A circuit is disclosed that includes a signal-forcing path, a discharging path, a contact probe, a monitoring probe and a switch module. The signal-forcing path is connected to a signal source. The discharging path is connected to a discharging voltage terminal. The contact probe contacts a pad module of an under-test device. The monitoring probe generates a monitored voltage associated with the pad module. The switch module is operated in a discharging mode to connect the contact probe to the discharging path when the monitored voltage does not reach a threshold voltage such that the under-test device is discharged and is operated in an operation mode to connect the contact probe to the signal-forcing path when the monitored voltage reaches the threshold voltage such that a signal generated by the signal source is forced to the under-test device.