I have developed a robust design methodology in a 0.18 [Mu]m commercial CMOS process
to circumvent the performance issues of the integrated Ions Sensitive Field Effect Transistor
(ISFET) for pH detection. In circuit design, I have developed frequency domain signal
processing, which transforms pH information into a frequency modulated signal. The
frequency modulated signal is subsequently digitized and encoded into a bit-stream of data.
The architecture of the instrumentation system consists of a) A novel front-end averaging
amplifier to interface an array of ISFETs for converting pH into a voltage signal, b) A high
linear voltage controlled oscillator for converting the voltage signal into a frequency
modulated signal, and c) Digital gates for digitizing and differentiating the frequency
modulated signal into an output bit-stream. The output bit stream is indistinguishable to a 1st
order sigma delta modulation, whose noise floor is shaped by +20dB/decade.
The fabricated instrumentation system has a dimension of 1565 [Mu] m 1565 [Mu] m. The chip
responds linearly to the pH in a chemical solution and produces a digital output, with up to an
8-bit accuracy. Most importantly, the fabricated chips do not need any post-CMOS
processing for neutralizing any trapped-charged effect, which can modulate on-chip ISFETs’
threshold voltages into atypical values. As compared to other ISFET-related works in the
literature, the instrumentation system proposed in this thesis can cope with the mismatched
ISFETs on chip for analogue-to-digital conversions. The design methodology is thus very
accurate and robust for chemical sensing.