A Low-Power Voltage-Clamped CMOS pH to Frequency Sensor

This paper presents a low-power voltage clamped CMOS pH readout circuit. The novelty of this design lies in an ISFET pH to frequency conversion using a voltage clamping topology. The proposed system, designed in a commercial 180 nm CMOS technology, achieves minimum supply voltage and minimum power consumption through subthreshold MOSFET operation. Simulation results of the circuit show that the designed ISFET provides an average sensitivity of 40 mV/pH for the ISFET front end readout and 6.92 MHz/V and 138 kHz/pH for the overall system with an ISFET sensing area of 80 μm × 80 μm over a 1-14 pH range. A 10 μm × 10 μm shows 27 mV/pH with 6.03 MHz/V and 120 kHz/pH over a 1-14 pH range for the pH to frequency system. The power consumption is 40.20 μW with an area of 0.0036 mm
²
. Monte Carlo simulations for the 80 μm × 80 μm electrode size shows the mean frequency for pH 4, 7, and 10 to be 5.82 MHz, 5.98 MHz, and 6.14 MHz with the maximum standard deviation of 346 kHz and the minimum standard deviation of 285 kHz.