses the LTC1040, LTC1041, and LTC1042 family of micropower comparator circuits.  And finally, Box Section C discusses the effects of test equipment on micropower circuits (or, stated another way, suggestions that might power your circuits from the input source: just turn up the amplitude on that pulse generator).

Best quote (from page AN23-19): For example, everyone "knows" that "MOS devices draw no current." Unfortunately, Mother Nature dictates that as frequency and signal swings go up, the capacitances associated with MOS devices begin to require more power. It is often a mistake to automatically associate low power operation with a process technology. While it's likely that CMOS will provide lower power operation for a given function than 12AX7s, a bipolar approach may be even better.

App Note 15

"Circuitry for single cell operation." 8 pages.

This app note suffers from a curious self-contradiction, to which Jim almost admits in the last section: "No commercially available logic, processor or memory family will operate from 1.5V. Many of the circuits described [herein] normally work in logic-driven systems." In other words, there are some great circuit designs here that work within 1.5-volt power supply, but what circuits interface to the ADC, VFC, and S&H? The A-to-D converter and the V-to-F converter have "logic" level outputs, and the sample-and-holds have "logic" level inputs (where "logic" means high (1.5V) and low (0V) voltages), but there is no logic family that works at 1.5V (or at least there wasn't in 1985; ironically, these circuits are probably more useful today than they were then).

But, if you go ahead and use Figure 13's flyback regulator (based on a Bob Widlar design) to produce a five-volt supply to power your logic, why not run the whole system from five volts? In fact, App Note 11 has already covered clever tricks for five-volt circuits, and App Note 8 has already covered power-supply design techniques for battery-powered systems (especially App Note 8 Figure 18). So what are we doing here?

Why do people climb Mt. Everest? Because it's there. Why does Jim design circuits for 1.5V operation? Because he can. Getting active circuits to work at 1.5V is deep magic, and Jim has it. Part of the magic is the LT1017/1018 comparator, part of the magic is Widlar's LM10 op amp, and part of magic is clever circuit design. Figures 1 and 3 show how to use capacitor charging and the low-voltage comparators to implement a V-to-F converter and a single-slope A-to-D converter, respectively. Figure 6 is a ramp-and-hold circuit using the LM10 op amp.

The best circuit is Figure 7, a more traditional sample-and-hold with a JFET switch and a charge pump to get enough negative voltage to turn the switch off. (Is a charge pump cheating? Maybe.) This S&H requires careful hand selection of the JFETs to get a below-500-millivolt pinch-off transistor for Q1, and 500-microvolt matching for Q2 and Q3. Do you want low voltage operation? You pay for it!

Figure 9 is another temperature-compensated crystal oscillator (see App Note 12 Figure 6), using a voltage-boost stage that is shown in Figure 11 here (and also App Note 8 Figure 18 and similar to App Note 6 Figure 7; he must have had a box of Triad transformers).

The App Note concludes with a box section on "Components for 1.5V operation" which basically just says "LT1017, LT1018, and LM10; good luck, sparky!" (He also mentions the LT1004 and LT1034 voltage references.) He discusses the pros and cons of silicon and germanium diodes and transistors (if you have a stash of germanium diodes and transistors, that is). There is also a graph of battery voltage versus life for carbon-zinc and mercury, which is quite interesting.

Best quote (for the specifications, from page AN15-8): "The LM10 op amp-reference runs as low as 1.1V; the LT1017/LT1018 comparator goes down to 1.2V. The LM10 provides good DC input characteristics, although speed is limited to 0.1V/us. The LT1017/LT1018 comparator series features microsecond range response time, high gain and good DC characteristics." Amazing.

App Note 14

"Designs for high performance voltage-to-frequency converters." 20 pages.

I'm surprised at the shear number of voltage-to-frequency converters that have appeared in these early application notes. Jim really had a V-to-F obsession going on here. So far, we've seen V-to-F converters in App Note 3 (1 circuit), App Note 7 (7 circuits), App Note 9 (3 circuits), App Note 11 (1 circuit), and App Note 13 (3 circuits), and now we've got a whole 20-page app note devoted to them (and there's more to come in future app notes). Did Linear Technology have that many requests for help with voltage-to-frequency applications? Or did Jim just love building them?

Of course, if the progression from App Note 7 Figure 8 (100 dB of dynamic range), to App Note 9 Figure 21 (150 dB), to Figure 1 of this App Note (160 dB) is any indication, he's getting good at designing V-to-F converters. Really good.

This app note is dedicated to ten more circuits that implement voltage-to-frequency converters. The box section (starting on page AN14-19) contains a brief tutorial, showing five different basic topologies (it's good introductory reading before the rest of the app note). Clearly, the best circuit is the star (and start) of the show, the "King Kong V-to-F" in Figure 1. This circuit is an amazing topology (compare it to Figure 21 in App Note 9). The text says "This circuit, similar to those employed in oscilloscope triggering applications..." and if you peek ahead at the references, you'll see a reference to the trigger circuit in the Tektronix 2235. Here's the relevant portion of the schematic from the 2235 service manual:

This circuit is an impressive example of Jim's talent for synthesis. Marrying together the brawn of speed (the trigger circuit with the ECL 10K and 10H gates) and the brain of linearity in the feedback path (the LTC1043 switched capacitor and the LTC1052 chopper amp), this Frankenstein monster really does achieve the best of both worlds. (Staying with the monster-movie theme: it was either that, or a gorilla joke.) A lesser engineer would have just ended up with a muddled mess. This circuit is an example of great design (the feedback path), outright theft (the trigger circuit), and a few gruesome hacks (the linearity correction and the one-gigaohm resistor in the collector of the 2N3904). Respect.

Figure 5 is another Pease-type V-to-F converter, but a "highly modified, high speed variant" of a Pease-type. Hmm. It would be interesting to check Bob Pease's publications in this time period and see if there was some friendly one-upmanship going on between these two former National Semiconductor colleagues.

The rest of the circuits cover a wide variety of V-to-F schemes and functions. Figure 8 is quartz stabilized for improved temperature coefficient. The "ultra-linear" design in Figure 10 includes additional inputs and hooks for a processor-driven gain-and-offset calibration scheme. Figure 12 is a 1.5-volt single-cell circuit exploiting the LT1017 comparator (pressed into service as an op amp, too, here). (More single-cell circuits are coming up in App Note 15 tomorrow). Figure 14 is another sine-wave output VCO (again abusing the under-appreciated AD639; see App Note 13 part 2). Figures 17 and 19 have 1/x transfer functions, and Figure 21 has an exponential transfer function (which, instead of volts-per-hertz, implements volts-per-octave for electronic-music applications). Finally, Figure 23 implements a ratiometric converter, measuring the ratio of two resistors.

The best quote is the (possibly) subtle tweak at Bob Pease (on page AN14-4): "The charge feedback scheme used is a highly modified, high speed variant of the approach originally described by R. A. Pease." In other words, Bob had a good idea, but it was too slow!

App Note 13 part 2

The applications section of App Note 13 includes a dozen more of his favorite circuits. There are three voltage-to-frequency converters. The first one (Figure 16) goes to 10 MHz and is based on a feedback charge pump (some similarities to Figure 8 in App Note 7). I really like the trick of replacing the input stage of the LT318A with discrete 2N4393 FETs connected to the op amp's balance terminals. Clever. The second V-to-F circuit (Figure 19) is a quartz-stabilized design that goes to 30 MHz, using a sampled-data DAC in the feedback path. Why doesn't this schematic include the oscillator circuit?

The third V-to-F is shown in Figure 21, with a sine-wave output using an AD639 as a triangle-to-sinusoid converter. I'm sad to see the (now-discontinued) AD639 used this way. The AD639 was a Barrie Gilbert's brilliant Universal Trigonometric Function Converter, and using it as a triangle-to-sinusoid converter is like using a Lamborghini to go get your groceries. It was capable of producing sine over a plus-and-minus 500 degree range, along with cos, tan, sec, csc, cot, arcsin, and other functions, using some really interesting bipolar-transistor tricks. The data sheet and Gilbert's JSSC paper are a fun read. It really was a brilliant chip; too bad nobody really figured out what to do with it.

(B. Gilbert, "A monolithic microsystem for analog synthesis of trigonometric functions and their inverses," IEEE Journal of Solid-State Circuits, vol. 17, no. 6, pp. 1179–1191, Dec. 1982.)

There are also three sample-and-hold circuits, continuing on a theme that he started in National Semiconductor App Note 294, "Special Sample and Hold Techniques." Figure 23 shows a high-speed ramp-and-hold circuit that uses a single-slope-integrator technique to match the input voltage. He uses a 2N5486 JFET to buffer the hold capacitor from the output (in this circuit and the next one). Figure 26 uses a similar topology to implement a track-and-hold circuit that oscillates around the input voltage. Oscillations on purpose? Scary.

Figure 29 revisits a sample-and-hold circuit for repetitive signals from AN-294, although it is improved from a 100-ns sampling window to a 10-ns window. This circuit samples a single voltage at an adjustable delay time after a zero crossing. Just think, with a bank of 32 of these circuits, you could build the front end of an analog sampling oscilloscope.

The next two circuits are analog-to-digital converters. Figure 31 is a two-speed SAR converter (a topology that he will revisit in App Note 17). Figure 33 is a single-slope-integrating converter. Several tricks are used to improve performance, such as the Schottky diode on the latch pin and the temperature-compensating resistor in the current source. Also note Q4, the 2N2222 being used in the reverse-active region to improve the capacitor reset voltage.

Figure 36 shows a high-frequency (2.5 MHz!) precision rectifier that uses a four-diode gate with level-shifted drive like Figure 5 of App Note 10. Figure 38 is a fiber-optic receiver, using a discrete-transistor amplifier for the high-frequency gain stage (Q1 through Q6), and a cute min-max detector (Q6 and Q7) that samples the peaks and valleys of the input signal and calculates the midpoint for the comparator threshold and the level-out monitor.

Figure 40 shows a 12-ns circuit breaker (I like the output stage of Q1, Q2, and the diode that is virtually the same topology as a discrete TTL gate). Figure 42 is a high-speed trigger circuit, using a discrete-transistor FET input buffer and showcasing the high speed of the LT1016 (50 MHz!).

Whew! I'll discuss the appendices tomorrow.

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Related:

• App Note 13 part 1
• App Note 13 part 3

App Note 11

"Designing linear circuits for 5V single supply operation." 16 pages.

This app note contains some clever tricks for circuits powered from a single 5V rail, most of them using LT1013/14 low-voltage op amps and LT1017/18 low-voltage comparators. The box section on page AN11-15 tells a little bit of the history of linear supply voltages, starting with the 300-volt supplies used in vacuum-tube analog computers, and going down from there. (How low can you go? Stay tuned: single-cell circuits are coming up in App Note 15.)

Again, the circuits here have interesting sensors. The platinum RTD in Figure 1 and the thermocouple in Figure 5 are old news, but Figure 2 shows a unique methane detector with a voltage-to-frequency output. A little translinear circuit is used to compensate for the sensor's square-root behavior. Figure 7 shows a strain-gauge circuit that uses a voltage inverter (LTC1044) and an op amp to servo the left side of the bridge to zero volts. (There are many more bridge interfaces coming up in App Note 28.)

The schematics in Figure 6 show instrumentation amplifiers for 5V operation, one using the standard-topology three-op-amp instrumentation amp with a LT1014 quad op amp, and the other circuit using the LTC1043 switched-capacitor block and LTC1052 chopper amp from App Note 9 Figure 11.

Figure 8 shows a "tachless" motor speed controller that senses the motor's back EMF to determine speed. This approach makes me nervous. Sensing the voltage this way is really noisy! The design of windings (and commutator brushes) is very different for a motor than for a tachometer. I think the key phrase here is "Q3 turns off and the motor's back EMF appears after the inductive flyback ceases." You can see all of the signal junk in Trace B of Figure 9, and the residual junk after filtering in Trace D. I'm always impressed when an engineer can get this noise-plagued approach to work even this well.

Circuits for systems requiring galvanic isolation are shown in Figures 11, 12, and 14. The best circuit is Figure 14, a fully isolated single-slope analog-to-digital converter. As in Figure 12, the transformer is used both to power the isolated circuits and to transmit data (in this case, the pulses from the A/D conversion) across the isolation barrier.

Best quote (from page AN11-1): "The difficulties encountered in maintaining the lowest possible levels of noise and drift in an analog system are challenging enough without contending with a digitally corrupted power supply."

App Note 9

"Application considerations and circuits for a new chopper-stabilized op amp."  20 pages.

This app note, discussing the LTC1052 chopper-stabilized op amp, includes a wealth of good advice for designing and building high-precision circuits. (The box section on the final two pages discusses chopper theory in more detail.) In particular, the discussion of the thermocouple effect starting on page AN9-2 is very valuable. Figure 5 is worthy of careful study to understand the deleterious effects of thermal EMFs on precision circuitry. This kind of attention to detail separates man from mouse in the microvolt regime. A thermal gradient across parasitic thermocouples created by copper connections to solder or Kovar can be a significant source of error. (What's Kovar, you ask? It's a magical alloy of nickel that makes vacuum tubes, light bulbs, and other glass packages for electronics possible. While you're looking it up, look up the other magical alloys of nickel, like invar, elinvar, constantan, and mu-metal.)

More vintage hardware: Figures 2 and 7 show strip-chart recordings of the low-frequency noise. A strip-chart recorder is another one of those valuable tools (particularly for low-frequency noise measurements) that I wish I had room for. Figure 9 shows a voltage reference using several components from Julie Research Labs, including a saturated-cell voltage reference and a Kelvin-Varley divider. Julie Research Labs : when you absolutely, positively, have to exceed your research budget.

Figure 15 shows another HP oscilloscope photo (also, Figure 22). Heresy.

Figure 18 shows how to use the LTC1052 to correct for offset and drift in an Analog Devices AD650 monolithic voltage-to-frequency converter. Figure 19 shows a discrete voltage-to-frequency converter (that improves upon the specifications of the AD650 part) that is very similar to the architecture used in Figure 2 of App Note 7. Now we start to see the rest of the V-to-F dynamic range iceberg (see the so-called "best circuit" from App Note 7): Figure 19 has a dynamic range of 120 dB, Figure 21 has a dynamic range of 150 dB ("This is by far the widest dynamic range and highest operating frequency of any V-to-F discussed in the literature at the time of writing."), and even more is promised in App Note 14. See the footnote on page AN9-14.

Figure 25 shows a 16-bit analog-to-digital converter, this time using a sigma-delta architecture.

For the best circuit, I'm going to have to go with the thermometer in Figure 27, which feels like a wicked prank. This application points out that the offset drift of an AD547J (the competition's op amp, now obsolete) is so bad that it can be used as a temperature sensor. (In reality, the max drift was only 5 uV/C, but that is one hundred times worse than the LTC1052 discussed here.)

Best quote (page AN9-2): "Any connection of dissimilar metals produces a potential which varies with the junction's temperature (Seeback effect). As temperature sensors, thermocouples exploit this phenomenon to produce useful information. In low drift amplifier circuits the effect is probably the primary source of error."

App Note 7

"Some techniques for direct digitization of transducer outputs." 16 pages.

This app note claims "direct digitization," but the circuits are based on voltage-to-frequency conversion rather than analog-to-digital conversion. Thus, I disagree with the title: V-to-F conversion isn't really a digitization technique; V-to-F conversion translates signals from one continuous domain (voltage) to another continuous domain (frequency). There's no quantization, so it's not really "digital". However, it is discretized in time (at the edges of the output waveform), so that's something.

Terminological quibbling aside, V-to-F conversion is an under-appreciated, but useful, trick in practical system design, particularly in instrumentation circuits, as demonstrated here. V-to-F isn't as sexy as A-to-D, and it doesn't get as much (if any) emphasis in college courses, but it's a life saver in some applications. Transmitting information via frequency (particularly over long cable runs) is often a good idea.

Figure 2 shows a V-to-F converter for a Type-K thermocouple. It's a good example of using a crude V-to-F converter in the forward path (the 74C04 inverter chain) and then linearizing it with a precision charge pump in the feedback path. Feedback is awesome. (Possible schematic error: is there a connection dot missing from the summing junction of A1? I think the 3300pF capacitor, the 33k resistor, and the 150k resistor should all be connected to the minus input of the op amp.)

The other applications are great, too. Figure 4 shows an acoustic thermometer (for those of you reading ahead, acoustic thermometry is also the topic of Jim's last app note twenty-five years later, App Note 131). Figure 6 shows a circuit for strain-gauge that produces a full-scale signal current of only 48 nA. Figure 8 shows a "Pease type charge pump" current-to-frequency converter with 100 dB of dynamic range (20 Hz to 2 MHz). Figure 11 shows a "humidity-to-frequency" converter (compare to Figure 8 in App Note 3). Figure 14 shows a level transducer with AC drive, and Figure 16 shows a circuit for a capacitive accelerometer. Again, the real story is the great assortment of sensors and transducers that Jim comes up with. I'm constantly jealous of his resourcefulness and his junk pile.

I think all these circuits are great, but the best circuit is the photodiode digitizer in Figure 8: Anything with a useful dynamic range of 100 dB deserves respect (and this result is just the tip of the V-to-F dynamic-range iceberg; stay tuned).

Best quote (page AN7-8): "At a minimum, careful layout and a clean PC board are required. The best practice is to use a Teflon stand-off for all summing point connections." Good advice. You know you're down in the picoamps when Telfon standoffs become necessary.

App Note 3

"Applications for a switched-capacitor instrumentation building block.'' 16 pages. 

First, we've got an oscilloscope photo with eight traces and two different time scales in Figure 16. It must be a dual-beam Tektronix 556 with two 1A4 plugins. Excellent.

This app note discusses the LTC1043, a board-level switched-capacitor building block, with applications involving platinum RTD temperature sensors, capacitive humidity sensors, and LVDT position sensors. You can see Jim's history in physics and biomedical instrumentation rear its head when he starts discussing instrumentation amps (Figures 2 and 3), and the lock-in amplifier (Figure 4). Unless you have background in experimental science, you probably don't even know what a lock-in amplifier is! (Hint: it's a powerful technique for extracting useful signals from noisy environments. It is the hidden ace in many experimental laboratories. See page 10 of the Analog Devices AD630 datasheet, or read the book by Meade (Lock-In Amplifiers: Principles and Applications, 1983).)

Another early appearance of a classic Williams theme: the schematic in Figure 10 (the LVDT signal conditioner) includes a Wien bridge oscillator (misspelled here as "Wein"), with a FET resistance for amplitude control. (We'll see this circuit again and again, I assure you.) More classic hardware: the tuning procedure for Figure 7 suggests a General Radio 1432-K precision resistor decade box (page AN3-7). Good luck finding one (Hello, flea market).

Frequency-to-voltage and voltage-to-frequency converters are shown in Figure 12. More foreshadowing of future topics, here. Another clever trick in these circuits is the use of a resistor with a temperature coefficient that is opposite of the tempco of the polystyrene capacitor. A single-slope analog-to-digital converter (first of many) is shown in Figure 15.

I think the best circuit is either the lock-in amplifier in Figure 4 (previously discussed) or the the temperature-compensated crystal oscillator with varactor in Figure 21, but there's not much discussion of the latter. Too bad.

Best quote (page AN3-4): "In this [lock-in amplifier] application, the signal source is a thermistor bridge which detects extremely small temperature shifts in a biochemical microcalorimetry reaction chamber." Wait, what?