Maybe you could use an ADC (Analog to Digital Converter). To get it out again, you could use a DAC (Digital to Analog Converter). You can find a lot of information about things like that at the chip manufacturers' websites, for example, http://www.national.com . They have many such ICs, each with a fairly-detailed datasheet that usually contains typical application circuit schematics. They also have many Application Notes, which often go into much more detail.
Of course, there are many details that I did not even hint at. You would probably also need either some other digital ICs, and/or a microprocessor of some sort, to manage the timing, addressing, etc etc, for the flow of data into the memory chip(s).
You could look into the PIC type of chips, and other similar types. Some of those include an on-board ADC. Some of their manufacturers' websites probably have application notes that come close to describing how to do what you asked about.
Someone with more digital knowledge than I have will probably be along, shortly, to give a more-coherent explanation and to fill in some of the gaps I left.