A testing system includes a subtractor and a divider. The subtractor is configured to receive a first voltage of a circuit being tested and a second voltage of the circuit, and to derive a difference between the first voltage and the second voltage. The divider is configured to receive the difference between the first voltage and the second voltage, and to derive a resistance of the circuit by dividing (i) the difference between the first voltage and the second voltage by (ii) a difference between a first current applied to the circuit and a second current applied to the circuit. The first voltage is corresponding to the first current, and the second voltage is corresponding to the second current.