WO2009070003A2

WO2009070003A2 - An analog read out interface circuit for ion sensor

Info

Publication number: WO2009070003A2
Application number: PCT/MY2009/000014
Authority: WO
Inventors: Yuzman Yusoff; Mohamad Faizal Hashim; Rohana Musa
Original assignee: Mimos Berhad
Priority date: 2007-11-30
Filing date: 2009-01-13
Publication date: 2009-06-04
Also published as: WO2009070003A3

Abstract

The present invention relates to an analog read out interface circuit for hydrogen ion sensors, an more particularly for ion-sensitive field transistor pH sensor. An analog read-out interface circuit for ion sensor, comprising a constant voltage constant current circuit; a tunable temperature sensor based on CMOS parasitic bipolar transistor; a summation circuit; characterised in that the analog read-out interface circuit is operable with an option to adjust temperature coefficient (Tc) of the tunable temperature sensor and output voltage level.

Description

An analog read out Interface circuit for ion sensor.

Field of Invention

The present invention relates to an analog read out interface circuit for hydrogen ion sensors, and more particularly for ion-sensitive field transistor pH sensor. The interface circuit of the present invention possess additional tunable CIΛOS temperature sensor based on CMOS parasitic bipolar transistor to further improve temperature dependence of ISFET.

Brief description of related art

Advancement in microelectronic and semiconductor technology has enable nisw in capabilities in field of sensor development, particularly of pH sensors based or field effect transistor. Ion sensitive field effect transistor (ISFET), a kind of micro-se ising device invented by Bergveld in 1970 and developed quickly thereafter. The I SFET pH sensors form semiconductor technology has the desired feature of low cost, good sensitivity, short response time and compatibility to CMOS technology.

ISFET structure is very similar to conventional MOSFET₁ except for the sta idard metal gate is replaced by reference electrode and electrolyte. The threshold voltage of ISFET is change due to the change in concentration of hydrogen ions (exposed by the pH charfge of the electrolyte), so that the voltage response is often usied as an output signal of ISFET.

To capture the electrical signal generated by a sensor, the sensor (ISFET) m jst be accompanied with an analog read-out interface. The present invention of an ϋinalog read-out interface is integrated together with temperature sensor to improve SFET temperature dependence characteristic.

The ISFET base pH sensor often has problem in reproducibility, stability, p H drift and temperature dependence. Certainly, others in the past have attempted to improve the problem faced by the ISFET base pH sensor. Every improvement in these aspects will undoubtedly increase the usage of ISFET. Additionally, referring to US Patent 6906524, the document discloses an ion sensing circuit comprises a bridge sensing circuit and a differential amplifying circuit. The bridge sensing circuit detects the ion concentration of constant voltage and constant current. The main features of the disclosed invention are that it ground the reference electrode and float the source terminal. It also include analog read-out interface for pH sensor and has the ability to compensate temperature effect.

Accordingly, there is still substantial need in the art for an a effective and a complete analog read out interface circuit for pH sensor with an option to adjust temperature coefficient (Tc) of temperature sensor, additional option to adjust the required output voltage level and utilising CVCC circuit. Which will greatly improve the current problem of reproducibility, stability, pH drift and temperature dependence. Further, the present invention provides good sensitivity, convenient form of response and certainly cost effective.

SUMMARY OF THE INVENTION

The present invention relates to an analog read out interface circuit for hydrogen ion sensors, an more particularly for ion-sensitive field transistor pH sensor. An analog read-out interface circuit for ion sensor, comprising a constant voltage constant current circuit ; a tunable temperature sensor based on CMOS parasitic bipolar transistor; a summation circuit; characterised in that the analog read-out interface circuit is operable with an option to adjust temperature coefficient (Tc) of the tunable temperature sensor and output voltage level.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature of the present invention, reference should be made to the following detailed description taken in connection with the accompanying drawings in which:

Figure 1A: An exemplified diagram of the preferred embodiment of analog read out interface circuit for pH sensor.

Figure 1B: An exemplified block diagram of the preferred embodiment of analog read out interface circuit for pH sensor.

Figure 2: An exemplified diagram of the preferred embodiment of of sensing circuit

Figure 3: An exemplified diagram of the preferred embodiment of circuit configuration of the present temperature sensor.

Figure 4: An exemplified diagram of the preferred embodiment of circuit configuration of summation circuit DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Shown throughout the figures the present invention is directed to an analog read out interface circuit for hydrogen ion sensors, an more particularly for ion-sensitive field transistor pH sensor. The interface circuit of the present invention possess additional tunable CMOS temperature sensor based on CMOS parasitic bipolar transistor to further improve temperature dependence of ISFET.

Looking particularly at Figure 1 A and 1B, Figure 1A illustrates an exemplified analog read out interface circuit 10 for pH sensor. It consists of three major components, first, a CMOS parasitic bipolar transistor temperature sensor 100, second, a constant current constant (CVCC) circuit 200 and third, a summation circuit 300. The principle of this read-out circuit 10 is to cancel the positive temperature coefficient of ISFET with the negative temperature coefficient of CMOS parasitic bipolar transistor 100. Temperature coefficient of both devices can be combined through summation circuit 300 and produce nearly zero temperature coefficient as illustrated in Figure 1B. Thus, the ISFET temperature dependence characteristic is improved.

Figure 2 illustrates an exemplified diagram of sensing circuit. The sensing circuit 200 detects the ion concentration of the solution with features of constant voltage/constant current operation mode, and floating reference electrode. The drain terminal 202 of the transistor ISFET 201 is connected to the output terminal 203 of the first amplifier OP1 204, where a constant voltage of V2, e.g 0.95 volts in the figure, is fed to its positive terminal 205. The negative terminal 206 is connected to the output terminal 203. The source terminal 207 of the transistor ISFET 201 is coupled with the ground 208 via variable resistor R3 209. By using variable resistor R3 209, the temperature coefficient of ISFET can be controlled. A constant voltage V1, e.g 0.75 volts in the figure, is fed to the positive terminal 211 of the second amplifier OP2 210. The output terminal 212 of the second amplifier OP2 210 is coupled with the reference electrode Ref 214 of the transistor ISFET 201. Another amplifier OP3 213 is connected as a buffer. With this configuration, two constant voltages inputs to the two positive terminals of the amplifiers 204, 210 cause the source terminal S 207 and the drain terminal D 202 of the transistor ISFET 201 to keep a constant drain-source voltage difference. The solution of the ion concentration creates the connection between the reference electrode Ref 214 and gate sensing membrane (terminal G) 215. The potential different between the gate sensing membrane 215 and the reference electrode 214 is determined by the ion concentration of the solution.

Figure 3 illustrates an exemplified diagram of circuit configuration of the present temperature sensor. To develop temperature sensor 100 using CMOS compatible process with no required additional mask, a CMOS parasitic bipolar transistor 101 is used. In addition the base emitter voltage of parasitic bipolar provides a negative temperature coefficient. A variable resistors, R1 102, and fixed resistor R2 103, are used to control the current and temperature coefficient. By adjusting R1 102 to the right value, the temperature coefficient of temperature sensor can be tune closely to ISFET temperature coefficient, but its value still negative. It is important to have this option because fabricated ISFET has high possibility to mismatch from expectation.

Figure 4 illustrates an exemplified diagram of circuit configuration of summation circuit 300. The circuitry is completed by feeding electrical signal generated by both temperature sensor 100 and CVCC circuit 200 into summation circuit 300 that mutually offset their temperature coefficient and produce a temperature independent output signal. The summation circuit consists of an operational amplifier 301 and four additional resistors 302,303,304,305. The output voltage level of summation circuit can be adjusted by controlling input voltage of Vref.

The above described embodiments of the present invention are intended to be examples only, it is intended that all matters in the foregoing description and shown in the accompanying drawings be interpreted as illustrative and not in a limiting sense. Alterations, modifications and variations may be effected to the particular embodiments by those skills in the art without departing from the scope of the invention which is defined solely by the claim appended hereto.

Claims

What is claim is:

1. An analog read-out interface circuit 10 for ion sensor, comprising a tunable temperature sensor 100 based on CMOS parasitic bipolar transistor; a constant voltage constant current circuit 200 ; a summation circuit 300; characterised in that the analog read-out interface circuit 10 is operable with an option to adjust temperature coefficient (Tc) of the tunable temperature sensor 100 and output voltage level.

2. An analog read-out interface circuit 10 of claim 1 characterised in that the analog read-out interface circuit is adaptable 10 in any ISFET base pH sensor.

3. An analog read-out interface circuit 10 of claim 1 characterised in that the constant voltage constant current circuit 200 comprising; a drain terminal 202 of a transistor ISFET 201 connected to an output terminal 203 of first amplifier 204 where a constant voltage is fed to its positive terminal 205; a negative terminal 206 connected to the output terminal 203; a source terminal 207 of the transistor ISFET 201 is coupled with the ground

208 via a variable resistor 209; constant voltage is fed to a positive terminal 211 of a second amplifier 210; an output terminal 212 of the second amplifier 210 is coupled with a reference electrode 214 of the transistor ISFET 201; an amplifier 213 is connected as a buffer; wherein this configuration provide two constant voltage input to the two positive terminals 205,211 of the amplifiers 204,210 cause the source terminal 207 and drain terminal 202 of the transistor ISFET 201 to keep a constant drain-source voltage difference.

4. An analog read-out interface circuit 10 of claim 1 characterised in that the summation circuit 300 comprising an operational amplifier 301; a plurality of resistor 302,303,304,305; characterised in that the summation circuit 300 output voltage level is adjustable by controlling input voltage.

5. An analog read-out interface circuit 10 of claim 1 characterised in that the tunable CMOS temperature sensor 100, further comprising; a CMOS parasitic bipolar transistor 101 to provides negative temperature coefficient; a variable resistor 102 to provides option to tune temperature coefficient of

CMOS parasitic bipolar transistor; and a resistor 103 to control the current.