Resonant noise-canceling current front-end for high-resolution impedance sensing

The ultimate resolution limit of the current front-end of wideband impedance detection circuits is set by the input total capacitance and input equivalent noise. We present a solution to reduce the noise of more than an order of magnitude when the capacitance cannot be further minimized. By introducing a properly-chosen inductor (and a proper and stable biasing network) it is possible to cancel the capacitive reactance at the resonance frequency (~10 MHz) which becomes the sensing frequency, thus significantly reducing the detection noise, while preserving the accuracy of the transfer function. A detailed analysis of the scheme, and its experimental validation with different inductors are presented. In all cases an improvement larger than 10 is achieved with an input-referred current spectral density of ~1 pA/VHz at the operating frequency of ~20 MHz.