Download V/F Converter ICs Handle Frequency to Voltage Needs (National Semiconductor, 1980, 8 pages, 187 KB pdf file)
The LM331 is a voltage to frequency (V/F) converter IC, but it can very easily be used as a frequency to voltage (F/V) converter if a few external components are arranged differently, and this is the topic of this app note. Pease first shows the basic F/V circuit and then discusses several improvements: Reduction of the nonlinearity to 0.006% (typ), adding an output buffer, reduction of the voltage ripple of the output, and compensation for the temperature coefficients of the external components. The last point should also be interesting if you are more interested in using the LM331 as a V/F converter, because Pease discusses properties of capacitors and resistors that are of general interest if you want to build precision circuits ( you can find some more information about this topic in AN348 Avoiding Passive Component Pitfalls ). The last circuit shows how you can build a (rather slow) frequency detector by adding two comparators.
Designing V/F converters was a specialty of Bob Pease. He has contributed 2 chapters to Analog Circuit Design and in one of these (chapter 29, p. 349 - 360) he explains how he designed his first V/F converter (a discrete circuit) back in 1967 and later moved on to ICs. The chapter is quite interesting historically and contains a few technical interesting facts, but the LM331 is only mentioned shortly. More important for practical purposes is Nationals datasheet of the LM331 / LM231 (the 231 is a close relative of the 331, one of the differences is the extended temperature range of the 231). Anyway, in the book Pease proudly mentions that "It's been pretty popular, and to this day, 13 years after it was introduced, the LM331 is still getting designed in to new circuits. People tell me they like the way it is versatile and offers good precision." This was written in 1991, and today the LM331 is still in production ! (only drawback: it is only available as DIP, not in SMT)
You might also be interested in the other app notes and columns by Bob Pease.
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