Measurement of Small Electrochemical Signals

Introduction to Low-Current Measurements

Potentiostats are often pushed to their limits when measuring very small electrochemical signals. Gamry Instruments' Reference 620, for example, can resolve currents as low as 1 femtoampere (fA), representing the flow of approximately 6000 electrons per second. Such measurements demand precision from the instrument, cell, cables, and the experimenter.

This application note delves into the critical factors that influence low-current measurements, with a particular focus on Electrochemical Impedance Spectroscopy (EIS), a primary application for low-current potentiostats.

Understanding Measurement Limitations

Accurate measurement of picoampere (pA) and femtoampere (fA) currents is challenging due to several physical limitations:

• Noise: Johnson noise, inherent in resistors, contributes significantly to measurement uncertainty, especially at low frequencies.
• Input Capacitance (Cin): Unavoidable capacitances in real circuits can shunt higher frequencies, limiting measurement bandwidth.
• Leakage Currents: Finite input resistance (Rin) and input leakage current (Iin) can introduce errors, requiring careful consideration of circuit design and insulation.
• Shunt Resistance and Capacitance: Imperfect insulators and physical proximity of components can create unwanted shunt paths, affecting signal integrity.

Understanding and mitigating these factors are essential for obtaining reliable results in sensitive electrochemical experiments.