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?


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