Projets - Polytechnique Montréal

Projets
GBM8320 – Dispositifs médicaux intelligents
(DMI2014)
Mohamad Sawan, professeur
Mohamed Zgaren, chargé de laboratoire
Section microélectronique
Département de génie électrique
Polytechnique Montréal
La présentation des projets
final aura lieu au
Local M5519
Pavillon Lassonde
28 avril 2014
14h00-17h00

GBM8320-Dispositifs Médicaux intélligents (DMI’14)
Présentation de projets
Lundi 28 avril 2014, 14h00-17h00
P#
Heure
Auteur
Titre du projet
Page
Programme
2
14h00 Début
1
14h25
Bouali, M.
Electrochemical Two-Electrode Amperometric
Biosensor for Neurotransmitters Analysis
3
2
14h50
Campbell, P.
Convoyeur de courant CCCII+ et transistor
DTMOS pour l'amplification de courant sous faible
tension
7
3
15h15
Collette, M.
Générateur de stimuli applicable pour stimulation
électrique fonctionnelle
11
15h40 Pause
4
16h00
Fartoumi, S.
Conception d’un bio-amplificateur à bande passante
et gain variable
15
5
16h25
Saha, S.
CMOS Transimpedence Amplifier with Automatic
gain control for Functional Near Infrared Spectroscopy
application
19
16h50 Fin

GBM8320 – Dispositifs médicaux intelligents (DMI 2014)
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Electrochemical Two-Electrode Amperometric
Biosensor for Neurotransmitters Analysis
Moez Bouali, Student Member, IEEE, and M. Sawan, Fellow, IEEE
Polystim Neurotechnologies Laboratory, Department of Electrical Engineering
Polytechnique Montreal, Quebec, Canada
[e-mail]
Abstract—This paper presents a highly integrated on chip
CMOS potentiostat for neurotransmitters sensing. The circuit
maintains a constant-potential amperometry between the
reference and working electrodes. Neurotransmitter concent-
ration is converted amperometrically to an equivalent output
voltage with a slop of 0. 12 V/µA. Redox currents ranging from 1
nA to 10 µA can be measured. Analog inputs are processed and
digital outputs are generated using delta-sigma analog to digital
converter. The circuit design is well studied with taken into
account power consumption, the input dynamic range as well as
the input referred noise. The proposed architecture is
implemented with 0.18 µm 1.8 V 1-poly 6-metal CMOS process
provided by TSMC. Simulations results show that the proposed
potentiostat shows a dynamic input current range of 1 nA to 10
µA, an input referred noise of 20.3 nV/ and consumes less
than 1mA, . The proposed ADC shows an input dynamic range of
5 mV and consumes 80µW.
Index Terms—Complementary metal-oxide semiconductor
(CMOS) integrated circuits, Potentiostat, Delta-sigma modulator,
Neurochemical sensing, Microfluidic.
I. INTRODUCTION
Neurotransmitters play a key role in neural communication.
When an action potential in a pre-synaptic neuron arrives to a
synapse, neurotransmitters are released to the synaptic cleft.
The neurotransmitters diffuse to the post-synaptic neuron and
bind to its receptors. This triggers an electrical signal in the
post-synaptic neuron, thus enabling neuron-to-neuron
messaging [1]. Neurotransmitters problems such as
neurotransmitters’ dysfunctions, concentration imbalance,
composition disorder, chemical activity … may cause various
neurological disorders. Thus, monitoring the composition and
the concentration of neurotransmitters such us dopamine,
glutamine and serotonin in real-time may help researchers from
the medical field to better understand the functioning of the
human brain in order to develop new treatments to various
neurodegenerative diseases such as Alzheimer, Parkinson,
Strokes, Sclerosis, Epilepsy and Schizophrenia.
Precise and fast measurement of neuro-chemical activity is
crucial for a better understanding of neurodegenerative
diseases. Several research groups introduced various solutions
for neurotransmitters detection’s such as neurotransmitter
imaging techniques’ and electrochemical biosensors. Although
the currently available techniques for neurotransmitters
imaging such as PET and SPEC [2] techniques are fast and
considered as low-risk platform, they remain expensive
approach that utilize bulky equipments. In recent years,
Biochemical integrated sensors in particular have received a
great attention due to their small probe/electrode size, high
sensitivity, low cost, accurate and fast measurement of
chemical activity and compatibility with CMOS process. As an
alternative, Miled et al. proposed a low-voltage laboratory-on
chip for dielectrophoretic manipulation and capacitive sensing
of nano and micro particles [3]. However, it still a complex and
expensive approach which requires a complex process
fabrication since it combines different modules such as a
Microfluidic structure, a Microelectronics chip and a packaging
system which increase the consumption and the size of the
Microsystem. Zadeh et al. focused on the sensing devices and
proposed a CMOS capacitive sensor for biochemical analysis
[4]. The limitation of the previous approach is that requires a
complex calibration and remains sensitive to parasites. Janata
and Hubert focused on chemically sensitive semiconductor
devices (CSSD’s) such as the ion-selective field effect
transistor (ISFET) which operate on potentiometric mode [5].
In recent years electrochemical amperometric microsystems
have been gaining popularity in manipulating the
neurotransmitters particles [6], [7] due to their low cost, high
sensitivity and small size. Potensiostat circuit is an exemple of
amperometric chemical sensor which measures the current
arise from the oxidation or reduction at the working electrode
(WE) which is lineary proportional to the concentration Cox
(Cred) of the oxidized (reduced) species in the test solution. The
current for cathodic , and anodic  processes are defined by
equation (1) and (2) respectively [8].
     (1)
     (2)
where z is the number of moles of electrons transferred per
mole of reactant, F is the Faraday constant = 96 487 C/mole, A
is the electrode area in square meters, Dox (Dred) is the diffusion
coefficient in m²/s for the oxidized (reduced) species, and Cox
(Cred) is the bulk concentration in moles /of the oxidized
(reduced) species in the test solution.
In this paper, the potentiostat circuit has been chosen as the
final project for the course of Smart Medical Devices. The
potentiostat system is described in Section II. Then in Section
III, the circuit level is detailed. Simulation results are presented
in section IV. Finally, Section V concludes the paper.

II. REVIEW OF CLASSICAL POTENTIOSTAT CIRCUIT
Fig. 1 shows potensiostat block diagram in its most basic
form. The input current, results of electrochemical reaction
between an electroactive species of interest and a working
electrode (WE), is measured through the WE, held at a fixed
potential to avoid the perturbation of the measuring redox
current. The direction of the current is depending on the type of
chemical reaction, reduction or oxidation. The reference
electrode (RE) is set to a constant voltage for constant-potential
amperometry (CA). The polarity (anode and cathode) of the
WE and the RE may be inverted depending on the reaction. A
high gain, wide-range, 2 stage operational amplifier (OA) is
used to control the drain current of the PMOS transistor MPMOS.
In equilibrium the OA controls the grid voltage of MPMOS. The
concentration’s variation of the neurotransmitter will generate
a proportional equivalent current described by (1) and (2)
through the drain of MPMOS. This current will change the output
impedance of MPMOS and hence the differential input voltage of
the OA which in turn moves Vgs to a new equilibrium value.
As it is easier to manipulate voltage more than current, a
current to voltage circuit based on Wilson current mirror circuit
is used to convert current to a proportional voltage with a
programmable offset by adding a trimming resistor RL. The
reference voltage Va is limited to the functional region of the
MPMOS transistor which is supposed to be working in the linear
region where the Ohm's law is respected. The output voltage
Vout is integrated with a sampling frequency modulated and
subsequently quantized in a multi-level quantizer into a digital
signal using a Delta-Sigma) analog to digital converter
(ADC). As part of our project for detecting neurotransmitters
using a miniaturized Potentiostat device, a ΣΔ ADC is among
the best candidates. Compared to Flash and Pipelined ADC
which are expensive and relatively complex,  modulators
offer a high resolution and they are more easily integrated into
a dedicated integrated circuit with minimum resources.
However, such converter requires a specialized signal
processing which may lead to a high conversion time.
Basically, these converters consist of an oversampling
modulator followed by a digital/ decimation filter that together
produce a high-resolution data-stream output. In this paper we
will look closely at  modulator.
The operating principle of the ΣΔ modulator is to encode
the variation in the output voltage of the Potensiostat Vp by a
series of binary pulses (Vout = 0 or 1). To do the coding
correctly, it is necessary that the sampling frequency is high
enough to consider the fastest input signal variations. The
analog signal applied to the input of the converter needs to be
relatively slow so the converter can sample it multiple times, a
technique known as oversampling [9]. Fig. 1. b shows the
block diagram of a first-order analogue ΣΔ modulator.
The Vp voltage attack a linear comparator (subtractor
circuit), which provides a signal e = D.Vp representing the
difference ("DELTA") between the input voltage at time ‘n’
and the output of the converter (which is the input voltage at
the preceding time ‘n-1’). This difference pass through an
integrator circuit then undergoes a coding by a clocked binary
comparator which produces the output voltage Vout. This
MPMOS
OA
Current to
voltage
converter
-
+
Vdd
Amperometric
Biosensor
Vref_b
Iox Comparator
Vp
integrator Digital output
Delta-Sigma
modulator
WERE
Vref
(b)
(a)
Fig. 1. Block diagram of the proposed potentiostat
voltage is fed back to the input. Finally, the final output is
obtained by introducing Vout through a digital/ decimation filter.
The latter removes noise outside the signal bandwidth
(quantization noise) and lowers the sampling frequency to a
value closer to 2x the highest frequency of interest, a technique
known as downsampling [10].
III. CIRCUIT DESIGN
The current through the WE, where the (1) or (2) is valid,
can vary from few pA to few microamperes depending on the
electrolyte. This variation on WE current generates a variation
on the drain current of MPMOS, and hence the output impedence
of MPMOS which in turn changes the differential of the OA. The
OA compares the feedback voltage Vfb and the reference
voltage Vref . The result of comparison controls the grid voltage
Vgs of the MPMOS transistor which moves it to a new
equilibrium value. A simple current to voltage circuit based on
a current mirror is used to convert the WE current to an
equivalent voltage which can be manipulated easy using a 
modulator.
A. The operational amplifier:
Since the OA is not dedicated to provide any current, a
two-stage amplifier is used. Fig.2 shows the OA which consists
of a differential input stage and a common source output stage
with a frequency compensation circuit of Cc in series of Rc. A
particular attention to the input referred noise of the OA must
take into account since we manipulate low currents. Thus, the
pMOS input stage is used to optimize Unity-Gain frequency
and minimize noise. All transistors are well sized to operate in
the saturation region with taking into account the final size of
the chip and the important parameters of the circuit.
V-
Cc Rc
Vdd
V+
Ibias
Fig. 2. Transistor level of the Two-Stage Operational Amplifier