Abstract:
A small, portable, and inexpensive potentiostat circuit that is suitable for wide-spread electrochemical analysis is disclosed. The potentiostat may be fabricated as a stand-alone electrical component or it may be fabricated in conjunction with a Programmable System-on-Chip (SoC) to facilitate on-the-fly calibration and configuration.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application No. 61/441,737 filed on Feb. 11, 2011, and entitled “Digital Potentiostat Circuit and System” which is incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The present document relates to a circuit and system for electrochemical analysis, and in particular to an inexpensive digital potentiostat that provides precision and accuracy in electrochemical analysis. 
     BACKGROUND 
     Potentiostats are common tools used in electrochemical analysis. Currently, a number of companies manufacture potentiostat instruments that deliver high precision and accuracy at an equally high cost. Typically, these instruments and their accompanying software cost anywhere from $5,000 to over $20,000. As a result, these instruments are not easily accessible for wide-spread use in academic settings or other initial research studies. Although there are a number of low-cost potentiostats, such potentiostats can only deliver a low-performance capability. As such, there is a need for an easily manufactured potentiostat circuit that is inexpensive while still providing high precision and accuracy. 
     SUMMARY 
     According to one aspect, an inexpensive potentiostat circuit is provided for performing electrochemical analysis. The potentiostat circuit includes a counter electrode, a working electrode, and a reference electrode. The circuit also includes a voltage source to supply an input voltage signal and a voltage feedback component to detect a reference voltage level at the reference electrode and a working voltage level at the working electrode. The voltage feedback component also generates a feedback voltage signal based on the reference voltage and working electrode levels. A voltage inverting component supplies a working voltage signal to the working electrode. In addition, a control amplifier receives the input voltage signal and the feedback voltage signal, generates a control voltage signal based on the input voltage signal and the feedback voltage signal, and supplies the control voltage signal to the counter electrode. A current measurement component measures a current level at the working electrode and generates an output measurement signal proportional to the current level measured at the working electrode. The output measurement signal indicates a change in the working voltage level, and therefore indicates an electrochemical property of a solution in contact with the counter electrode, the working electrode, and the reference electrode. The circuit also includes an output device to receive the output measurement signal which is used to determine the electrochemical properties of the solution. 
     According to another aspect, a potentiostat circuit includes an electrochemical cell having a counter electrode, a working electrode, and a reference electrode. The circuit also includes a digital-to-analog converter connected to a non-inverting control amplifier input of a control amplifier and an inverting input of an inverting amplifier. The control amplifier includes the non-inverting control amplifier input, an inverting control amplifier input, and a control amplifier output, wherein the control amplifier output provides a voltage to the counter electrode to maintain a specific voltage difference between the reference electrode and the working electrode. 
     A voltage feedback system includes a first instrumentation amplifier having a first input, a second input, and a voltage feedback output. The first instrumentation amplifier is connected to an offset ground and the voltage feedback output is connected to the inverting input of the control amplifier and an analog-to-digital converter. The voltage feedback system removes a common-mode voltage of the reference electrode and the working electrode. 
     The inverting amplifier includes a number of resistors, a non-inverting input, an inverting input, and an output. The non-inverting input is connected to the offset analog ground and the inverting output is connected to the digital-to-analog converter through at least one of the resistors. The output of the inverting amplifier is connected to the working electrode and the inverting input through another of the resistors. 
     The circuit also includes a current measurement system having a measurement resistor and a second instrumentation amplifier. The measurement resistor is connected to the working electrode in series with the inverting operation amplifier and the second instrumentation amplifier measures a voltage difference across the measurement resistor. The output of the second instrumentation amplifier is connected to the analog-to-digital converter. 
     In yet another aspect, a method of using a potentiostat circuit includes generating a first voltage at a digital-to-analog converter. The first voltage is provided to a first input at an operational amplifier to amplify the difference between the first voltage and a feedback voltage. The first voltage is also provided as another input to an inverting operational amplifier to generate an inverted voltage. The amplified difference between the first voltage and a feedback voltage is provided to a counter electrode of a three-electrode cell, while the inverted voltage is provided to a working electrode of the three-electrode cell. A reference electrode voltage and a working electrode voltage are received from the three-electrode cell at a first instrumentation amplifier. 
     The feedback voltage is generated at the first instrumentation amplifier and the feedback voltage is provided to a second input at the operational amplifier which then provides the feedback voltage to an analog-to-digital converter. A voltage difference is measured across a resistor in series with the working electrode. The measured voltage difference is provided to a second instrumentation amplifier to determine a current received at the working electrode. A voltage signal proportional to the determined current is then provided to the analog-to-digital converter. The analog-to-digital converter generates an output measurement signal that is provided to an output device that generates output measurement data for displaying, storing, and/or printing. Electrochemical properties of a solution within the three-electrode cell are then determined based upon the displayed output measurement signal. 
     In other aspects, the potentiostat circuit and method are implemented in conjunction with a system-on-chip or a programmable system-on-chip, such as the Programmable System-on-Chip (PSoC®) by Cypress MicroSystems, Inc. 
     Additional objectives, advantages, and novel features will be set forth in the description which follows or will become apparent to those skilled in the art upon examination of the drawings and detailed description which follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a potentiostat circuit; 
         FIG. 2  is a circuit diagram of a potentiostat circuit; 
         FIG. 3  is a block diagram of a potentiostat system including the potentiostat circuit shown in  FIG. 1 ; 
         FIG. 4  is another circuit diagram of a potentiostat circuit; and 
         FIG. 5  is a flowchart of a method to use the potentiostat circuit to conduct electrochemical analysis. 
     
    
    
     Corresponding reference characters indicate corresponding elements among the view of the drawings. The headings used in the figures should not be interpreted to limit the scope of the claims. 
     DETAILED DESCRIPTION 
     Referring to the drawings, an embodiment of a potentiostat circuit is illustrated and generally indicated as  100  in  FIGS. 1-3 .  FIG. 1  is a block diagram of an embodiment of a potentiostat circuit  100  that may be used to conduct an electrochemical analysis of a solution with a three-electrode cell  102 . The three-electrode cell  102  includes a counter electrode  102 A, a reference electrode  102 B, and a working electrode  102 C. 
     The potentiostat circuit  100  includes a voltage generator  104  that provides analog voltages to an input of an inverting amplifier  106  and a control amplifier  108 . In one embodiment, the voltage generator  104  includes a digital-to-analog converter (DAC) that provides an analog signal in response to a digital input. 
     The inverting amplifier  106  inverts the analog voltage received from the voltage generator  104  and provides the inverted voltage to the working electrode  102 C. Another input of the inverting amplifier  106  is connected to an offset ground  112  and is held at a voltage level that is offset from ground (i.e., offset ground)  112 . The offset ground  112  is an artificial ground provided by a voltage source. In one embodiment, the offset ground  112  may be provided as another voltage by the voltage generator  104 . 
     According to one aspect, the offset ground provided to the inverting amplifier  106  is equal to one-half of a nominal power supply voltage of approximately 5V. For example, if the nominal power supply is approximately 5V, the offset ground is approximately 2.5V. In various embodiments, the nominal power supply voltage may be a nominal power supply voltage range, such as between about 4.75V and 5.25V. As the nominal 2.5V reference voltage is present at all relevant points in the system, the inverting amplifier  106  and other instrumentation amplifiers treat this as a common-mode voltage to be subtracted from the gathered measurements and/or control signals. 
     As explained in more detail below, according to another aspect, the potentiostat circuit  100  includes a System-on-Chip (SoC) system. In this aspect, a power supply, similar to the voltage generator  104 , supplies the nominal supply voltage of approximately 5V to the SoC system and the SoC system generates the analog ground voltage of approximately 2.5V. As explained above, the analog 2.5V is present at all relevant points in the system and is treated as a common-mode voltage by the inverting amplifier  106  and other instrumentation amplifiers. The SoC system also produces a tightly regulated and temperature-independent reference voltage of approximately 1.3V that is used to operate analog-to-digital converters and the digital-to analog converters included in the potentiostat circuit  100 . This temperature-independent reference voltage is also referred to as an internal bandgap reference. Accordingly, the analog voltage generated by the SoC system is constrained to operate at a range of 2.5 volts±1.3 volts, or a total range from 1.2 to 3.8 volts. 
     According to one aspect, the inverting amplifier  106  includes two resistors, such as an input resistor and a feedback resistor. In one example, the input resistor and the feedback resistor have resistances of 10K and 15K, respectively. The ratio of the feedback resistor to the input resistor is 1.5; therefore, the output voltage of the inverting amplifier  106  is −1.5 times the voltage applied to the control amplifier  108 . As such, the inverting amplifier  106  expands the range and sensitivity of the potentiostat circuit  100 . By driving the voltage for the working electrode −1.5 times the voltage supplied by the control amplifier  108 , range of the voltages applied and detectable by the potentiostat circuit  100  is increased by approximately 2 volts. Therefore, the voltage range between the counter electrode  102 A and the working electrode  102 C is now ±4.5 volts. This expanded range is achieved without the need for additional circuitry. 
     The control amplifier  108  is connected to the counter electrode  102 A and drives the voltage of the counter electrode  102 A until the reference electrode  102 B and the working electrode  102 C are held at desired voltages. In one embodiment, the counter electrode  102 A is driven until the difference between the voltages at the reference electrode  102 B and the working electrode  102 C is equal to the voltage provided by the voltage generator  104 . 
     The control amplifier  108  also receives a voltage feedback signal from a voltage feedback system  110 . The voltage feedback system  110  is connected to the reference electrode  102 B and the working electrode  102 C of the electrochemical cell  102 . Accordingly, the control amplifier  108  provides a voltage to the counter electrode  102 A that is related to the difference of the voltages between reference electrode  102 B and the working electrodes  102 C. As such, the voltage provided by the control amplifier  108 , may be any voltage that achieves the desired difference between the voltage at the reference electrode  102 B and the voltage at working electrode  102 C. 
     In one embodiment, the voltage feedback system  110  is an instrumentation amplifier configured to determine the voltage levels of the reference electrode  102 B and the working electrode  102 C and generate an output that removes the common voltage shared by the electrodes  102 B-C. Therefore, the output from the voltage feedback system  110  is equal to V reference −V working  (V diff ). To insure that this output is positive, the voltage feedback system  110  is also connected to an offset ground  112 . In this embodiment, the offset ground  112  is the analog offset ground. V diff  may be positive or negative and have maximum magnitude of 1.3V. Therefore, V diff  ranges from about −1.3 to 1.3. The absolute voltages seen by the voltage feedback system  110 , however, are all relative to the nominal 2.5V offset ground  112 . For example, if V diff =−1.3, the voltage feedback system  110  sees 1.2V (2.5V−1.3V=1.2V) Similarly, if V diff =1.3, the voltage feedback system  110  3.8V (2.5V+1.3V=3.8V). Since the analog portion of the potentiostat circuit  100  system is designed around the offset ground  112 , when the analog portion measures an absolute voltage of 2.5V, it interprets this voltage to be at the offset ground, i.e. 0 volts relative to the measurement being taken. Similarly, a digital-to-analog converter will produce an absolute voltage of 2.5V (analog ground) when it is instructed to generate an output of 0 volts relative to the measurement being taken. 
     As described above, the voltage feedback signal is representative of the difference between the voltage of the reference electrode  102 B and the working electrode  102 C and is provided to the control amplifier  108 . In order to keep the analog voltage from the voltage generator  104  and the voltage feedback signal as close as possible, the control amplifier  108  controls the voltage level provided to the counter electrode  102 A, such that the difference between the voltages of the reference electrode  102 B and the working electrode  102 C (V reference −V working ) equals the voltage generated by the voltage generator  104 . 
     The voltage feedback signal is also provided to an analog-to-digital convertor (ADC)  114 , where an output measurement signal, as indicated by arrow  115 , is generated for subsequent analysis by an output device  116 . The ADC  114  has one or more inputs to receive signals from the other components of the potentiostat circuit  100 . In yet another embodiment, the functionality of the ADC  114  may embodied by two or more separate but synchronized ADC components that sample the signals at the same time such that they can synchronize the voltages, signals, currents and or other data received at that same instant. The 2.5V reference voltage is used by both the analog-to-digital converters, (e.g., ADC 114 ) and digital-to-analog converters (DAC), such as a DAC that may operate as the voltage generator  104 , to make both measurements and the generated voltages absolute even though the common-mode voltage may vary. The inverting amplifier  106  and the control amplifier  108 , however, are not constrained by the 2.5 volts±1.3 volts operating range. As such, the differential voltage range between the counter electrode  102 A and the working electrode  102 C is approximately +/−4.5V. The 0.5V difference between the nominal power supply voltage and the differential voltage range is provided as headroom for the components of the potentiostat circuit  100  to keep them operating in their linear region. 
     The output device  116  receives the output measurement signal from the ADC  114  and may display and/or record the signal. In various embodiments, the output device  116  is an oscilloscope, a microprocessor, or any other data acquisition device, including audio, graphical, and text-based devices. In one embodiment, the output device is in communication with and controls the voltage generator  104  to synchronize the generation and sampling of the voltages and signals. 
     The current measurement system  118  measures the current that is supplied to the working electrode  102 C. As the voltage received at the working electrode is variable, the current is measured by determining the voltage differences across a resistor that is placed in series with the working electrode  102 C. 
     According to one aspect, the current measurement system  118  includes an instrumentation amplifier to determine the voltage difference across the resistor. From this voltage difference, the current flowing to the working electrode  102 C can be determined as a function of the voltage difference. The voltage difference as determined by the current measurement system  118  is also sent to the ADC  114 . In one embodiment, the output measurement signal may be sent to another ADC  114  that is synchronized with the ADC  114 . 
       FIG. 2  is an exemplary circuit diagram of the potentiostat circuit  100  illustrated in  FIG. 1 . In this embodiment, the control amplifier  108  includes an operational amplifier (op-amp)  200  that is connected to the counter electrode  102 A through a controllable switch  202 . The switch  202  prevents the control amplifier  108  from driving counter electrode  102 A when measurements are not being taken. 
     In one embodiment, the switch  202  allows a user of the potentiostat circuit  100  to monitor the voltage between the reference electrode  102 B and the working electrode  102 C. The switch  202  may be controlled manually or automatically to measure the intrinsic voltage of a chemical solution in the electrochemical cell and to perform electrochemical titration experiments. 
     The op-amp  200  receives an analog voltage at the non-inverting input from the voltage generator  104  and receives another voltage at the inverting input from the voltage feedback system  110 . 
     The voltage feedback system  110  includes an instrumentation amplifier  204  that is connected to the offset ground  112 A. The instrumentation amplifier  204  receives a voltage at the non-inverting input from the reference electrode  102 B and another voltage at an inverting input from the working electrode  102 C. As described above, the instrumentation amplifier  204  provides a voltage feedback signal that represents the difference between the voltages of the reference electrode  102 B and the working electrode  102 C. When the voltages of the reference electrode  102 B and the working electrode  102 C are equal or the difference (V diff ) is negative, the magnitude of the voltage feedback signal may be equal to the offset ground  112 A or another value relative to the nominal 2.5V offset ground. The voltage feedback system  110  also provides the voltage feedback signal to the ADC  114 . 
     The voltage generated by the voltage generator  104  is also provided to the working electrode  102 C; however, this voltage is linearly inverted with respect to the voltage at the counter electrode  102 A. The inversion is provided by the inverting amplifier  106 . The inverting amplifier  106  includes an op-amp  206  where the non-inverting input is connected to the offset ground  112 B. The inverting input receives an analog voltage from the voltage generator  104  in series with an input resistor  208 , and a feedback voltage from the op-amp  206  in series with a feedback resistor  210 . The voltage output from the op-amp  206  is equal to the voltage from the voltage generator  104  times the ratio of the feedback resistor  210 /input resistor  208 . 
     In order to analyze the electrochemical properties of a solution in the three-electrode cell  102 , the potentiostat circuit  100  determines the current flowing to the working electrode  102 C by measuring the voltage drop across a resistor  212  at the current measurement system  118 . The current measurement system  118  includes an instrumentation amplifier  214 , connected to the offset ground  112 C, that measures the voltage difference across the resistor  212  connected in series with the working electrode  102 C. The current measurement system  118  also generates an output measurement signal and provides it to the ADC  114 , where it is further provided to the output device  116 . 
     In various embodiments, the potentiostat circuit  100  also includes a triggering signal input component (not shown) and a triggering signal output component (not shown). The triggering signal input component receives an externally generated signal to initiate the measurement gathering by the potentiostat circuit  100 , such that the measurements may be synchronized with an external event. Similarly, the triggering signal output component generates a triggering signal to initiate an action at an external device to coincide with measurement gathering by the potentiostat circuit  100 . In one embodiment, both triggering signal components are combined into a single component. 
       FIG. 3  is a block diagram of the potentiostat system  300  illustrating another embodiment of a potentiostat circuit  302 . In this embodiment, the potentiostat circuit  302  is formed on a system-on-chip system (SoC)  304 , such as the Programmable System-on-Chip (PSoC®) by Cypress MicroSystems, Inc. For example, the SoC  304  can be programmed to perform the functions of the various circuit components shown in  FIGS. 1 and 2 . In another embodiment, the potentiostat circuit  302  may be configured on a printed circuit board that is in communication with the SoC  304 . The SoC  304  provides flexibility in both calibrating and configuring the potentiostat circuit  302 . 
     The SoC  304  includes a mixed-signal microprocessor  306  configured to generate and receive both digital and analog signals, manage data acquisition and control the potentiostat functionality. The microprocessor  306  may be an 8-bit Microcontroller Unit (MCU) processor) and voltage source. In one embodiment, the microprocessor  306  may incorporate the functionalities of the voltage generator  104 , the offset ground  112 , and/or the ADC  114  (See  FIG. 1 ). In this embodiment, the potentiostat circuit  302  includes a digital-to-analog convertor (DAC)  308  that converts a digital signal from the microprocessor  306  into an analog signal. The SoC  304  may also include an interface (e.g., RS-232, universal serial bus (USB), or other computer interface) configured to provide communication with a host computer (not shown) through which a user may configure the circuitry of the potentiostat circuit  302  on the SoC  304 , define a protocol for an electrochemical study, and/or monitor and analyze the results of the study. The host computer may further include a graphical user interface (GUI) that enables a user to modify the structure and functionality of the potentiostat circuit as configured on the SoC  304 . 
     In one embodiment, the microprocessor  306  is a commercially available microcontroller chip that generates voltages and signals sent to the DAC  308 , generate the offset ground  112 , and receive the output of ADC  114 . The microprocessor  306  may also include one or more counters for synchronizing and controlling the integration time of the DAC  308  and ADC  114  to minimize the on-chip digital circuitry, thereby reducing errors and signal noise in the analog blocks. 
       FIG. 4  illustrates a circuit diagram of one embodiment of a potentiostat circuit  400 . The potentiostat circuit  400  includes a stabilizing circuit  402  that prevents or dampens oscillations when the potentiostat circuit  400  is driving the cell  102  during a measurements phase. For example, the stabilizing circuit is configured to sense a voltage signal flowing through the control amplifier  108  and to provide a stable voltage feedback signal from the voltage feedback system  110  to the control amplifier. In one embodiment, the stabilizing circuit  402  stabilizes the differential output voltages from the control amplifier  108  and the voltage feedback system  110 . 
     The potentiostat circuit  400  also includes a switch array  404  within the current measurement system  118 . In one embodiment, the switch array  404  includes a bank  406  of digitally controlled analog switches to select the most appropriate resistor to sense the current flowing into the working electrode  102 C. This allows the potentiostat circuit  400  to sense a wide range of currents and have the capability to automatically select the best resistor (i.e. auto-ranging). The switches of the bank  406  may be selectively closed in order to identify the voltage drop across the switch array  404  and therefore the current flowing to the working electrode  102 C. In this embodiment, each of the switches includes a resistor of a different resistance to provide greater precision and accuracy. The switch array  404  and the switch  202  may be controlled by microprocessor  306 . 
     In another embodiment, the components and functionality of the offset ground  112 , the ADC  114 , and the DAC  308 , shown in  FIGS. 3 and 4 , are incorporated into the microprocessor  306 . In yet another embodiment, the components and functionality of the voltage feedback system  110 , the offset ground  112 , the ADC  114 , the current measurement system  118 , and the DAC  308  are incorporated into the microprocessor  306 . In these embodiments, the microprocessor  306  includes a number of input/output (I/O) pins or ports that allow the microprocessor to generate and receive digital and analog signals. In various other embodiments, the functionality of components that may be internal to the microprocessor  306 , such as the offset ground  112  and the ADC  114 , may supplemented by an external component. 
       FIG. 5  is a flowchart illustrating a method  500  of using the potentiostat circuit to conduct an electrochemical study. At  501 , a three-electrode cell  102  of a potentiostat circuit is placed in a solution which contains a chemical selected to undergo electrochemical property analysis. As described above, the three-electrode cell  102  includes, for example, a counter electrode  102 A, a reference electrode  102 B, and a working electrode  102 C. At  502 , an input voltage signal is generated. In one embodiment, the input signal is generated by the voltage generator  104  (See  FIGS. 1-2 ). In another embodiment, the input voltage is generated by the microprocessor  306  (See  FIG. 3 ). In one embodiment, the input signal is generated as a digital signal that is converted to an analog signal by the DAC  308 . In yet another embodiment, the input signal may be an analog signal generated by the microprocessor  306 , thereby eliminating a need for the DAC  308 . 
     At  504 , the control amplifier  108  amplifies the difference between the input voltage signal and a feedback voltage. The amplified difference is provided to the counter electrode  102 A of the three-electrode cell  102 , at  506 . At  508 , the input voltage signal is also inverted by the inverting amplifier  106 . The inverted voltage signal is provided to the working electrode  102 C at  510 . In one embodiment, the amplifications at  504  and  508  occur simultaneously. 
     A reference electrode voltage signal and the inverted voltage signal are received at the voltage feedback system  110 , at  512 . A feedback voltage signal is generated by the voltage feedback system  110 , at  514  based upon the signals received at  512 . At  516 , the feedback voltage signal is provided as a second input to the control amplifier  108  and provided to the ADC  114 . 
     The current measurement system  118  measures the voltage difference across the resistor  212  in series with the working electrode  102 C and generates an output measurement signal, at  518 , based on the current received at the working electrode  102 C. At  520 , the output measurement signal is received at the ADC  114 . At  522 , the output measurement signal is provided to the output device  116  where it is analyzed to determine the electrochemical properties of a solution within the three-electrode cell  102 . 
     In one embodiment, the method  500  may further include providing a user of the potentiostat circuit  100  or a device containing the circuit, an output signal that identifies and/or quantifies an analog cell voltage and analog cell current. This analog output signal may be represented as a voltage proportional to the cell current and allows the user to view the raw analog output without it being sampled by the ADC  114 . 
     In another embodiment, the method  500  includes receiving a trigger-in signal from the user or an external device. The trigger-in signal can be used to initiate the measurement gathering by the potentiostat circuit  100 , such that the measurements may be synchronized with an external event. The trigger-in signal is received at an input of the microprocessor  306 . 
     In yet another embodiment, the method  500  includes generating a trigger-out signal. The trigger-out signal is used to initiate an action at an external device to coincide with measurement gathering by the potentiostat circuit  100 . The trigger-out signal is generated by the microprocessor  306 . 
     It should be understood from the foregoing that, while particular embodiments have been illustrated and described, various modifications can be made thereto without departing from the spirit and scope of the invention as will be made apparent to those skilled in the art. Such changes and modifications are within the scope and teachings of this invention as defined in the claims appended hereto.