A MEMS based Microelectrode Sensor with Integrated
Signal Processing Circuitry
Angan Das, Prashant R. Bhadri,
Alla S.Kumar, Jin-hwan Lee, Ian Papautsky,
Fred R. Beyette Jr.
Am Jang1, Paul L. Bishop1, William Timmons2
1
Department of CEE, College of Engineering
University of Cincinnati
Cincinnati, USA
2
Entera-Tech Inc.
Columbus, OH, USA
Department of ECECS, College of Engineering
University of Cincinnati
Cincinnati, OH, USA
dasan@ececs.uc.edu
necessary data acquisition circuitry. The chip is embedded in
a Printed Circuit Board (PCB) to develop a fully integrated
system along with the microelectrodes designed to meet the
purpose [5]. Amperiometric testing of the microelectrode
probes is conducted to acquire the current flowing through
the probes. The current sensed varies with the altering
conditions of the external environment.
Abstract - Microelectrodes have been developed over the last
few years with tip diameters of 1-10 um, but they are fragile
and susceptible to electrical interference. In addition, they are
difficult to manufacture and operate, and are often unsuitable
for measurement in small volumes of liquid or in soils. This
limits their use to specialized laboratories under highly
controlled conditions. The paper introduces a robust, selfcontained, inexpensive MEMS based microelectrode sensor
that can be used in situ environments. It deals with the design,
analysis and performance of circuitry for a microelectrode
sensor. The primary focus is to design, implement and
integrate a CMOS circuit with the MEMS device to process,
amplify and transmit the signal from the microelectrode to a
measuring instrument. A current sensing circuit is developed
for amperiometric measurement with the microelectrode
array. The magnitude of the output signal is dependent on the
characteristics of the liquid being evaluated by the system. A
Printed Circuit Board (PCB) has been built to integrate the
microelectrode sensor array along with the sensor chip with
the aim of producing a fully integrated system.
I.
The paper is arranged into the following sections. Section
II provides the system level design of the integrated sensor,
including a brief introduction of the various components.
Section III introduces the design of the circuitry for sensing
the signal. Section IV discusses the PCB that is designed and
fabricated to contain the chip. Section V details the testing
procedure adopted and Section VI reports the testing results.
Finally the conclusion is highlighted in Section VII.
II.
INTRODUCTION
Many environmental applications like wastewater
treatment reactors, stream or lake sediments, bioremediation
process of hazardous waste sites and water distribution
systems require substantial monitoring. Previous work [1, 2,
3] indicates, that the microelectrode sensors that have been
used till now for these applications are fragile; difficult to
manufacture and operate; and susceptible to electrical
interference. This constrains their use to specific laboratories
under highly controlled conditions. Thus, there is a great
need for robust integrated microelectrode sensors with
integrated signal processing circuitry that can be used in situ
for environmental monitoring [4]. In situ monitoring is also
required in bio-films and laboratory reactors, both to
determine existing environmental conditions and to properly
control them. The sensor should function in an efficient
manner, so that the signal can be accurately sensed and
measured. This paper introduces a sensor chip that has the
0-7803-9197-7/05/$20.00 © 2005 IEEE.
SYSTEM LEVEL DESIGN
Environmental
Conditions
Printed Circuit
Board
Microelectrode
Sensor
Sensor Chip
Figure 1 System Level Block Diagram of the Microelectrode Sensor
The system level block diagram for the sensor is shown
in Figure 1 which essentially consists of two components.
The first one is the microelectrode that monitors the external
environmental conditions. These electrodes sense the signal
and pass it on to the second component, the sensor chip
embedded in the Printed Circuit Board. The chip contains the
necessary circuitry to process the signal. The output signal
from the chip is measured with suitable measuring
instruments.
363
III.
gain is equal to 1. The advantage of the differential amplifier
is that the overall gain can be varied by varying only a single
resistor RGAIN. The other advantage of the differential
amplifier relates to its high input impedance and noise
elimination.
CIRCUIT DESIGN METHODOLOGY
C. Unity Gain Inverting Amplifier
The unity gain inverting amplifier used in the last stage
has both the resistors of value 1kΩ. This circuit acts as a
buffer amplifier and ensures impedance matching and signal
isolation.
The amperiometric circuit is intended for measurement of
current. But the circuit is designed so that it produces an
output voltage, which is measured, instead of the current.
Since the expected value of the current flowing through the
probes is of the order of nanoamperes, accurate measurement
of this low value current, even with a precision picoammeter, poses a challenge due to the noise levels
introduced. Therefore voltage is measured at the output pin
(Pin_26). This necessitates the calibration of the circuit
before conducting test experiments. The calibration step
comprises of providing known values of current at the input
pin of the circuit and measuring the output voltage. The input
current, fed from a current source, is varied from 1-100 nA
and the output voltage then measured. The results
demonstrate that the voltage obtained for the different values
of current fed bears a linear fit to the current, with the value
of the degree of determination (R2) equal to 0.95, which is
statistically acceptable. As the voltage bears a linear relation
to the current, so instead of measuring current, the
measurement of voltage suffices the need to determine the
response of the circuit to varying conditions of the solution
in which the probes are placed.
Figure 2: Sensor Circuit for Amperiometric Testing
The chip contains the circuitry for amperiometric
measurement of the microelectrode probes. It is a 40 pin DIP
chip laid out using the Tanner Tools L-Edit layout editor in
the 1.5um process and fabricated through the MOSIS
foundry. The section of the chip shown in Figure 2 contains
the amperiometric measurement circuit. It essentially
consists of the following three stages connected in
succession.
A. Transimpedance Amplifer
The transimpedance amplifier is a current to voltage
converter. The current I sensed from the probes is fed into
Pin_13 (Amperiometric In) of the chip (from the probes) and
converted through the resistance R into its corresponding
voltage V2 that is governed by the equation:
V2 = - R · I (R = 1kΩ)
IV.
(1)
Therefore, the voltage produced bears a linear relation to
the input current that is fed. It is then passed on to the next
stage of the circuit. The other transimpedance amplifier has
its input pin (Pin_14) grounded and therefore provides a 0V
reference voltage (V1).
B. Differential Instrumentation Amplifier
The next stage of the circuit is a buffered differential
amplifier stage with three resistors connecting the two buffer
circuits together. It establishes a voltage drop across RGAIN
equal to the voltage difference between V1 and V2. The
RGAIN in this circuit is variable (RGAIN is provided from a
potentiometer on the PCB) and is typically equal to 1kΩ.
The voltage drop produced between points 3 and 4 V3-4 is
given by:
V3-4 = (V2 – V1) (1 + 2R/RGAIN)
PRINTED CIRCUIT BOARD
As discussed in the previous section, the circuit senses
current in the range of nanoamperes and voltages in the
range of millivolts. Initial testing of the chip using a breadboard demonstrated high noise. Furthermore, bread board
testing necessitates the use of various active elements like
voltage supply for the chip as well as passive elements like
resistors and capacitors to be connected externally. In order
to have all the components needed for testing the chip on a
common platform, a Printed Circuit Board (PCB) has been
designed and laid out. The two layered PCB is designed
using the Express PCB software [6] and fabricated through
the foundry service of Express PCB. The schematic layout of
the PCB is shown in Figure 3. The main components of the
PCB are as follows:
(2)
The regular differential amplifier in the next part of this
sub-circuit then takes this voltage drop V3-4 and amplifies it
by the gain. Since all the resistors used are of value 1kΩ, this
364
•
Power Supply: The chip requires a voltage supply
for its proper operation. This is provided from a 9V
battery placed in a battery holder.
•
Voltage Regulator Circuit: For providing a 5V
constant voltage to the chip from the 9V supply, the
required voltage regulation is achieved through an
adjustable Low Dropout voltage regulator (National
V.
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
12.
Working Electrode Probe Socket
20 Pin DIP Socket
Reference Probe Socket
Chip
I/O Ribbon Cable Connector
I/O Ribbon Cable Connector
20 Pin DIP Socket
3V Battery
9V Battery
18 Pin DIP Socket
Voltage Regulator Circuit
Potentiometer
Figure 4: Block Diagram of the Test set up for Amperiometric Testing
The amperiometric testing of the chip is carried out to
evaluate the current flowing through the probes. This helps
in determining the external conditions where the probes are
placed. As detailed in Section III, the measurement of
voltage suffices the purpose of current measurement. The
block diagram of the test setup is given in Figure 4. The 3V
battery supply on the PCB provides the varying voltage
supply through the potentiometer (POT’). This voltage is
applied across the working and reference electrodes placed in
chlorine dioxide (ClO2) solution. In amperiometric
measurement, the probes sense the oxygen content of the
solution under test. So the solution chosen must be stable and
should be such that the oxygen content can be conveniently
measured. These criteria are met by chlorine dioxide making
it an obvious choice for the solution under test. It behaves
like a resistor to the current flowing through it. This current
enters the chip at Pin_13 and the output voltage at Pin_26 is
measured. The bias potential is varied from 0.1 to 0.5V
insteps of 0.1V and the strength of the solution is varied from
5mg/dL to 25mg/dL in steps of 5mg/dL. Therefore, two
testing procedures are adopted:
Figure 3: Layout of the Printed Circuit Board
Semiconductor LM1086 3-lead TO-220 package).
The circuit consists of two adjustable resistors 5 kΩ
and 1 kΩ along with two 10uF capacitors. One of
the capacitors is a decoupling capacitor at the
output and the other is provided at the input for
ripple rejection.
•
40 Pin DIP socket for holding the chip.
•
3V battery power supply placed in a holder for
providing the necessary voltage to be applied across
the microelectrode probes. This voltage is required
to drive the current through the probes. It acts as the
bias potential.
•
Two ribbon cable connectors acting as the
Input/Output interface for the PCB.
•
Resistor divider circuit consisting of two 10kΩ
potentiometers for obtaining any desired voltage
from the 3V battery power supply.
•
Working electrode and Reference electrode sockets
for holding the working and reference electrodes
respectively.
•
A 20 pin and an 18 pin DIP socket for providing
necessary wire connectivity that is done through the
use of jumper wires. One more 20 pin DIP socket is
also provided for future provision of an Analog to
Digital converter.
TESTING METHODOLOGY
•
Bias potential is kept constant and the concentration
of the ClO2 solution is varied.
•
Concentration of the ClO2 solution is kept constant
and the bias potential is varied.
In addition, it is to be noted that the current flowing
across the solution is affected highly by external noise. To
have an idea of the effects of noise, a Faraday’s cage is
employed and the testing is carried out in two different
situations:
365
•
All of the testing equipments and the probes are kept
outside the Faraday’s cage.
•
All of the testing equipments and the probes are kept
inside the Faraday’s cage.
VI.
RESULTS AND DISCUSSION
The results of amperiometric testing for both the
situations, inside and outside Faraday’s cage, are tabulated
below in Table I.
TABLE 1
Bias
Potential
(Volts)
0.1
0.2
0.3
0.4
0.5
AMPERIOMETRIC TESTING RESULTS
(mg/dL)
Measured
Voltage
Outside
Faraday’s cage
(Volts)
5
10
15
20
25
5
10
15
20
25
5
10
15
20
25
5
10
15
20
25
5
10
15
20
25
0.58
0.591
0.604
0.626
0.641
0.608
0.622
0.635
0.653
0.673
0.649
0.659
0.668
0.681
0.695
0.675
0.679
0.685
0.699
0.709
0.683
0.689
0.693
0.704
0.714
Solution
Strength
Measured
Voltage
Inside
Faraday’s
cage
(Volts)
0.851
0.863
0.875
0.887
0.907
0.871
0.883
0.895
0.902
0.913
0.876
0.886
0.896
0.908
0.915
0.881
0.901
0.908
0.914
0.918
0.883
0.902
0.912
0.919
0.925
Figure 5: Plot of Measured Voltage vs. Solution Strength for different
values of applied bias potential (0.1 - 0.5 V) outside Faraday’s Cage
VII. CONCLUSIONS
The paper demonstrates MEMS based integrated
microelectrode sensor system for in situ measurements in the
range of nanoamperes (corresponding to a voltage in the
range of millivolts). In addition, the system is portable and
works for an increased resolution. Furthermore, it is mobile
and can be deployed in field tests. It helps in evaluating the
environmental conditions around the probes from a
knowledge of the signal measured. The application of this
microelectrode sensor can be extended to areas such as
biomedical applications where the solution under test may be
a bio-film.
ACKNOWLEDGMENT
The authors owe their acknowledgment to all the
members of the Microelectrode Research Group at the
University of Cincinnati for their significant contribution.
Their thanks also extend to the personnel of Entera-Tech Inc.
for providing all kinds of technical and financial support
throughout the execution of the project.
REFERENCES
[1]
The results indicate, that for a constant bias potential,
when the concentration of the solution is gradually increased,
the current (I=f (VMEASURED)) flowing through the solution
increases. With the increasing concentration, the oxygen
content of the solution increases that in turn decreases the
resistance offered by the solution, thereby increasing the
current that flows through it. Also, for constant solution
strength, the current flowing through the solution gradually
increases as the bias potential increases. This shows, that for
a definite value of applied potential, the solution strength can
be accurately determined from the measured values of
voltage. It is also observed that the results obtained without
Faraday’s cage are comparable to that obtained with the
shielding provided. The sensor chip thereby eliminates the
need of any external shielding mechanism for the sensor.
[2]
[3]
[4]
[5]
[6]
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