Abstract:
An electronic sensing circuit for an electrochemical gas sensor cell providing high dynamic range is described. The electronic circuit utilizes an amplifier with a resistive feedback element to provide high sensitivity linear sensing of low detected gas concentrations. The electronic circuit also provides the ability to source sufficient current to prevent the electrochemical gas sensor cell from becoming biased under very high gas concentration conditions. This operation is enabled by adding a non-linear element in the feedback path of the amplifier, which non-linear element begins to conduct during the high gas concentration conditions.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to electrochemical sensing circuits and more particularly to electrical circuits which sense cell current flow from a three terminal electrochemical cell. 
   BACKGROUND OF THE INVENTION 
   Three terminal electrochemical cells are used for a variety of gas monitors. These monitors comprise a cell in which a gas to be analyzed is introduced and three spaced apart electrodes. The three electrodes comprise a main pair across which the cell current is generated and a reference electrode which enables a potential at a predetermined point in the cell electrolyte measured. The cell current is proportional to the concentration of the compound or element being sensed by the cell, which may, for example, be carbon monoxide. 
   Known three-terminal electrochemical cells can be stabilized using the circuit shown in  FIG. 1 . In order to stabilize the cell, the “working” and “reference” electrodes, labeled “W” and “R” respectively must be brought to the same electrical potential. No current is taken from the reference electrode. Instead, current is injected into the counter electrode, labeled “C”, by the amplifier A 1  until both the reference and working electrodes, R and W respectively, are the same potential. The current which flows in both the counter and working electrodes is the cell current. Due to the internal operation of the cell, this is proportional to the concentration of the compound being sensed by the cell. 
   Referring to  FIG. 1 , the amplifier A 1  maintains the reference electrode at 0V by feeding back current to the counter electrode. Amplifier A 2  maintains the working electrode at 0V, since the input of amplifier A 2  is at 0V. The cell current is driven by amplifier A 1  but is sensed by amplifier A 2 , because the cell current passes through resistor R 2  to develop the voltage V out. 
   A disadvantage of this prior known circuit shown in  FIG. 1  is that it is prone to oscillation because the virtual earth impedance of each amplifier appears as part of a feedback path of the other amplifier. This can lead to oscillation at high frequencies where the virtual earth impedances are not well defined. A further disadvantage is that, whereas Vout is normally positive when gas is being sensed, the counter electrode charges negatively, requiring the output of amplifier A 1  to go negative. Therefore the circuit shown in  FIG. 1  requires both positive and negative supplies (shown as +V and −V). 
   To overcome these disadvantages and to allow the detector and associated circuitry to be operated from a single battery source, the circuit of  FIG. 2  was developed. This circuit is described in U.S. Pat. No. 6,277,255, entitled Electrochemical Sensing Circuits, to Green, et al. This patent describes an electrochemical cell having a working electrode, a counter electrode and a reference electrode in an electrolyte as shown in  FIG. 2 . The cell is constructed such that in use, when a gas to be analyzed is introduced into the cell, a current flows between the counter electrode and the working electrode. A potential at a position in the electrolyte is sensed by the reference electrode. The circuit also includes a power supply for applying an offset voltage to the counter electrode relative to the working electrode. An amplifier circuit monitors the voltage difference between the reference electrode and the working electrode. This amplifier circuit feeds back a current to the working electrode through a feed back loop in order to maintain the working electrode at substantially the same potential as the reference electrode. The current fed back by the amplifier to the working electrode is monitored as a measure of the cell current flowing between the working electrode and the counter electrode. The current is monitored as a voltage across the resistor in the feed back loop indicated as Vout. 
   While this circuit overcame the shortcomings of the prior monitoring circuits and allowed usage of a single polarity DC source such as a battery, emerging agency standards, e.g. UL 2034 and EN 50291, are placing increasing demands on the electrochemical cell sensing circuitry. Specifically, since electrochemical gas sensors are current output devices, as the gas of interest is introduced into the sensing cell, e.g., carbon monoxide (CO), the amount of current produced by the cell increases. To allow the sensor to work properly, the output current must always be maintained at a desired bias condition. As with the circuitry described above, for products such as CO detectors, the output current is usually converted into a voltage by an amplifier. This voltage can then be read by a microprocessor. While the microprocessor can read this output voltage in a variety of ways, one of the most common methods is to supply the voltage to an analog to digital (A/D) converter. 
   In order to meet the agency standards, the circuitry used in a typical CO detector must be able to respond to very low levels of CO, in the 10&#39;s of parts per million (ppm) concentration levels. However, the typical carbon monoxide detector must also react, albeit in a faster time, to CO concentrations in the 1000&#39;s of ppm exposure. Since most microprocessors utilize a A/D converters that are 8 or 10 bits, the resolution available is either 256 or 1024 possible concentrations, respectively. In many applications the A/D resolution in and of itself does not impose too much of a limitation despite the wide range of input conditions that the detector must be able to sense accurately. Instead, because many such carbon monoxide detectors are battery powered devices, the number and magnitude of the power supply voltages powering the electronics is the limiting factor. 
   In the simplest of implementations, the current to voltage converter and the microprocessor are both powered by the same power supply. For practical purposes in a consumer device, e.g., to conserve battery life, this supply is typically 3.3 volts DC. With such a limitation, the problem is to allow the amplifier, e.g., a transimpedance amplifier (TIA) to convert the sensor output current for low level exposures to CO to be read by the microprocessor, and still allow the amplifier to have enough capacity such that it can source the current produced by the electrochemical cell at the tens of thousands of ppm exposure to CO gas for over exposure conditions. If the sensing electronics are unable to source this higher current, the sensor typically becomes biased and is unable to recover quickly from such high concentration exposures. 
   There exists, therefore, a need in the art for an electronic circuit having a high dynamic range such that it is capable of accurately monitoring the output of an electrochemical gas sensor cell that responds to gas concentrations in the 10&#39;s of ppm through the tens of thousands of ppm of CO concentration. More particularly, there is a need in the art for an amplifier configuration that allows maximum sensitivity while allowing for sensor recovery to very high gas concentrations for an electrochemical sensor. 
   BRIEF SUMMARY OF THE INVENTION 
   In view of the above, it is an object of the present invention to provide a new and improved electronic circuit that senses cell current flow from a three terminal electrochemical gas sensor cell that overcomes the above problems existing in the art. More particularly, it is an object of the present invention to provide a new and improved amplifier configuration that allows high sensitivity to the electrochemical gas sensor cell output during periods of low gas concentration while allowing for rapid sensor recovery when the electrochemical gas sensor is exposed to very high gas concentrations. Specifically, it is an object of the present invention to provide a new and improved electronic circuit for sensing the output of a electrochemical gas sensor that has a very high dynamic range such that operation of the hazardous condition detector in which it is utilized may meet agency standards. 
   In the electronic circuit of one embodiment to the present invention the circuitry provides high resolution of measurements while limiting the power supply needed for high concentrations of gas. In this embodiment, this is achieved by utilizing a non-linear element in the feedback of an amplifier. As such, at low levels of gas concentration the output signal is defined by the feedback resistance. At the point that the non-linear element begins to conduct, the gas concentration is high and the area of interest for linear signals has passed. Under such conditions the amplifier circuitry is still able to maintain the current sourced by the electrochemical gas sensor with a much smaller power supply. 
   In a preferred embodiment of the present invention, the electronic sensing circuitry utilizes a resistive feedback from the output of an amplifier to the working electrode of a three terminal electrochemical gas sensor cell. In addition to this feedback resistor, a pair of series connected transistors are coupled in parallel to the feedback resistor. These two transistors act as a single diode in that they do not conduct in negative polarity. However, in positive polarity there exists a region of potential in which no current will flow through these devices, i.e., the feedback resistor is the only element through which current may flow. If the voltage potential increases beyond the turn on voltage of the transistors, they will begin to conduct current. This conduction will allow the amplifier to maintain the current through the electrochemical sensor without requiring additional voltage potential across the feedback resistor. This provides an increased dynamic range of the amplifier while still utilizing the low voltage supply of the battery. 
   Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
       FIG. 1  is a simplified single line circuit schematic of a prior electrochemical sensing circuit; 
       FIG. 2  is a simplified single line circuit schematic of an improved prior electrochemical sensing circuit; and 
       FIG. 3  is a simplified single line circuit schematic of an electrochemical sensing circuit constructed in accordance with the teachings of the present invention. 
   

   While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 3 , the electrochemical cell  30  is of conventional construction and comprises a chamber, or cavity  32  into which a gas to be monitored is introduced. Three spaced electrodes W, C, R are located in the cavity  32 . Electrode R constitutes a reference electrode and is connected to an input  34  of an operational amplifier  36 . A second input  38  of amplifier  36  is connected to the electrode W. The output  40  of the amplifier  36  is connected to electrode W through a feedback loop which includes resistor  42 . The counter electrode, C, is connected to a reference voltage generated by resistor  44  and diode  46  which is connected to the circuit ground. 
   The amplifier  36  feeds back current through resistor  42  to maintain the working and reference electrodes, W and R respectively, at the same potential. 
   When gas is sensed by the cell  30 , the output of amplifier  36  goes positive to deliver a positive current into the working electrode W, and at the same time, the electrode W charges positive with respect to the counter electrode C. These two effects ensure that the output  40  of amplifier  36  remains positive relative to the circuit ground at all times. 
   The voltage across resistor  42  is an accurate output which is proportional to gas concentration in the cell  30 . If the +V supply is provided by an isolated battery (not shown), the terminals  48 ,  50  of the resistor  42  can be taken as the output with one terminal connected to an external isolated ground. Where accuracy is less important, it may be sufficient to sense the output of amplifier  36  relative to the circuit ground, but the output voltage of amplifier  36  includes a component due to the offset voltage between the working electrode, W, and the counter electrode C. The offset is normally a very small fraction of a volt. 
   The offset voltage on the counter electrode, C, is generated by resistor  44  and diode  46  and protects those cells which would otherwise have an in-built tendency for the counter electrode to be positive relative to the working electrode. The offset voltage also allows for exposure to gases which would otherwise spuriously provoke a reverse response from the cell. If the cell is designed to have an inbuilt positive bias from the counter electrode, C, to the working electrode, W, it may not be necessary to impose an offset voltage on counter electrode C. 
   In addition to the resistor  42 , the circuit of the present invention also utilizes a non-linear element in the feedback path of amplifier  36 . Specifically, in one embodiment of the present invention the non-linear element may be implemented as a pair of transistors  52 ,  54  coupled in parallel to resistor  42 . However, those skilled in the art will recognize that a single transistor may be utilized depending on circuit parameters. This non-linear element operates such that it is non-conducting at low levels of signals corresponding to low detected gas concentrations. As such, the voltage at terminals  48 ,  50  is defined solely by the output current multiplied by the resistance  42 . However, as the concentration of detected gas increases, a larger current will begin to flow. As the voltage generated across feedback resistor  42  increases due to this increasing current, the non-linear elements begin to conduct, effectively reducing the feedback path resistance. The amplifier is then able to maintain the current through the electrochemical sensor  30  while still utilizing only the small power supply sourced from the battery. 
   In the environment of the present invention illustrated in  FIG. 3  the two transistors  52 ,  54  may be thought of as a single diode that, in the negative polarity, do not conduct. However, in the positive polarity these transistors  52 ,  54  may begin to conduct. That is, in the positive polarity there is a region of potential across resistor  42  in which the transistors  52 ,  54  do not conduct. This is the linear sensing region of the circuit. However, as the potential across resistor  42  increases due to the sensing of a high concentration of detected gas, the area of interest for linear sensing has passed. In other words, at such high concentrations the carbon monoxide detector will have already sounded its alarm. However, unlike prior circuits the circuits of the present invention will provide rapid sensor recovery once the very high gas concentration is no longer present. That is, once the very high gas concentration of detected gas has cleared, the sensor will rapidly return to its linear sensing operation. Since the amplifier  36  is able to source enough current during the very high detected gas concentration, the sensor does not become biased, which typically results in the slow recovery once the concentration has cleared. In the circuit of  FIG. 3 , once the voltage potential across resistor  42  increases beyond the turn on voltage of the transistors  52 ,  54 , these transistors will enter their conduction phase to allow additional current to flow therethrough without an additional voltage potential needed across the resistor  42 . This provides the increased dynamic range of the amplifier while enabling the use of the low supply voltage, e.g., a battery. 
   All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
   The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
   Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.