ISFET Readout Circuit using Flipped Voltage Follower with Temperature Compensation

Sensors and devices dependent in temperature variations can present significant problems that may lead to incorrect analysis of results. Ion-Sensitive Field Effect Transistors are sensors that can measure the pH level of a solution. A low-power flipped voltage follower readout circuit was used to acquire the corresponding voltage level output. However, the output changes as temperature varies, resulting to a proportional-to-absolute-temperature output voltage. A diode-connected MOSFET was used to provide a complementary-to-absolute-temperature voltage along with the flipped voltage follower to construct a circuit with temperature compensation. The proposed circuit was simulated from a temperature range of 0 °C to 70 °C, and pH levels of 4, 7, and 10. Temperature coefficient with a 91.8% improvement was obtained from an uncompensated average of 685.47 ppm/°C to a compensated 6.29 ppm/°C output. The circuit was designed in 0.18 μm technology with a supply voltage of 1.8V and an overall power consumption of 27.76 μW.