There's no way to perform natural log directly. When I had to perform the natural log function on the response from an A/D converter I calculated the natural log function for each A/D 3-bit point in an excel file and put it in a lookup table. Since the lookup table allows 255 entries (3-bits) this worked out.

If this method won't work you can use linear interpolation. Here's a website about it with many other helpful hints:

http://www.emesys.com/BS2math3.htm