I recently purchased an ADC-200 unit and there does not seem to be anywhere to change the AC/DC Coupling. The help file states that there should be AC/DC combo boxes on the instrument tool bar, which there are not.
Before I downloaded the latest software, the switches appeared on the toolbar if I did not have the unit connected to the PC. Now they don't appear at all.
Information from the Help|About dialog is as follows
Release 5.11.6 - 32 bit
ADC200 Driver V4.01
ADC200-20 V5 on LPT1