The conventional amplitude measurement of electrolyte-insulator–semiconductor-capacitor (EISCAP) sensor is convenient but susceptible to noise interference, which can deteriorate the measurement accuracy. In this study, a new amplitude-measurement method based on voltage-frequency–voltage (V/F/V) conversion is proposed, which is composed of impedance conversion, hysteresis comparison, and frequency-to-voltage conversion. The impedance conversion technique increases the equivalent capacitance of EISCAP; thus, the measurement sensitivity is improved obviously. The change of EISCAP output voltage is converted into the frequency variation of a periodic pulse signal by the hysteresis comparator. Compared to the amplitude of a signal, its frequency is less susceptible to circuit noise, so that the anti-noise ability of the measurement system is enhanced. Voltage output is more convenient and flexible; at the end of the detection circuit, the frequency-to-voltage conversion is performed. With this proposed amplitude-measurement method, the relative error of measurement is decreased from 2.2643% to 0.0045%. After using impedance converter, the detection sensitivity is increased by 88.66% from 0.0344 to 0.0649/pH at fixed bias voltage mode and by 64.94% from 39.53 to 65.20 mV/pH at fixed amplitude mode, respectively. In addition, the linearity was improved by 7.77%, and the repeatability standard deviation was decreased by 20% from 0.0045 to 0.0036.