Hey Andrea,
Before you worry too much about impedance problems (which are explained <a HREF=http://digital.ni.com/public.nsf/websearch/B9BCDFD960C06B9186256A37007490CD?OpenDocument>here</a>) first make sure that you can read in dc voltages correctly. Other things to check: 1.) Is the referencing set in MAX equal to the referencing set on the board? 2.) Is the board configured to use the BNC-2120 in MAX? 3.) Will low impedance sources work? If all of these point towards the source impedance of the photodiode being the problem, then the only real solution is a buffer. <a HREF=http://zone.ni.com/devzone/conceptd.nsf/webmain/CD57A73721E0612586256BAE0055CDD9?opendocument>Here</a> is a link to information about using a unity gain buffer. Let us know if you have any further questions! Justin T. National Instruments
