Dear Ki-Rak, Can you send me the manual for the Lock-in Amplifier you are using, I'd like to know what exactly are ADC1, ADC2, A, OSC Out. Here is my guess: ADC1 is to read in the modulated data, in this case, tunneling current. OSC Out is the AC drive signal, in this case, the dV. "A" got be the measured oscillation amplitude, here, dI