Noise CMOS ISFETs Using In-Pixel Chopping

Ion sensitive field effect transistors (ISFETs) are CMOS-compatible pH sensors which have been adopted for a wide range of biochemical sensing applications. Drift and low-frequency noise are perennial challenges for these small charge-sensitive devices. However, ISFET designers have often avoided the common circuit solution of chopper stabilization due to understandable concern that the switching will disturb the sensing gate. Here we introduce a new configuration which modulates the source and drain voltages of the ISFET, reducing 1/f noise and drift with negligible disturbance of the sensing gate. We experimentally demonstrate this in-pixel chopping scheme with titanium nitride ISFETs in 180-nm CMOS technology. Using in-pixel chopping, the circuit achieves a three-fold reduction in drift along with suppression of 1/f noise.