  Relaxation oscillator-based circuits are widely used for interfacing various
resistive and capacitive sensors. The electrical equivalent of most resistive
and capacitive sensors is represented using a parallel combination of resistor
and capacitor. The relaxation oscillator-based circuits are not suitable for
parallel R-C sensors. In this paper, we propose a modified circuit for parallel
R-C sensors. The proposed relaxation oscillator-based circuit is based on a
dual-slope and charge transfer technique to measure the resistance and
capacitance of parallel R-C sensors separately. In addition, the paper provides
a detailed analysis and design considerations for the oscillator design by
taking into account the various sources of non-idealities. A method to reduce
the error by using single-cycle averaging is also introduced. To verify the
analyzed design criteria, the circuit is tested with multiple operational
amplifiers with different non-idealities. Experimental results verify the
performance of the proposed circuit. The circuit is tested for a range from 10
pF to 42 pF and 100 k$\Omega$ to 1 M$\Omega$ for parallel R-C sensors with an
error of less than 1.5\%. The circuit is tested with a fabricated water-level
sensor. The result confirms the efficacy of the proposed circuit.
