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Voltage to Frequency Conversion ADC

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Is there a way to get rid of the dead time? Yes. We can integrate in one direction until we reach one threshold level, then integrate in the opposite sense until a second threshold (possibly zero, though usually a bit above zero to avoid noise in the zero level) is reached. We continue switching back and forth, with the error due to switching time typically small compared to dead time error. We get one count for each combined up/down cycle. One might think that needing two comparators would be expensive and complex. Fortunately, early in the era of integrated circuits, the 555 timer was fabricated. It goes "High" when an input voltage is 2/3 of a reference, then switches "Low" when the input to a second input drops to 1/3 of the same reference. The reference can be the power supply potential (cheap, simple, readily available) or a separate, carefully controlled reference (more precise). Here's the circuit:

555-based V to F

A 555 timer can toggle reliably at at least 100 kHz, so 1 s integrations can have 5 significant figures. 10 s integrations? 6 significant figures, provided only that there's no drift in the values of R, C, Vref, or the mean of Vin during this time.



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