Front end with offset-free symmetrical current source optimized for time domain impedance spectroscopy

Fast impedance measurements are often performed in time domain utilizing broad bandwidth excitation signals. Other than in frequency domain measurements harmonic distortion cannot be compensated which requires careful design of the analog front end. In order to minimize the influence of electrode polarization and noise, especially in low-frequency measurements, current injection shows several advantages compared to voltage application. Here, we show an active front end based on a voltage-controlled current source for a wide range of impedances. Using proper feedback, the majority of the parasitic capacitances are compensated. The bandwidth ranges from dc to 20 MHz for impedance magnitude below 5 kΩ. The output is a symmetric signal without dc-offset which is accomplished by combination of a current conveyor and a voltage inverter. An independent feedback loop compensates the offset arising from asymmetries within the circuitry. We focused especially on the stability of the current source for usage with small metal electrodes in aqueous solutions. At the monitor side two identical, high input impedance difference amplifiers convert the net current through the object and the voltage dropping across into a 50 Ω symmetric output. The entire circuitry is optimized for step response making it suitable for fast time domain measurements.