Chapter 8 -- Measurement of Small Signals--Measurement System Model and Physical Limitations
8 - 4
resistance between +15 volts and the input, the leakage current is 15 pA. Fortunately, most sources of
leakage current are DC and can be tuned out in impedance measurements. As a rule of thumb, the DC
leakage should not exceed the measured AC signal by more than a factor of 10.
The Interface 1000 uses an input amplifier with an input current of around 1 pA. Other circuit
components may also contribute leakage currents. You therefore cannot make absolute current
measurements of very low pA currents with the Interface 1000. In practice, the input current is
approximately constant, so current differences or AC current levels of less than one pA can often be
measured.
Voltage Noise and DC Measurements
Often the current signal measured by a potentiostat shows noise that is not the fault of the current
measurement circuits. This is especially true when you are making DC measurements. The cause of the
current noise is noise in the voltage applied to the cell.
Assume that you have a working electrode with a capacitance of 40 µF. This could represent a 1 cm
2
polished bare metal immersed in an electrolyte solution. You can roughly estimate the capacitance of the
electrical double layer formed by a metal/electrolyte interface as 20 µF/cm
2
. The area is the microscopic
area of the surface, which is larger than the geometric area, because even a polished surface is rough. The
impedance of this 40 µF electrode, assuming ideal capacitive behavior, is given by:
Z = 1/jωC
At sixty Hertz, the impedance magnitude is about 66 Ω.
Apply an ideal DC potential across this ideal capacitor and you get no DC current.
Unfortunately, all potentiostats have noise in the applied voltage. This noise comes from the instrument
itself and from external sources. In many cases, the predominant noise frequency is the AC power line
frequency.
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.
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