Different piezoelectric transducers have different dynamic resistance, capacitance, inductance and static resistance, capacitance. These parameters can be obtained from the corresponding admittance circle. Two factors of quantizing error of A/D converter and measurement resistance in the admittance circle measurement have been analyzed in this paper. In order to improve measurement accuracy of admittance circle, different piezoelectric transducers need to choose different measurement resistances to ensure that two sinusoidal signals have right amplitude ratio and phase difference when the quantizing error is a constant. When the measurement resistance is in the value between 0.15 to 3.5 times of piezoelectric transducer dynamic resistance, the relative error of the admittance circle radius is less than 0.5%. When the measurement resistance is 0.7 times of the dynamic resistance, the admittance circle has the minimum relative error.