10 February 2012

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."

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