Abstract:
A smoke detection sensor ion chamber has a leakage current that is dependent upon the permittivity of the ionized gas (air) in the chamber. Smoke from typical fires is mainly composed of unburned carbon that has diffused in the surrounding air and rises with the heat of the fire. The permittivity of the carbon particles is about 10 to 15 times the permittivity of clean air. The addition of the carbon particles into the air in the ion chamber changes the permittivity thereof that is large enough to detect by measuring a change in the leakage current of the ion chamber.

Description:
RELATED PATENT APPLICATION 
       [0001]    This application claims priority to commonly owned U.S. Provisional Patent Application Ser. No. 61/570,436; filed Dec. 14, 2011; entitled “Method and Apparatus for Detecting Smoke,” by Benjamin T. Cooke, Joseph Julicher and Keith Edwin Curtis; and is a Continuation-In-Part of U.S. patent application Ser. No. 13/633,686; filed Oct. 2, 2012; entitled “Differential Current Measurements to Determine Ion Current in the Presence of Leakage Current,” by Joseph Julicher, Keith Curtis and Paul N. Katz; both of which are hereby incorporated by reference herein for all purposes. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates to smoke detection devices, and more particularly, to a smoke detection device that uses a delta-sigma analog-to-digital converter for determining when smoke is present in ion chamber. 
       BACKGROUND 
       [0003]    A smoke detector generally uses an ionization chamber containing a radioactive ion source that is coupled to a high input impedance operational amplifier.  FIG. 1  shows a typical ionization chamber used in a smoke detector to produce a very small current (nA) that is reduced in the presence of smoke particles. Operational amplifiers are used to convert this current to a voltage that is then measured to determine the presence of smoke. Elevated temperatures cause increased leakage currents on the inputs of the operational amplifier in the smoke detector. This affects overall performance of the ionization chamber smoke detection function. Thus, such increases in leakage currents can pose a variety of problems such as inaccuracy, etc. which may require further compensation circuits when designing a smoke detector and therefore may increase the cost of the device. 
         [0004]    Furthermore, the impedance of the ion chamber is extremely high, and any leakage currents, e.g., printed circuit board leakage current, masks the ion chamber current. Smoke detection ion chambers therefore require a complex manufacturing process where pins of the sensing integrated circuit operational amplifier are bent and directly welded in mid-air to the ion chamber. As mentioned above, special low leakage circuits are required to detect the small current change through the ion chamber caused by the presence of smoke therein. 
       SUMMARY 
       [0005]    Therefore, a need exists for a way to detect smoke in an ion chamber of a smoke detector that does not require sensitive and expensive components nor complex manufacturing processes. 
         [0006]    According to an embodiment, a method for detecting smoke may comprise the steps of: determining a voltage on a conductive screen with a delta-sigma analog-to-digital converter, wherein the conductive screen may be located between a first ion chamber and a second ion chamber, wherein the first ion chamber may be open to smoke ingress and the second ion chamber may be closed to smoke ingress; and detecting a presence of smoke when the voltage on the conductive screen changes by a certain amount. 
         [0007]    According to a further embodiment of the method, may comprise the steps of: applying a first voltage potential to the first and second ion chambers at a first polarity; determining a first voltage on the conductive screen caused by the first voltage potential at the first polarity; applying a second voltage potential to the first and second ion chambers at a second polarity; determining a second voltage on the conductive screen caused by the second voltage potential at the second polarity; determining a voltage difference between the first and the second voltages; and detecting the presence of smoke when the voltage difference changes by a certain amount. According to a further embodiment of the method, the voltage on the conductive screen changes by the certain amount within a certain time. 
         [0008]    According to a further embodiment of the method, the step of determining the voltage on the conductive screen may comprise the steps of: comparing the voltage on the conductive screen to a reference voltage from a voltage reference with a voltage comparator; charging a capacitance coupled between the conductive screen and the voltage reference when the voltage on the conductive screen may be less than the reference voltage; discharging the capacitance coupled between the conductive screen and the voltage reference when the voltage on the conductive screen may be greater than the reference voltage; counting the number of times the capacitance may be charged during a sample time; and comparing the number of times the capacitance may be charged during the sample time to determine whether a count number of any one or more of the subsequent sample times has changed by a certain number of counts. 
         [0009]    According to a further embodiment of the method, the step of determining the voltage on the conductive screen may comprise the steps of: comparing the voltage on the conductive screen to a reference voltage from a voltage reference with a voltage comparator; charging a capacitance coupled between the conductive screen and the voltage reference when the voltage on the conductive screen may be less than the reference voltage; discharging the capacitance coupled between the conductive screen and the voltage reference when the voltage on the conductive screen may be greater than the reference voltage; counting the number of times the capacitance may be discharged during a sample time; and comparing the number of times the capacitance may be discharged during the sample time to determine whether a count number of any one or more of the subsequent sample times has changed by a certain number of counts. 
         [0010]    According to a further embodiment of the method, may comprise the step of compensating for temperature change with temperature information from a temperature sensor. According to a further embodiment of the method, may comprise the step of compensating for relative humidity change with relative humidity information from a relative humidity sensor. According to a further embodiment of the method, may comprise the step of compensating for voltage change with voltage information from a voltage sensor. 
         [0011]    According to a further embodiment of the method, may comprise the steps of: counting clock pulses from a clock generator in a first counter when the capacitor may be being charged; counting clock pulses from the clock generator in a second counter during the sample time; and comparing a first count value from the first counter of the number of clock pulses counted therein with a second count value from the second counter of the number of clock pulses counted therein; wherein if the first count value may be less then the second count value by a certain amount then generating a smoke alarm. 
         [0012]    According to a further embodiment of the method, may comprise the steps of: subtracting the first count value from the second count value to produce a difference value; and dividing the first count value by the second count value to produce a proportional count value. 
         [0013]    According to another embodiment, an apparatus for detecting smoke may comprise: an ionization chamber having a radiation source and comprising first and second chambers with a conductive screen therebetween, wherein the first chamber may be open to smoke ingress and the second chamber may be closed to smoke ingress; a voltage comparator having a first input coupled to the conductive screen and a second input coupled to a voltage reference; a capacitor coupled between the first and second inputs of the voltage comparator; a flip-flop having a D-input coupled to an output of the voltage comparator and a clock input coupled to a clock generator, wherein each time a clock signal may be received from the clock generator a logic value at the D-input may be transferred to a Q-output of the flip-flop; a feedback resistor coupled between the Q-output of the flip-flop and the first input of the voltage comparator for charging and discharging the capacitor; wherein when a voltage on the first input of the voltage comparator may be greater than a voltage from the voltage reference the output of the voltage comparator may be at a logic low and the capacitor may be discharged, and when the voltage on the first input of the voltage comparator may be less than the voltage from the voltage reference the output of the voltage comparator may be at a logic high and the capacitor may be charged; a first counter for counting a first number of clock pulses from the clock generator when the Q-output of the flip-flop may be at a logic high during a certain time period; and a second counter for counting a second number of clock pulses from the clock generator during a certain time period; wherein when the first number changes a certain amount within the certain time period a presence of smoke may be detected within the first chamber. 
         [0014]    According to a further embodiment, circuits may be provided for alternately coupling the ionization chamber to voltage potentials at first or second polarities, wherein the second polarity may be opposite the first polarity. According to a further embodiment, the circuits for alternately coupling comprises voltage multiplexers. According to a further embodiment, the ionization chamber may be coupled to a microcontroller having a digital processor and memory, and the microcontroller performs all of the aforementioned functions for detecting the presence of smoke. According to a further embodiment, an alarm circuit may be coupled to the digital processor. 
         [0015]    According to a further embodiment, a temperature sensor may be coupled to the digital processor and a temperature compensation look-up table stored in the memory coupled to the digital processor and used to compensate temperature induced changes of the capacitance of the ionization chamber. According to a further embodiment, a humidity sensor may be coupled to the digital processor and a humidity compensation look-up table stored in the memory coupled to the digital processor and used to compensate humidity induced changes of the capacitance of the ionization chamber. According to a further embodiment, a voltage sensor may be coupled to the digital processor and a voltage compensation look-up table stored in the memory coupled to the digital processor and used to compensate voltage induced changes of the capacitance of the ionization chamber. 
         [0016]    According to a further embodiment, an audible alert may be actuated by the presence of smoke in the ionization chamber. According to a further embodiment, a visual alert may be actuated by the presence of smoke in the ionization chamber. According to a further embodiment, a microcontroller having a low power sleep mode may be provided, wherein the digital processor and memory of the microcontroller go into the low power sleep mode during counting by the first and second counters. According to a further embodiment, the second counter may be a sleep wake-up timer for the digital processor and memory of the microcontroller. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    A more complete understanding of the present disclosure may be acquired by referring to the following description taken in conjunction with the accompanying drawings wherein: 
           [0018]      FIG. 1  illustrates a schematic diagram of an ion chamber having a radiation source and used as a smoke detection sensor; 
           [0019]      FIG. 1A  illustrates schematic diagrams of an ion chamber having a radiation source and showing current flows therethrough for different polarity voltage source connections thereto; 
           [0020]      FIG. 2  illustrates a schematic elevational view of a typical two chamber smoke detection sensor having a radiation source; 
           [0021]      FIG. 3  illustrates a schematic block diagram of a smoke detector using a differential delta-sigma analog-to-digital converter (ADC), according to a specific example embodiment of this disclosure; and 
           [0022]      FIG. 4  illustrates a schematic block diagram of a smoke detector using a differential delta-sigma analog-to-digital converter (ADC) and having rejection of common mode leakage current, according to another specific example embodiment of this disclosure. 
       
    
    
       [0023]    While the present disclosure is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the disclosure to the particular forms disclosed herein, but on the contrary, this disclosure is to cover all modifications and equivalents as defined by the appended claims. 
       DETAILED DESCRIPTION 
       [0024]    A radioactive source in an ion chamber causes some of the gas (e.g., air) molecules in the chamber to ionize. The results is a higher than normal permittivity of the gas due to the higher than normal number of electrically polarized (ionized) gas molecules. Therefore, when a voltage is placed across two of the ion chamber electrodes (see  FIG. 1 ) a small current will flow through the this ionized gas. When smoke enters the ion chamber, the smoke reacts with the ionized gas molecules thereby changing the permittivity, ε, thereof, and reduces the number of ionized gas molecules. This results in a lower leakage current through the ion chamber. The ion chamber current leakage will vary with temperature, relative humidity and voltage variations. But these variations are very slow to change. However, smoke causes a sudden change in the ion chamber leakage current (reduces the leakage current). Therefore according to various embodiments, a delta-sigma converter may be used to detect changes in this leakage current. 
         [0025]    By using a sigma delta analog-to-digital converter (ADC), the accuracy of analog-to-digital conversion of the leakage current through the ion chamber can be increased to a level sufficient to resolve a leakage current change generated by the presence of smoke. By using a differential technique according to various embodiments, the parasitic leakage can be subtracted from the “smoke” signal and the “smoke” signal amplitude increased by a factor of 2. The parasitic leakage currents can be subtracted from the signals present on an ion chamber by using differential techniques and a high resolution sigma delta ADC. This makes it possible to measure the presence of smoke without additional external components beyond the ion chamber. Eliminating the external circuitry and special manufacturing processes can save a smoke detector vendor a considerable amount of money. 
         [0026]    Temperature and battery voltage variations can make significant differences in the permittivity of the gas (air) with corresponding variations in the leak current of a first ion chamber. By providing a second ion chamber that is sealed from smoke entering, a comparison of the measured leakage current values of each of the first and second ion chambers can be used to compensate for these variations and provide a sensitive way of detecting smoke particles. For example, subtracting the first ion chamber leakage current value from the second ion chamber leakage current value and then dividing by the second ion chamber leakage current value, removes the temperature and battery voltage effects, leaving a resultant value with is primarily affected by the presence of smoke in the first ion chamber. 
         [0027]    Temperature, relative humidity (RH) and/or battery voltage sensors may be incorporated into a smoke detection system for determining the compensation necessary for the leakage current measurements of the ion chamber used for smoke detection. Permittivity variations due to temperature, RH and/or voltage changes generally are over a longer time period than a sudden change in the amount of contaminates (carbon particles, etc.) in the air of the ion chamber used for smoke detection. Another less sensitive way to ignore permittivity variations due to temperature, RH and/or voltage changes, would be to use an envelope detection or averaging process to ignore the slow drift of ion chamber leakage current due to voltage and/or temperature changes but recognize a more abrupt (rapid) change of the permittivity of air due to carbon particles suddenly showing up in the ion chamber. A mixed signal (analog and digital functions) microcontroller may used for leakage current measurements using a delta sigma ADC, doing the calculations necessary to determine whether smoke is present in the ion chamber, and compensate for and/or average out permittivity changes due to temperature, RH and/or battery voltage changes. 
         [0028]    Referring now to the drawing, the details of specific example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix. 
         [0029]    Referring to  FIG. 1 , depicted is a schematic diagram of an ion chamber having a radiation source and used as a smoke detection sensor. The ion chamber  102  may be characterized as two electrodes some ionized gas molecules therebetween, e.g., electrodes  104  and  106 . The gas molecules are ionized by the radiation source and when a voltage is applied between the two electrodes  104  and  106  a current will flow through the ionized gas characterized as a very high resistance, R chamber , and a resistor  108  connected in series with the electrodes  104  and  106 . This current produces a voltage across the resistor  108 . By measuring the voltage across the resistor  108 , the permittivity, ε, of the gas may be determined. Smoke in the ion chamber will cause an abrupt change in the permittivity, ε, causing an abrupt change in the current flow and voltage across the resistor  108 . This voltage is measured by a very high impedance operational amplifier (not shown) which requires complex circuitry and manufacturing processes. 
         [0030]    Referring to  FIG. 1A , depicted are schematic diagrams of an ion chamber having a radiation source and showing current flows therethrough for different polarity voltage source connections thereto. The ion chamber  102  may be characterized as three electrodes, e.g., electrodes  104 ,  106  and  210 , having some ionized gas (e.g., air) molecules therebetween. The gas molecules are ionized by a radiation source  108 . When a voltage potential  112  is applied between the two electrodes  104  and  106  at a first polarity (positive to electrode  106  and negative to electrode  104 ), a positively biased ionization electron current  116 , I chamber , will flow through the ionized gas. When the voltage potential  112  is applied between the two electrodes  104  and  106  at a second polarity (positive to electrode  104  and negative to electrode  106 ), substantially no negatively biased ionization electron current  116   a  will flow through the ionized gas since now the electrode  104  will repel the ionized gas electrons. However, leakage current  114 , I leakage , e.g., printed circuit board contaminates, grease, dust, etc., will flow irrespective of the connected polarity of the voltage potential  112 . 
         [0031]    Thus when the voltage potential  112  is connected at the first polarity across chamber  102  electrodes  104  and  106 , the total current flow through the current meter  110  is the ionized electron current  116 , I chamber , plus the leakage current  114 , I leakage . And when the voltage potential  112  is connected at the second polarity across chamber  102  electrodes  104  and  106 , the total current flow through the current meter  110  is substantially no ionized electron current  116   a  plus the leakage current  114 , I leakage , which results in substantially only the leakage current  114 , I leakage . Therefore, by subtracting the leakage current  114 , I leakage , from the total current flow, the actual ionized electron current  116 , I chamber , may be determined. This allows more sensitive measurements of any change in the ionized electron current  116 , I chamber , without these changes being masked by the undesired leakage current  114 , I leakage . It is contemplated and within the scope of this disclosure that any fluid, e.g., gas or liquid, that can be ionized by the ion source  108  will function as described hereinabove. 
         [0032]    Referring to  FIG. 2 , depicted is a schematic elevational view of a typical two chamber smoke detection sensor having a radiation source. The ion chamber  102  is comprised of two chambers  102   a  and  102   b.  The top chamber  102   a  is open to ingress of smoke therein, and the bottom chamber  102   b  is closed to smoke ingress. A conductive screen  210  is located between the two chambers  102   a  and  102   b.  The radiation source  210  proximate to or in the ion chamber  102  causes some of the gas in the chambers  102   a  and  102   b  to ionize. This ionization of the gas causes an ionization current  116 , I chamber , to increase between the two electrodes  104  and  106  of the ion chamber  102 . 
         [0033]    When smoke is present in the top chamber  102   a,  it combines with the ionized gas, neutralizing some of the ionized gas from the current path of the ionization current  116 , I chamber . As a result, when smoke is present, the permittivity of the top chamber  102   a  is smaller than it would be in the lower chamber  102   b.  But since the ionized gases of the two chambers  102   a  and  102   b  are located in series with the current flowing between the electrodes  104  and  106 , the ionization current  116 , I chamber , will be reduced. Since the conductive screen  210  is electrically floating, the top chamber  102   a  may be represented by a first resistance, Ra, and the bottom chamber  102   b  may be represented by a second resistance, Rb. When there is substantially the same number of ionized gas molecules in each of the two chambers  102   a  and  102   b,  the first resistance, Ra, will be substantially the same value as the second resistance, Rb, and the voltage drop across each ion chamber will be substantially the same. When smoke is introduced into the first chamber  102   a,  the first resistance, Ra, will be greater than the second resistance, Rb. Since the leakage current  114 , I leakage , must always be the same through both chambers  102   a  and  102   b,  there will be a first voltage between the electrode  106  and the conductive screen  210 , and a second voltage between the conductive screen  210  and the electrode  104  that will change as the permittivity of each of the chambers  102   a  and  102   b  varies, i.e., the ion chamber having the lower permittivity will have a high voltage across its respective electrode and the conductive screen  210 . Sensitivity in detecting changes between the first and second voltages may be reduced by the leakage current  114 , I leakage , since the desired current change is the change in the ionization electron current  116 , I chamber . In the configuration of (b) by comparing the voltage at terminal [b] when only the leakage current  114 , I leakage , is present from the configuration of (a) voltage at terminal [b] when both the ionized electron current  116 , I chamber , and the leakage current  114 , I leakage , are present. For example: 
         [0000]        Vb =( Rb /( Ra+Rb ))*( I   chamber   +I   leakage ) 
         [0000]        Vb =( Rb /( Ra+Rb ))*( I   chamber )+( Rb /( Ra+Rb ))*( I   leakage ) 
         [0000]        Vb ′=( Rb ′/( Ra′+Rb ′))*( I   leakage )
 
         [0000]        Vb   chamber   =Vb−Vb ′=( Rb /( Ra+Rb ))*( I   chamber )
 
         [0034]    Referring to  FIG. 3 , depicted is a schematic block diagram of a smoke detector using a differential delta-sigma analog-to-digital converter (ADC), according to a specific example embodiment of this disclosure. A smoke detector, generally represented by the numeral  300 , may comprise a smoke detection sensor ionization chamber  102 , a digital processor and memory  314 , an alarm driver  316 , an audible/visual alert  318 , a clock generator  326 , a first counter  332 , a second counter  328 , a voltage comparator  336 , a D flip-flop  334 , a feedback resistor  338 , an internal capacitor  340 , and a voltage reference  342 . All of the aforementioned elements except for the ionization chamber  102  and the audible/visual alert  318  may be provided in an integrated circuit microcontroller  330 . When the digital processor  314  determines that there is smoke present, the alarm driver  316  will actuate the audible/visual alert  318 . 
         [0035]    The ion chamber  102  electrodes  106  and  104  may be coupled to a power supply V DD  and the power supply common (e.g., V SS ) or any other voltage source that will cause a leakage current to flow between the two electrodes  106  and  104  (see  FIG. 2 ). When the permittivity of each chamber  102   a  and  102   b  are the same, the voltage at the conductive screen  210  terminal [b] will be approximately V DD /2. When the permittivity of the upper chamber  102   a  is reduced because of the presence of smoke therein, the voltage at the conductive screen  210  terminal [b] will be less than V DD /2. 
         [0036]    The comparator  336  has high impedance differential inputs and a low impedance output providing logic low “0” and logic high “1” levels. The positive input of the comparator  336  is coupled to a voltage reference  342  that provides a reference voltage of approximately V DD /2. Other reference voltages may be provided by the voltage reference  342  and are contemplated herein for all purposes. The capacitor  340  is coupled between the negative and positive inputs of the comparator  336  and will charge to the voltage difference between the reference voltage of approximately V DD /2 and the voltage at the conductive screen  210  terminal [b]. When the voltage at the negative input of comparator  336  is greater than (or equal to) the voltage at the positive input of comparator  336  the output thereof will be at a logic “0”, e.g., substantially VSS. When the voltage at the negative input of comparator  336  is less than the voltage at the positive input of comparator  336  the output thereof will be at a logic “1”, e.g., substantially V DD . 
         [0037]    The output of the comparator  336  is coupled to the D-input of the flip-flop  334  and every time a clock signal from the clock  326  is received at the clock input of the flip-flop  334  the logic level at the D-input will transfer to the Q-output of the flip-flop  334 , e.g., substantially VDD or VSS. The feedback resistor  338  is coupled between the Q-output of the flip-flop  334  and the negative input of comparator  336  which is also coupled to the top of the capacitor  340 . When the Q-output of the flip-flop  334  is at a logic “1” the capacitor  340  will charge to a higher voltage, and when Q-output of the flip-flop  334  is at a logic “0” the capacitor  340  will discharge to a lower voltage. Quiescent equilibrium will be reached when the negative and positive inputs of the comparator are at substantially the same voltages. For quiescent equilibrium during no smoke detection, with the voltage reference  342  at substantially V DD /2, the logic 1/0 outputs of the flip-flop  334  will be at substantially a 50 percent duty cycle. If the voltage reference  342  output is less than V DD /2, then the quiescent duty cycle will be less than 50 percent, and if the voltage reference  342  output is greater than V DD /2, then the quiescent duty cycle will be greater than 50 percent. During quiescent conditions in the ion chamber  102 , e.g., no smoke present in the upper chamber  102   a,  the conductive screen  210  terminal [b] will be at substantially V DD /2. Smoke in the upper chamber  102   a  will cause the voltage at the conductive screen  210  terminal [b] to be less than V DD /2 and the output of the comparator  336  be at a logic “1” (V DD ) more often than at a logic “0” until the negative and positive inputs of the comparator are at substantially the same voltages again. The comparator  336 , flip-flop  334 , feedback resistor  338  and capacitor  340  form a sigma-delta modulator. 
         [0038]    The clock inputs of the first and second counters  332  and  328  are coupled to the clock generator  326  and increment each time a clock signal is received, except for the first counter  332  which will only increment when enabled. The enable input of the first counter  332  is coupled to the Q-output of the flip-flop  334  and its count is thereby controlled to count only when the Q-output is at one or the other logic level, e.g., at a logic “1”. The maximum count values of the first and second counters  332  and  328  may be as large as necessary, e.g., 16 bits. The first and second counters  332  and  328  may also be concatenated, e.g., a plurality of first and second counters  332  and  328 . The larger the count value, the greater the resolution but also an increase in the time required for analog-to-digital conversion. By applying an appropriate clock speed, and appropriate values for the feedback resistor  338  and capacitor  340 , very high resolution may be obtained that will allow the digital processor to easily discern when there is a smoke detection event in the smoke detection ionization chamber  102 . 
         [0039]    Since the first counter  332  will only count when the Q-output of the flip-flop  334  is at one of the logics, e.g., logic “1” for the following explanation, for a reference voltage at approximately V DD /2 and no smoke in the upper chamber  102   a  the count value will be approximately half the count value of the second counter  328  which counts continuously. When there is smoke present in the upper chamber  102   a  and the voltage at the conductive screen  210  terminal [b] is less than V DD /2, the Q-output of the flip-flop  334  will be at a logic “0” more often than at a logic “1”. Therefore, the count value of the first counter  332  will be less than half of the count value of the second counter  328 , e.g., more zeros “0s” than “1s”. 
         [0040]    The digital processor  314  reads the first and second count values of the first and second counters  332  and  328 , respectively, then resets them to begin counting again. From the read first and second count values the digital processor  314  can determine when a smoke event has occurred. The digital processor may also do decimation of these count values, averaging, etc. For example, the first count value is subtracted from the second count value to produce a difference value for the top chamber  102   a  and the bottom chamber  102   b.  The value for the top chamber  102   a  is then divided by the value for the bottom chamber  102   b  to produce an output value. By dividing the two chamber values by one another, any shift due to battery voltage or temperature change is removed and the remaining value is a proportional value of the relative leakage of the two chambers  102   a  and  102   b.    
         [0041]    The smoke detector  300  may further comprise a temperature sensor  320 , a relative humidity sensor  322 , and/or a voltage sensor  324  coupled to a power supply, e.g., battery (not shown). Wherein the digital processor  314  may compensate for leakage current measurements that may change under different temperature, humidity and/or voltage conditions, e.g., using look-up tables that contain calibration and compensation data for the smoke sensor ion chamber  102 . In addition, the digital processor  314  may perform smoothing, time averaging, noise suppression, over sampling, decimation, and/or digital signal processing to enhance the leakage current change detection sensitivity and/or reduce noise pick-up. 
         [0042]    Referring to  FIG. 4 , depicted a schematic block diagram of a smoke detector using a differential delta-sigma analog-to-digital converter (ADC) and having rejection of common mode leakage current, according to another specific example embodiment of this disclosure. A smoke detector, generally represented by the numeral  400 , measures the voltage at the conductive screen  210  terminal [b] in substantially the same way as the smoke detector  300  described hereinabove. A further refinement to the operation of the smoke detector  400  is the removal of the common mode leakage current  114  that reduces the smoke detection sensitivity of the ionization chamber  102 . The smoke detector  400  further comprises multiplexers  450  and  452  that reverse the voltage polarity on the ionization chamber  102  (see  FIG. 1A ). The digital processor  314  controls the multiplexers  450  and  452 , when a first voltage at the conductive screen  210  terminal [b] is measured at a chamber first polarity, and a second voltage at the conductive screen  210  terminal [b] is measured at a chamber second polarity, wherein the chamber second polarity is opposite the chamber first polarity. These voltage measurements are stored in the memory of the digital processor  314  for further processing to enhance the voltage change sensitivity and thereby increased the detection sensitivity of the smoke detector  400 . 
         [0043]    Also a further enhancement to more reliable smoke detection is to require that the change in leakage current occurs in less than or equal to a certain time period so as to reject slow leakage current change due to changes in temperature, relative humidity and/or supply voltage (e.g., battery not shown). 
         [0044]    It is contemplated and within the scope of this disclosure that the digital processor and memory  314  may go into a low power sleep mode while the first and second counters  332  and  328  are counting, and only wake up to read the count values therefrom and do appropriate calculations in determining whether there is smoke in the first chamber  102   a.  All other functions and circuits described hereinabove remain in an active mode but are all very low power. Also the second counter  328  may be a wake-up timer inherent with a low power, standby sleep mode function in a microcontroller. This sleep mode further increase battery life of the smoke detector  300 . 
         [0045]    While embodiments of this disclosure have been depicted, described, and are defined by reference to example embodiments of the disclosure, such references do not imply a limitation on the disclosure, and no such limitation is to be inferred. The subject matter disclosed is capable of considerable modification, alteration, and equivalents in form and function, as will occur to those ordinarily skilled in the pertinent art and having the benefit of this disclosure. The depicted and described embodiments of this disclosure are examples only, and are not exhaustive of the scope of the disclosure.