> However antoher idea is to use something cheap and standard like
> MAX232. Has anybody tried to use this chip just as a voltage converter?

Hi WZab. Yes, I've done this and it worked fine (for me)

http://home.clear.net.nz/pages/joecolquitt/white_noise.html

The +/- 8V is taken from a MAX232 between the PIC (a random
number generator) and a PC

> Unfortunately I can't find the output characteristics for V+ and V- pins

As there as so many clones and variants it might be best to pick a chip
and run some tests. If the only significant load is an op-amp then RC
filters and/or reservoir caps will probably be sufficient. Otherwise there
are LDO regulators with low quiescent currents for providing stable rails

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