A current branch circuit is electrically coupled with a Faraday cup and an operation amplifier separately. The Faraday cup, the current branch circuit and the operation amplifier are formed as a portion of an ion implanter. When the Faraday cup is electrically coupled with a ground through a conductive structure formed by an ion beam received by the Faraday cup, a current flows from the output of the operation amplifier to the current branch circuit to balance another current flow from the current branch circuit through the Faraday cup to the ground. By dynamically monitoring the voltage of the output of the operation amplifier, current flows through the Faraday cup to the ground and through the resistance of the conductive structure can be dynamically monitored. Accordingly, the difference between the ion current measured by the Faraday cup and the real ion current implanted to the wafer can be dynamically acquired to avoid overdosage of the wafer with the ion beam.