An Oscillator Based Potentiostat with Switch-Cap Feedback for Current Sensing Applications

This paper presents an oscillator based potentiostat with switch-cap feedback for current sensing applications. In this work, a sigma-delta modulator is realized using current as input and charge as the feedback. The current input is integrated at the input capacitance and the voltage difference between the integrated value and reference input is converted to current via a transconductor. This current is then fed into a current controlled ring oscillator and the frequency output drives the switch-cap circuits to compensate the input current. Therefore, the current input can be directly readout as frequency deviation, or can be further converted to digital output via a frequency to digital converter. The proposed system is implemented in a typical 0.18µm CMOS technology, with total area of 80 × 250µm2. A differential structure was implemented to minimize the parasitic and kick back influence, with oscillator base frequency at 491MHz. A counter based frequency to digital converter with an additional CIC filter was implemented to convert the differential frequency signal to digital domains at 16MHz sampling frequency. Simulation results demonstrated that a dynamic range of 52dB was achieved with input range of ±2.5µA.

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