Abstract:
A system of measuring pH of a solution having a calibration device to counteract time-drift effect. The calibration device of the system adjusts a compensation voltage to zero a measuring voltage of a first sensor and only respond to time-drift voltage of the first sensor. The calibration device has a differential operation amplifier receiving measuring voltages from the first sensor and a second sensor of the system to eliminate the time-drift voltages of the first and second sensors, thereby achieving calibration of the time-drift effects.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a measuring system, and in particular to a system of measuring pHs of solutions and method for calibrating time-drift effects of sensors thereof. 
     2. Description of the Related Art 
     Extend Gate Ion Sensitive Field Effect Transistor (EGISFET) connects a sensing electrode, for example a titanium nitride electrode, to gate of metal oxide semiconductor field transistor (MOSFET). EGISFET can be fabricated using CMOS standard process, and is developed from Ion Sensitive Field Effect Transistor (ISFET). 
     Theorem and related knowledge of ISFET are detailed in the following list of documents: 
     1. U.S. Pat. No. 4,879,517, inventors: Connery, James G., Shaffer Jr., Earl W; 
     2. U.S. Pat. No. 6,617,190, inventors: Chou Jung Chuan, Chiang Jung Lung; 
     3. U.S. Pat. No. 5,309,085, inventor: Byung Ki Sohn; 
     4. U.S. Pat. No. 4,657,658, inventor: Alastair Slbbald; 
     5. US patent publication No. 20020180609, inventors: Kang Ding, W. E. JR. Seyfried, Zhong Zhang; 
     6. US patent publication No. 20030054177, inventor: Ping Jin; 
     7. US patent publication No. 20040075578, inventors: Olaf Dudda, Christian Oldendorf; 
     8. U.S. Pat. No. 4,691,167, inventors: Hendrik H. V. D. Vlekkert, Nicolaas F. de Rooy; 
     9. US patent publication No. 20030093011A1, inventors: Jalisi Marc Mehrzad; 
     10. U.S. Pat. No. 5,130,265, inventors: Massimo Battilotti, Giuseppina Mazzamurro, Matteo Giongo. 
     EGISFET comprises separated gate, is capable of being fabricated using CMOS standard process and has advantages of low cost, simple structure and easy package, making it suitable for biomedical application. 
     Many materials can act as detecting membranes for ISFETs, such as Al 2 O 3 , Si 3 N 4 , Ta 2 O 5 , amorphous WO 3  (a-WO 3 ), amorphous Si:H (a-Si:H) and others. Response time, hysteresis effect, time-drift effect, and light effect are important factors that influence performance of ISFET. EGISFET is developed from ISFET, having similar sensing principle, and therefore its performance is inevitably influenced by such non-ideal effects such as time-drift effect and hysterersis effect. 
     BRIEF SUMMARY OF INVENTION 
     Accordingly, an exemplary embodiment of the invention provides a system of measuring pHs of solutions, the system comprising: a first sensing unit provided in a solution, measuring pH of the solution to generate a first voltage; a second sensing unit provided in the solution, measuring pH of the solution to generate a second voltage, wherein the first and second sensing units having the same time-drift effect; and a calibration device for calibrating the time-drift effect of the first and second sensing units. 
     The calibration device comprises an offset voltage compensator outputting an adjustable compensation voltage, a first differential amplifier coupling the first voltage and the compensation voltage and outputting a third voltage, wherein the third voltage is substantially zeroed by adjusting the compensation voltage and only responsive to the time-drift effect of the first sensing unit, a second differential amplifier coupling the second voltage and a reference voltage, outputting a fourth voltage, and a third differential amplifier coupling the third voltage and the fourth voltage to counteract the time-drift effect of the first and second sensing units, thereby outputting a fifth voltage corresponding to pH of the solution. 
     Another exemplary embodiment of the invention provides a method for calibrating time-drift effect of a pH measuring system, providing a first sensing unit in a solution to measure pH of the solution and obtain a first voltage, providing a second sensing unit in the solution to measure pH of the solution and obtain a second voltage, the first and second sensing units having the same time-drift effect, and providing a calibration device to receive the first and second voltages to calibrate the time-drift effect of the first and second sensing units, wherein the calibration device comprises an offset voltage compensator, a first differential amplifier, a second differential amplifier and a third differential amplifier. 
     The method further comprises using the first differential amplifier to receive the first voltage and the compensation voltage output from the offset voltage compensator and output a third voltage, adjusting the compensation voltage to substantially to zero the third voltage and make the third voltage only respond to the time-drift effect of the first sensing unit, using the second differential amplifier to receive the second voltage and a reference ground and output a fourth voltage, and using the third differential amplifier to receive the third voltage and the fourth voltage to counteract the time-drift effect of the first and second sensing units, thereby outputting a calibrated fifth voltage corresponding to pH. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  shows block diagrams of a system of measuring pH values of solutions according to an exemplary embodiment of the invention. 
         FIG. 2  shows a possible embodiment of the offset voltage compensator of the pH measuring system. 
         FIG. 3  shows a relation curve of the output voltage V out  of the calibration device with respect to time. 
         FIG. 4  shows responses of an original time-drift output voltage including original time-drift voltage and a calibrated output voltage including calibrated time-drift voltage, where both output voltages are respect to pH 6. 
         FIG. 5  shows a voltage-time relation curve at different pH values using the pH measuring system  100 . 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  shows block diagrams of a system of measuring pH values of solutions according to an exemplary embodiment of the invention. The pH measuring system  100  comprises a first sensing unit  101 , a second sensing unit  102 , a calibration device  103 , a light-isolating container  104  accommodating a testing solution  105 , and a reference electrode  106 . The reference electrode  106  is provided in the solution  105 , connected to a reference ground through a conduction line to provide a ground reference voltage. In this embodiment, the reference electrode  106  is a silver/silver-chloride (Ag/AgCl) electrode. The light-isolating container  104  reduces light-sensitivity effect to the first and second sensing units  101  and  102 . 
     The first and second sensing units  101  and  102  are provided in the solution  105  to measure pH of the solution  105  and generate a first voltage V 1  and a second voltage V 2 , respectively. It is noted that the first and second sensing units  101  and  102  have the same time-drift effect. 
     The calibration device  103  comprises an offset voltage compensator  103   a  outputting an adjustable compensation voltage V adj , a first differential amplifier D op1  coupling the first voltage V 1  and the compensation voltage V adj  and outputting a third voltage V 3 , a second differential amplifier D op2  coupling the second voltage V 2  and the reference ground, outputting a fourth voltage V 4 , and a third differential amplifier D op3  coupling the third voltage V 3  and the fourth voltage V 4  to counteract the time-drift effect of the first and second sensing units  101  and  102 , thereby outputting an output voltage V out  corresponding to pH of the solution  105 . 
     The adjustable compensation voltage V adj  is controlled by the offset voltage compensator  103   a  to zero the third voltage V 3  such that the third voltage V 3  merely responds to the time-drift effect (or time-drift voltage) of the first sensing unit  101 . The fourth voltage V 4  responds to combination of pH of the solution  105  and the time-drift effect (time-drift voltage) of the second sensing unit  102 . Therefore, the third differential amplifier D op3  can eliminate the common time-drift effect of the first and second sensing units  101  and  102  and output the output voltage V out  corresponding to pH of the solution, thereby achieving calibration to time-drift effect. 
     The offset voltage compensator  103   a  comprises at least a resistor, a variable resistive unit and a buffer; wherein the variable resistive unit comprises at least a variable resistor connected in series with the resistor, and an output of the buffer is coupled to a connection node of the resistor and the variable resistor.  FIG. 2  shows a possible embodiment of the offset voltage compensator  103   a . In  FIG. 2 , the offset voltage compensator  103   a  comprises a variable resistive unit  300  having three variable resistors  304  to  306  connected in series, two resistors  302  and  303 , and a buffer  301 . The variable resistive unit  300  and the resistors  302  and  303  are connected in series and provided between two voltage nodes V a  and −V a . The adjustable compensation voltage V adj  can be controlled by changing resistances of the variable resistors  304  to  306 . 
     The pH measuring system  100  further comprises a first buffer  103   b  coupled between the first sensing unit  101  and the first differential amplifier D op1  and a second buffer  103   c  coupled between the second sensing unit  102  and the second differential amplifier D op2 , to increase input impedances of the first and second differential amplifiers D op1  and D op2 . In this embodiment, the first and second sensing units  101  and  102  are EGISFETs and are manufactured using the same fabricating process and packaging condition. In addition, electrodes connecting to gates of the EGISFETs of the first and second sensing units  101  and  102  are titanium nitride electrodes. 
     A 1.0 cm×1.0 cm silicon wafer is used as a substrate to form the EGISFETs with titanium nitride gate electrodes. The substrate is cleaned in alcohol or deionized water (DI water) and cleaned by an ultrasonic oscillator. Then, nitrogen gas is sprayed to clean the surface of the substrate to ensure no water remained on the substrate. Titanium nitride electrode (or membrane) of the EGISFET is formed using sputtering with titanium target material of pure grade 99.995% and mixed gas of argon (Ar) gas and nitrogen gas (N 2 ) with 80 sccm and 10 sccm, respectively. The vacuum (or atmosphere pressure) inside the process chamber is adjusted to 10 −6  Torr before deposition of titanium nitride. Further, surface of the titanium target is cleaned using RF power of 150 W for 10 minutes at ambient of 10 m Torr to prevent oxide thereon from being sputtered to the substrate. 
     After sputtering, the wafer is cleaned in deionized water and cleaned by ultrasonic oscillator to remove dust and particles therefrom. The titanium nitride membrane formed on the 1.0 cm×1.0 cm substrate is divided into four 0.5 cm×0.5 cm units. The four units are packaged to form four titanium nitride electrodes, such that the titanium nitride electrodes and the EGISFETs with such electrodes have the same time-drift effect. 
     Long time-drift experiment is carried out to test stability of the calibration device  103  of the pH measuring system  100 . First, input terminals of the first and second differential amplifiers D op1  and D op2  are connected to reference ground. Then, the output voltages V out  of the third differential amplifier D op3  are recorded for a long time, obtaining a relation curve of the output voltage V out  with respect to time (i.e., voltage-time relation curve) as shown in  FIG. 3 . Average drift voltage of the calibration device  103  can be obtained from the recorded output voltages V out  in  FIG. 3 . The average drift voltage of the calibration device  103  is only about −12.2 μV, much less than the drift voltage of the sensing electrodes, so as to not be considered. 
       FIG. 4  shows responses of an original time-drift output voltage including original time-drift voltage and a calibrated output voltage including calibrated time-drift voltage, where both output voltages are respect to pH 6. It can be seen that time-drift voltage of the original output voltage without calibration is reduced from 34.18 mV (2.85 mV/H) to 5.73 mV (0.48 mV/H). 
     In addition, a method of measuring sensitivity of a sensing electrode for use in the pH measuring system  100  is disclosed. The method comprises using titanium nitride as electrodes of the first and second sensing units  101  and  102 , contacting the titanium nitride electrodes with a test solution, changing pHs of the test solution at a fixed temperature, using the calibration device to measure pH and recording the output voltages V out  output from the calibration device  103 , obtaining a voltage-time relation curve at different pH levels, and obtaining sensitivity of the titanium nitride electrodes according to the voltage-time relation curve. 
       FIG. 5  shows a voltage-time relation curve at different pH levels using the pH measuring system  100 . The sensitivity (S=V/pH) of the sensing units or electrodes can be obtained from  FIG. 5  and is about 57.65 mV/pH. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.