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).