Chapter 9 -- Measurement of Small Current Signals--Hints for System and Cell Design
9 - 5
Assume a realistic noise voltage, Vn, of 10 µV (this is lower than the noise level of most commercial
potentiostats). Further, assume that this noise voltage is at the US power line frequency of 60 Hz. It will create
a current across the cell capacitance:
I = Vn/Z ≈ 10 x 10
-6
/ 66 ≈ 150 nA
This rather large noise current will prevent accurate DC current measurement in the low nA or pA ranges.
In an EIS measurement, you apply an AC excitation voltage that is much bigger than the typical noise voltage,
so this is not a factor.
Shunt Resistance and Capacitance
Non-ideal shunt resistance and capacitance arise in both the cell and the potentiostat. Both can cause
significant measurement errors.
Parallel metal surfaces form a capacitor. The capacitance rises as either metal area increases and as the
separation distance between the metals decreases.
Wire and electrode placement have a large effect on shunt capacitance. If the clip leads connecting to the
working and reference electrodes are close together, they can form a significant shunt capacitor. Values of 1 to
10 pF are common. This shunt capacitance cannot be distinguished from "real" capacitance in the cell. If you
are measuring a paint film with a 100 pF capacitance, 5 pF of shunt capacitance is a very significant error.
Shunt resistance in the cell arises because of imperfect insulators. No material is a perfect insulator (one with
infinite resistance). Even Teflon, which is one of the best insulators known, has a bulk resistivity of about 10
12
ohms•m. Worse yet, surface contamination often lowers the effective resistivity of good insulators. Water films
can be a real problem, especially on glass.
Shunt capacitance and resistance also occur in the potentiostat itself. The Reference 3000 Potentiostat Mode
specifications in Appendix A contain equivalent values for the potentiostat's R
shunt
and C
shunt
. These values can
be measured by an impedance measurement with no cell.
In most cases, the cell's shunt resistance and capacitance errors are larger than those from the potentiostat.
Hints for System and Cell Design
The following hints may prove helpful.
Faraday Shield
A Faraday shield surrounding your cell is mandatory for very low-level measurements. It reduces both current
noise picked up directly on the working electrode and voltage noise picked up by the reference electrode.
A Faraday shield is a conductive enclosure that surrounds the cell. The shield can be constructed from sheet
metal, fine mesh wire screen, or even conductive paint on plastic. It must be continuous and completely
surround the cell. Don't forget the areas above and below the cell. All parts of the shield must be electrically
connected. You will need an opening in the shield large enough to allow a cell cable to enter the shield.
The shield must be electrically connected to the Reference 3000's floating ground terminal.
An additional connection of both the shield and the Reference 3000 floating ground to an earth ground may
also prove helpful.
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