How Cabling and Signal Amplitudes Affect EIS Results
Gamry Instruments prides itself on performing accurate EIS. We publish an Accuracy Contour Plot (ACP) for every instrument we sell. ACPs describe a region of accuracy over a given range of impedances and frequencies under a defined set of conditions.
Why Generate ACPs?
Why do we do this? Two main reasons:
- So that you understand the ranges and limitations of our instruments when performing EIS under typical conditions
- ACPs can change depending on cable length and signal amplitude
Generating an ACP typically starts with examining the open lead curve and the shorted lead curve. The open lead curve is meant to describe the absolute capacitive limits of the entire potentiostat and cable under a stated amplitude. Any result that you get from an EIS experiment that is at or above open lead curve should be thrown out no matter how nice the data appear. You are measuring the capacitance of your measurement system and not your sample. Extremely well insulating coatings can be one example.
Potentiostatic experiments are typically performed at 10 mV rms or smaller in order to maintain a linear response. Remember that in order for EIS results to be valid, your system needs to be linear, stable, and casual. Linearity, stability, and causality can assessed using Gamry’s built-in Kramers-Kronig function found in our analysis software. Galvanostatic experiments are a little bit different in that the current amplitudes can get larger as long as the voltage response maintains linearity – i.e. passes the Kramers-Kronig test.
ACPs of the Interface 1000
ACPs are only valid under the applied conditions. For example, the ACP for the Interface 1000 as seen below shows that you can measure impedances from 3 GΩ down to less than 1 mΩ at greater than 99% accuracy. The lower impedance limit is useful to know when examining energy storage and conversion devices while the upper impedance limit is useful for corrosion-resistant materials and well-coated samples. It is also helpful to know the capacitive limit, seen as an increasing line as frequency decreases, for well-coated samples. If you were to use a longer cable you would expect a decrease in bandwidth due to the added R and C.
Gamry’s standard cell cable is 60 cm but we also have 1.5, 3, and 10 m cables available as options. Since the open lead curve is the measure of the instruments capacitive limit for a measurement, we measured the open lead curve for the 3 and 10 m cables also. Additionally, we measured the open lead curve without a cable. As shown in Figure 2, the maximum applied frequency decreases as a function of cable length. The capacitive region of the ACP decreases slightly as the cable length is increased. The No Cable line falls in the middle due to the unshielded shunts we used to short the pins on our cell cable connector. Notice too that the maximum impedance limit decreases as a function of cable length due to the increased R of the cable.
Want a PDF version of this application note?
Please complete the following form and we'll mail a link to your inbox!