How can I check, if the EPC9 produces the correct command potential?
You can easily check if the EPC9 produces the correct command potential by measuring the voltage monitor output, which is the command that will also be output at the probe. To do this you can connect the VOLTAGE MONITOR output of the EPC9 to an external oscilloscope or a potentiometer, generate a command potential and read back the value. Alternatively you can read back this value by using the EPC9 itself, just connect the VOLTAGE MONITOR with AD0 which is the default voltage input (this can be changed in the Configuration dialog of PULSE). In the EPC9 panel you should now read the potential in the V-mon field. You can change the precision of that display by clicking on it with CAPSLOCK on and CTRL+ALT (MacOS: CTRL+COMMAND) pressed. Set the type of the field to engineering, text to V and precision to 7 digits. The difference between the command and the output you read back should be less than 1 mV.
