artem,
Yep, that's what I was going to suggest.
V+ ---- R1 ----- ADCin ----- R2 ------- V-
Then just adjust the divider ratio.
To the asker:
Vx = (Rx/Rt)Vt
so (...where's the "therefore" character thingy??)
VADCin - V- = (R2/(R1+R2))(V+ - V-)
An example, using a +5v and -5v rail:
+5v ----- R ----- tap ----- R ----- -5v
Vtap - -5v = (R/(R+R))(5v - (-5v)) = (R/2R)(5v + 5v) = (1/2)(10v) = 5v
Solvng for Vtap,
Vtap - -5v = 5v
Vtap +5v = 5v
Vtap = 5v - 5v = 0v
And this makes sense, since 5v are dropping across R (the right side of the equation) and the total supply was +5 --> -5v, or 10v.
So, you can see that you only need decrease the R to the +v supply to send Vtap positive or decrease the R to the negative supply to send Vtap negative. Voltage dividers are neato. ...at least I think.
Oh, and what value is R? Start with 10k and 10k or 100k and 100k. That'll give you 0v, and then you can lower (or raise of course) one of them to get whatever middle voltage you want.