Multi-range current source for biocompatible impedance spectroscopy measurement

The impedance spectroscopy system is a widely used technique for measuring the impedance of various samples, by injecting current into the samples and measuring the voltage as a response in a certain frequency range. Thus, a precise and stable current source has a significant role in the impedance spectroscopy system. The multi-range current source in this study is part of the electrochemical impedance spectroscopy (EIS) system under development. The circuit is based on the Enhanced Howland Current Source with a current selector to change the current value. The three current ranges used are 100 μA, 200 μA, and 400 μA, that relatively small and biocompatible. The high-speed op-amp OPA843 with lead-lag compensation is used in the circuit. The data was obtained by experiments and simulations using the Spice simulator of TINA-TI software from Texas Instruments. The design of a stable current source with broad bandwidth and high output impedance is an important consideration. The current source is applied to a load of discrete resistor, then the load voltage is measured, and the output impedance is calculated. The output impedance and the output current were stable at frequencies 0.1Hz to 300kHz. There is some difference in the measured load voltage compared to the simulation results caused by the difference in the electronic components' tolerance. We also implemented the current source to several load of complex samples (parallel circuit of resistor and capacitor) and KCl solutions with varying concentrations, then measured the load voltage. The measurement of load voltage of KCl solutions shows that the higher the concentration, the lower voltage due to the lower impedance value.