A Potentiometric Sensor System with Integrated Circuitry for in situ Environmental Monitoring

The need to monitor the conditions prevailing in the sense the signal reproduced by the fmicroelectrodes and external environment in order to control environmental provide convenient means of measuring the sensed signal. A pollution is one of the prime necessities of the modern age. This CMOS ASIC sensor circuitry is developed to perform the paper introduces a robust, self-contained, inexpensive potentiometric analysis of the solutions under test. integrated potentiometric sensor that can be used for in situ Potentiometric testing is conducted to measure the voltage environmental monitoring. The primary focus is to design, developed across a working probe (mnicroelectrode) and a implement and integrate a potentiometric CMOS circuit with reference probe under varying environmental conditions. It is the sensor device to measure the potential developed across a shown that the voltage measured by the sensor system varies working probe (microelectrode sensor) and the reference wt h hnei h xea odtosi hc h probe. The magnitude of the output voltage signal is dependent withethe plchang inThe externalembconditions aPinte whirchth on the characteristics (pHT) of the solution being evaluated by probsaret pladced Th chipyistembaeddedstin alPrnte Circuite the system. A Printed Circuit Board has been built to integrate Bioardto rodueap ulyitertebyseeaogsih.h the microelectrode sensor device and the sensor chip with the mcolcrd rbs aim of producing a fully integrated system. The microelectrode The system level block diagram representation of the sensor device may be replaced by a NEMS based sensor device integrated sensor system is shown in Fig. 1. It broadly in areas that require further miniatunization like biomedical cnit of two components. The first one is the applications. ~~~~~~~~microelectrode sensor device that monitors the external Keyord -pteniomtri, snso, itegate, niseenvironmiental conditions. These electrodes produce a Keywrds potntioetri, snsor intgrae4 nisevoltage signal and pass it on to the second component, the

2Entera Tech Inc. Columbus, OH, USA Abstract-The need to monitor the conditions prevailing in the sense the signal reproduced by the fmicroelectrodes and external environment in order to control environmental provide convenient means of measuring the sensed signal. A pollution is one of the prime necessities of the modern age. This CMOS ASIC sensor circuitry is developed to perform the paper introduces a robust, self-contained, inexpensive potentiometric analysis of the solutions under test. integrated potentiometric sensor that can be used for in situ Potentiometric testing is conducted to measure the voltage environmental monitoring. The primary focus is to design, developed across a working probe (mnicroelectrode) and a implement and integrate a potentiometric CMOS circuit with reference probe under varying environmental conditions. It is the sensor device to measure the potential developed across a shown that the voltage measured by the sensor  The rest of the paper is arranged into the following stream or lake sediments that require substantial and sections. Section II introduces the design of the uninterrupted monitoring. This kind of constant monitoring potentiometric circuitry for sensing the voltage signal. of the environment necessitates a robust, portable and Section III discusses the PCB that is designed and fabricated integrated sensor capable of producing a fast response and tocnanhehialgwthhemroetodpobs having a high sensitivity. The  battery and a necessary voltage regulatory circuit. It is a 40 pin DIP chip laid out using the Tanner Tools L-Edit layout editor in the .5um process and fabricated through the * Voltage Regulator Circuit: The required voltage MOSIS foundry [4]. Fig. 2 shows the potentiometric circuit regulation for providing a 5V DC supply to the chip in the chip. It essentially consists of the following two stages from the 9V battery supply is achieved through an connected in succession.
A. Voltage Follower Circuit * Two ribbon cable connectors (10 pin and 14 pin) The first part of the potentiometric circuit is a set of two acting as the Input/Output interface for the PCB. separate voltage follower circuits. The input impedance of * 40 Pin DIP socket for holding the chip. the operational amplifier used in the circuit is designed to be very high. This provides an effective isolation of the output * Appropriate sockets with proper connectivity for from the input signal source. Loading effects are avoided and holding the working and reference electrodes. very little power is drawn from the signal source. The working probe is connected to Pin 4, input of one voltage * A 20 pin and an 18 pin DIP socket for providing follower circuit, and the reference probe to Pin 3, input of necessary wire connectivity. the other voltage follower circuit.
IV. TEST PROCEDURE B. Differential Instrumentation Amplifier The second stage of the circuit consists of a buffered A. Potentiometric Testing differential amplifier with three resistors linking the two The purpose of potentiometric measurement with the buffer circuits together. It establishes a voltage drop across chip is to measure the Oxidation Reduction Potential (ORP) the resistor RGAIN equal to the voltage difference between V1 of the solution in which the two electrodes are immersed [3]. and V2. The resistor RGAIN is connected externally to the This potential generated across the electrodes is directly sensor chip between Pin_6 and Pini 1 by connecting Pin 6 related to metal used for the working electrode and the to the fixed end of a 100k-Ohm potentiometer and Pin_1 to chemical nature of the solution. The working and reference the variable end. The regular differential amplifier in the next electrode probes, placed in the solution, are connected to the part of this sub-circuit takes the voltage drop V34 and two input pins, pin_4 and pin 3, of the potentiometric amplifies it further. However, since all resistors used are of circuit. The output voltage is obtained at Pin_3 1. The test value 1 k-Q, this gain is equal to unity. The advantage of the setup is shown in Fig. 3. Two types of solutions are used for differential amplifier lies in the fact that the overall gain can the test -225 mV standard solution and pH7 solution. In be varied by varying only a single resistor RGAIN. The other order to replicate the known value of the potentials that are advantages are related to its high input impedance, high expected with the given solution, the variable resistor RGAIN common mode rejection ratio (CMRR) and noise elimination is kept open (pins 6 and 1 disconnected), i.e. overall gain of capability. The output differential voltage is obtained at the circuit equal to unity. The output voltage readings are Pin_3 1.
taken with a precise digital multimeter and monitored using the National Instruments LABVTEW automated software program designed for the measurement purpose.  Celsius. An LED display may be One of the primary purposes of the chip along with the attached to the output of the chip to read the voltage signal PCB is the elimination of extenal inoise effect as far as roduced Comparable results are obtained inside and possibleOl. So to have an idea about thle effccts of noise and outside the Faraday's cage that justifies the usefulness ofthe thereby to ensue its proper elimination, two kinds of testing microelectrode sensor system in eliminating the necessity of procedures are adopted. Th ley are:a kind of external shielding mechanism. In othert words, * All of the testing equipments and the probes are the system is immune to the effects of external noise.
placed outside the Faraday's cage.
* All of the testing equipments and the probes are tp placed inside the Faraday's cage.
To note the change in behavior of the probes over time, the measurements are taken for a time interval rarging fom Agaln to verin the appropriateness of the PCB in 0-300 seconds. The transient behavior of the probes for the eliminating noise in addition to that already eliminated by two solutions is shown in Fig. 4. It is seen that the measured the chip, both of the above testing procedures are adopted for potential attalns a high value as soon as the probes are placed two different kinds of situations, approximately: * 37 seconds for 225mV standard solution The root-mean-squiare (rms) valuie of the noise signal obtained is measured continuously over the time range (0-* 32 seconds for pH7 solution 300s) and then an average of the rms values obtained is noted. The measurements are obtained with a high precision digital multimeter and momntored using ani automted 250 LABVIEW measurement software program. The voltage values obtained in the potentiometric testing > are given in Table I. It is seen that the potentiometric circniit noise distribution is shown in Fig. 5(a)