Chapter 7: Stability In Potentiostat Mode; Capacitive Cells And Stability - Gamry Instruments Interface 1000 Operator's Manual

Chapter 7: Stability in Potentiostat Mode--Capacitive Cells and Stability

Chapter 7: Stability in Potentiostat Mode

Capacitive Cells and Stability

All potentiostats can become unstable when connected to capacitive cells. The capacitive cell adds phase-shift
to the potentiostat's feedback signal (which is already phase-shifted). The additional phase-shift can convert the
potentiostat's power amplifier into a power oscillator.
To make matters worse, almost all electrochemical cells are capacitive because an electrical double-layer forms
next to a conductor immersed in a solution.
Potentiostat oscillation is an AC phenomenon. However, it can affect both AC and DC measurements.
Oscillation often causes excessive noise or sharp DC shifts in the system's graphical output. The Interface 1000
Potentiostat can be stable on less-sensitive current ranges and unstable on more-sensitive current ranges.
Whenever you see sharp breaks in the current recorded on the system, you should suspect oscillation.
The Interface 1000 has been tested for stability with cell capacitors between 10 pF and 3000 F. In all but its
fastest control amplifier speed-setting, it is stable on any capacitor in this range—as long as the impedance in
the reference electrode lead does not exceed 20 k. With reference-electrode impedances greater than 20
k the Interface 1000 may oscillate. The RC filter formed by the reference-electrode impedance and the
reference terminal's input capacitance filters out the high-frequency feedback needed for potentiostat stability.
Longer cell cables make the problem worse by increasing the reference terminal's effective input capacitance.
Even when the system is stable (not oscillating), it may exhibit ringing whenever there is a voltage step applied
to the cell. The Interface 1000's D/A converters routinely apply steps, even when making a pseudo-linear ramp.
While this ringing is not a problem with slow DC measurements, it can interfere with faster measurements. The
steps taken to eliminate potentiostat oscillation also help to minimize ringing.
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