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The paper presents a very high sensitivity transimpedance amplifier in standard CMOS 0.35 mum technology suited for sensing current signals from molecular and nanodevices systems. The circuit, based on an integrator followed by a differentiator configuration, features i) a low-noise time-continuous feedback loop to cope with possible standing currents from the device under test as high as few tens of nA without limiting the signal dynamic range; ii) active current-reducers to implement very high value equivalent resistances of hundreds of GOmega with high linearity irrespective to the current direction and characterized by a shot noise current level (2qI) which is, for low standing current, few orders of magnitude smaller than a physical resistor of equal value and iii) nested-Miller compensation networks to ensure strong stability over a bandwidth of few MHz. Thanks to the ability to draw large standing currents, the circuit is suitable for a use in biological systems where physiological medium is co-present. The measured input equivalent noise of 4 fA/radic(Hz) at about 100 kHz, recorded when the input dc current is lower than 10 pA, allows the chip to be used, among others, in impedance spectroscopy measurements at the nanoscale with a capability of detecting capacitance variations in sub-attofarad range to cope with the challenges of single-chip instrumentation.
Date of Publication: May 2009