Abstract:
An electrochemical cell has two terminals. One of the terminals is connected to a pulse-width-modulated (PWM) power supply and to a voltmeter. The other terminal is connected to circuitry capable of switching between amperometric and potentiometric measurement modes. A sequence of successive approximations permits selection of a PWM duty cycle giving rise to a desired voltage at the terminal connected with the power supply. In this way a stable excitation voltage is supplied to the cell even in the face of supply voltage instability or drift or instability in electronics coupled with the cell.

Description:
BACKGROUND 
   It is not easy to make repeatable and accurate measurements in analytical systems such as consumer devices using an electrochemical cell. Many constraints contribute to the difficulty of this task. The consumer device must be light in weight, small and reliable. The price cannot be too high. The device may be running on a new battery or an old one, and the user cannot be relied upon to perform manual calibration steps. The repeatability and accuracy of the measurements must be preserved even in the face of temperature changes and user decisions such as whether or not to use a display backlight. 
   SUMMARY OF THE INVENTION 
   An electrochemical cell has at least two terminals. One of the terminals is connected to a pulse-width-modulated (PWM) power supply and to a voltmeter. Another of the terminals is connected to circuitry capable of switching between amperometric and potentiometric measurement modes. A sequence of successive approximations permits selection of a PWM duty cycle giving rise to a desired voltage at the terminal connected with the power supply. In this way a stable excitation voltage is supplied to the cell even in the face of supply voltage instability or drift or instability in electronics coupled with the cell. 

   
     DESCRIPTION OF THE DRAWING 
     The FIGURE shows an exemplary circuit according to the invention. 
   

   DETAILED DESCRIPTION 
   The FIGURE shows an analytical system  39  with an electrochemical cell  22 . The analysis is performed under the control of a microcontroller  21 . The microcontroller  21  has a pulse-width-modulated output  27  as well as analog inputs  28 ,  32  and  29 . The analog inputs  28 ,  32 ,  29  are (in an exemplary embodiment) connected by means of a multiplexer internal to the microcontroller  21  to an analog-to-digital converter also internal to the microcontroller  21 . PWM signal  27  controls transistors  25 ,  26  which, through filter  24 , develop a voltage at point  40  (called V 2 ) from input  290 . This voltage passes through buffer  23  to electrode  37  which, in an exemplary embodiment, is a working electrode. The voltage V 2  can be measured by the microcontroller  21  via line  28 . 
   The other electrode  38  of the cell  22  is connected by switches  33 ,  34 ,  35  to a reference voltage VREF (from input  36 ) at point  41  and to an operational amplifier  31 . The voltage at point  41  can be measured by the microcontroller via line  32 . 
   Depending on the positions of switches  33 ,  34 ,  35 , the amplifier  31  is able to serve as a voltmeter or an ammeter. When it serves as an ammeter it is measuring the current through electrode  38  and thus through the reaction cell  22 , and it gives rise to a voltage at point  42  that is indicative of the current. When it serves as a voltmeter it is measuring the voltage at electrode  38 , and this gives rise to a voltage at point  42  that is indicative of the voltage. In either case, the microcontroller  21  is able, via line  29 , to measure the voltage at point  42 . Low-pass filter  30  is provided. 
   As a first step, the microcontroller  21  measures the voltage at the counter electrode  38 . This measurement is relative to the working electrode  37 , meaning that the microcontroller  21  will need to measure the voltages on lines  28  and  29  nearly contemporaneously. 
   It will be appreciated that both of the operational amplifiers  23 ,  31  are on the same chip. Thus to a first approximation the offsets and temperature drifts for the two op amps are likely to be about the same. 
   Next the microcontroller  21  guesses at a PWM duty cycle that may give rise to a desired voltage at the working electrode  37 . (The choice of an initial duty cycle may be preconfigured in the microcontroller firmware or may be based upon past experience.) The duty cycle is applied and time is allowed to pass so that the PWM filtered voltage is stable. 
   Next the microcontroller  21  measures the voltage at  40  again. If the voltage at  40  is higher or lower than desired, then in a recursive way the PWM duty cycle is adjusted to come closer to the desired voltage at  40 . 
   This cycle may be repeated several times. 
   In the case where the apparatus is being used to analyze a bodily fluid or other analyte, this sequence takes place: 
   Before the analyte has been introduced into the cell, V 2  (the voltage at  40 ) is calibrated. The voltage V 1  (the voltage at  41 ) is monitored. 
   Next the analyte is introduced into the cell  22 . The microcontroller  21  performs the calibration again. It monitors V 2 . It monitors V 1 . The microcontroller  21  measures the output of the second op amp  31 . In this way analytical measurements are carried out with respect to the analyte in the cell  22 . 
   This sequence of events may be carried out as described in copending U.S. application Ser. No. 10/907,790, which application is hereby incorporated herein by reference for all purposes. 
   An exemplary sequence of steps will now be described in greater detail. These steps make the following assumptions. 
   The offset is assumed to be stable after the calibration sequence. 
   The offset of the two amplifiers is assumed to be the same because they are on the same chip and are under the same conditions. 
   The potential at the working electrode  37  is assumed to be the voltage at  28  plus the offset. 
   The potential at the counter electrode  38  is assumed (during sample introduction, recalibration, and amperometry) to be the same as the voltage at  32 . 
   The potential at the counter electrode  38  is assumed (during potentiometry) to be the same as the voltage at  29 , minus the offset. 
   Calibration. During a calibration phase, switch  35  is on and switches  33  and  34  are off. The PWM is adjusted so that the desired applied voltage is developed. Eventually the voltage at  40  is stable. The microcontroller also monitors any changes in the voltage at  32 , and measures the voltage at  29 . The difference between the two is the measured offset within amplifier  31 . The assumption is then made that the offset within amplifier  23  is the same or nearly the same. 
   Sample introduction. Next the system is readied for introduction of the sample in the cell  22 . Switches  33 ,  35  are on and switch  34  is off. When current flows, this is an indication that the sample has been introduced. In an exemplary embodiment the sample is human blood (or a reference solution for calibration) and the cell  22  contains a glucose oxidase. 
   Recalibration. During this phase the switches remain as previously set. The PWM is adjusted as needed to give rise to the desired applied voltage, defined as the difference between the voltages at  37  and  38 . The voltage at  28  should be stable. Changes in the voltage at  32  are monitored as this affects the applied voltage. The current through the cell  22  is measured (by noting the voltage at  29 ). 
   Amperometry. For the amperometry phase, the switches are as before. The PWM monitors the voltages at  32  and  28  to ensure that the applied voltage at the cell  22  is at the desired level. The current through the cell  22  is measured as before. 
   Potentiometry. Switches  33 ,  35  are turned off. Switch  34  is turned on. The system measures the potential difference between the working and counter electrodes  37 ,  38  (the cell voltage), by measuring the voltages at  29  and  28 . The difference between those two voltages (plus two times the offset) is the measure of the cell voltage. 
   This approach uses inexpensive components and thus helps to minimize cost. 
   Those skilled in the art will have no difficulty devising myriad obvious improvements and variations upon the embodiments of the invention without departing from the invention, all of which are intended to be encompassed by the claims which follow.