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the input impedance to DC is extremely high because any change of input voltage does not cause any extra current to flow. This is because the input current is the same as the output current which is set by Vout which is constant. As the output voltage is constant, the load is constant, the output curent is constant, so the input current is constant. However to measure it set uo your regulator with a load, and a low value resistor in series with the input lead. Measure the Vin and Iin to the regulator. Increase the Vin to Vin2, you will get another input current Iin2. So the input impedance is (Vin2 -Vin)/(Iin2 -Iin).