Instrumentation & Measurement Magazine 25-4 - 30

amplification of the input signal, the most relevant part of the
chain is the front-end, and noise is conveniently referred to the
input of the chain to be directly comparable with the input signal.
Noise is treated as a statistical quantity with zero mean and
it is frequency dependent. It is described by its spectral density,
whose integral across the measurement bandwidth sets
the RMS level of the fluctuations. This relationship highlights
the first message: the wider the measurement bandwidth, the
higher the noise. Thus, filtering and signal shaping, both in
analog and digital domains, are pivotal in reducing the bandwidth
to the minimum required to track the signal dynamics.
Filtering strategies are different for current sensing (continuous
signals) and charge pulses (discrete packets). In either
scenario, maximization of the SNR is pursued both by looking
for solutions to increase the numerator and adopting counter
measurements to decrease the denominator.
Applications
High-resolution current measurements find several applications
in the bio-medical field, especially in the area of
biosensors (Fig. 2). In most cases, the current flows through
an electrochemical interface between an electrode and an electrolytic
(saline) solution that hosts a biological entity. Charge
can be hopping between molecules in solution and a properly
biased metal in the case of redox reactions (Faradic interface)
or simply induced in the case of a non-Faradaic interface (i.e.,
with ions in solution being attracted in the double-layer region
of the interface). Thanks to the advancements in microfabrication
of planar electrodes and microfluidics, it is possible
to fabricate structures in the micrometer size range, matching
with the dimensions of individual biological entities such
as biological cells [1]. Focusing only on the bottom end of the
size range, i.e., on the sub-micrometric scale, we can mention
the amperometric detection of molecules, in particular of
neurotransmitters and glucose, patch-clamp and nanopores
techniques, and affinity biosensors.
Electrochemical detection of molecules is a label-free approach
opposed to marker-based techniques, such as optical
fluorescence, that require the chemical linking of a marker (the
fluorophore) to the molecule for its detection and quantification.
Clearly, the avoidance of the labeling step represents an
advantage in terms of the sample preparation process. From
Fig. 2. Examples of current measuring applications in bio-medicine. (a) Direct detection of redox molecules and indirect detection; (b) mediated by an enzyme;
(c) tracking current flowing through an ion channel in patch clamp; or (d) nano-pore. Impedance detection of: (e) DNA strands; (f) antigens binding to antibodies;
(g) cells growing on planar electrodes; and (h) flowing in a micro-channel.
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IEEE Instrumentation & Measurement Magazine
June 2022

Instrumentation & Measurement Magazine 25-4

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-4