CMOS pulse-width-modulation readout circuit with a wide modulation range for ion-sensitive FET-based sensors. (11th March 2015) - Wang, Ruey-Lue
Abstract: A CMOS pulse-width-modulation readout circuit for sensors is presented. An input voltage, V sen, which comes from a sensor, is converted into a current I sen and then the I sen is used to generate a single pulse by a procedure of constant-time charging and then constant-current discharging. With two signals RESET and SW, a control signal generator produces two signals MS1 and MS2 to control the charging and discharging of the capacitor C p, and hence the sensitivity of the pulse width can be tuned by changing the period of the SW. The pulse width linearly depends on the V sen with a linearity of at least 99.996%. The integration of the readout circuit with an ion-sensitive field effect transistor (ISFET) exhibits a measured transfer characteristic of pulse width versus pH value with a sensitivity of −31.6 µs/pH and a linearity of 99.35% after a charging time of 500 µs at 25 °C.