Measurement of Small-current Signals – Overview
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Chapter 10:
Measurement of Small-current Signals
Overview
The Gamry Instruments Reference 3000 is a high-performance measurement instrument used for all types of
electrochemical testing. Unlike many other electrochemical instruments, it offers outstanding performance for
both tests with small-current signals and high impedances, and for tests involving large currents and very small
impedances.
Chapter 11 is a discussion of the latter problem, when currents are large, voltages and cell impedances are
small, and inductance limits the measurements. These measurements include those encountered in research
on:
Batteries
Fuel cells
Super-capacitors
Problems in the measurement of very small currents are discussed here. Examples of this type of testing
include:
Cells for testing painted metal samples
Cells for testing corrosion of bare metals
Microelectrode cells
Most cells for fast CV (cyclic voltammetry)
Super-capacitors
Description of the Problem
The Reference 3000 is a very sensitive scientific instrument. It can theoretically resolve current changes as small
as 100 femtoamperes (1 × 10
-13
A). To place this current in perspective, 100 fA represents the flow of only
about 600 000 electrons per second!
The small currents measured by the Reference 3000 place demands on the instrument, the cell, the cables, and
the experimenter. Many of the techniques used in higher-current electrochemistry must be modified when
used to measure pA currents. In many cases, the basic physics of the measurement must be considered.
This chapter discusses the limiting factors controlling low-current measurements. It includes hints on cell and
system design. The emphasis is on EIS (Electrochemical Impedance Spectroscopy), a highly demanding
application for the Reference 3000.
Measurement System Model and Physical Limitations
To get a feel for the physical limits implied by very sensitive current measurements, consider the equivalent
circuit shown in Figure 10-1. We are attempting to measure the cell impedance given by
Z
cell
. This model is
valid for analysis purposes even though the real Reference 3000 circuit topology differs significantly.
In Figure 10-1:
E
S
ideal signal source
Z
cell
unknown cell impedance
I
cell
“real” cell current
R
m
current measurement circuit’s current-measurement resistance
R
shunt
unwanted resistance across the cell
