Hi Fausto,
Thanks for your response.
Yes, I have two signal sources; one is connected to a function generator, and the other one is connected to a photodetector.
"Switch to the photodetector" refers that the acquired data from DAQ is correct when "CH0" and "CH1" are both connected to a function generator, but when I connect "CH1" to a photodetector, the acquired data shows incorrect value.
I think I may need to get an operational amplifier to make my DAQ work properly. Thanks for you assistance.
Qiwen Sheng