square wave
Apparently, xvibe didn't understand your question because of the rather confusing specification. I understand now,
that you're requesting a voltage-to-frequency converter. To use it with a bipolar voltage, you have to apply an input offset.
Because most recent uP have on chip ADC, voltage-to-frequency conversion is a somewhat outdated method. As you
correctly said, a VCO is required as basic building block. Chips as National LM331 can provide a low part count solution.
You'll also find VCO circuits in many manufacturer application notes, e.g. National LM139 data sheet.