MODIFIED Σ-Δ VOLTAGE TO FREQUENCY CONVERTER

Milan Štork

Abstract

Voltage to frequency converter (VFC) is an oscillator whose frequency is linearly proportional to control voltage. There are two common VFC architectures: the current steering multivibrator and the charge-balance VFC. For higher linearity, the charge-balancing method is preferred. The charge balanced VFC may be made in asynchronous or synchronous (clocked) forms. The synchronous charge balanced VFC or "sigma delta" (&Sigma;-&Delta;) VFC is used when output pulses are synchronized to a clock. The charge balance VFC is more complex, more demanding in its supply voltage and current requirements, and more accurate. It is capable of 16 to 18 bit linearity.<br /> &Sigma;-&Delta; modulator can be used for synchronous VFC (SVFC). The synchronous behaviour is good in many applications, but the output of SVFC is not a pure tone (plus harmonics) like a conventional VFC, but it contains components harmonically related to the clock frequency. The SVFC produces a change in probability density of output pulses <i>N</i> and <i>N</i>+1 clock cycles after the previous output pulse.<br /> In this paper, the modified SVFC (MSVFC) is described. This MSVFC works similarly as conventional SVFC but it has a pure tone on output (for constant input voltage). Therefore, it is possible to measure the period of MSVFC output (this does not work for SVFC).

Keywords

voltage to frequency converter, charge balanced, sigma delta modulator

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IMEKO TC

TC4 - Measurement of Electrical Quantities

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