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OPA228: Op amp used in cyclic voltammetry application

Part Number: OPA228
Other Parts Discussed in Thread: TIDA-00854, OPA197, OPA2197, ADS1115

In cyclic voltammetry application (as per circuit design given in image), the voltage difference between reference electrode (RE) and working electrode (WE) will be constant.

As per REDOX process (Reduction & Oxidation), the current will flow between counter and working electrode. 

The circuit given in image, is looks open loop circuit. To maintain the constant voltage between RE & WE, the circuit must be close loop. 

We have used I to V convertor, to convert the working electrode's current into voltage. 

How do I take voltage difference between RE & WE and give back to control the current flow between CE & WE? I need support to understand how to make close loop circuit. 

Thanks,

Nirav

  • Hi Nirav,

    Below is a circuit example for a three-terminal unbiased CO sensor from the TLV6002 datasheet page 15 or OPA391 datasheet page 17.  The same circuit topology applies to other electrochemical cell applications. See below a circuit example:

    As you have mentioned, the WE and RE electrodes must be maintained at a fixed potential by adjusting the bias on CE.  The voltage at which to maintain RE is set by the non-inverting input of U1, which is labeled as VREF. The non-inverting input of the transimpedance amplifier (TIA) U2, is also biased at VREF; and therefore the TIA keeps WE biased at the same potential VREF.

    The servo feedback action maintains the RE pin at a potential set by VREF, in other words, the loop maintains the proper bias potential at RE, which adjusts the voltage on the CE to maintain the balance. 

    In addition to the TVL9002 datasheet application example, you may also refer to the TI Design TIDA-00854: "Micropower Electrochemical Gas Sensor Amplifier Reference Design" that documents the design procedure of another circuit example in more detail, where the potential between WE and RE is kept constant, controlled by VREF and VWORKING:

    Please let me know if you have additional questions.

    Thank you and Regards,

    Luis

  • Hello Luis,

    This is fine. In this case, I think, fixed reference voltage applied.

    I have searched out another method to control the current flow between working electrode and counter electrode. The potential difference between RE & WE is measured and given feedback as per given image.

    Please suggest your view.

    Thanks,

    Nirav

  • As per your suggestion, I have made full circuit, as per attachment. 

  • Hi Nirav,

    There are different topologies on how to implement a potentiostat circuit, and the simplified block diagram above on your post prior to the last makes sense.

    Questions on the last post, where you show a schematic:

    The last schematic shows a circuit where U7 is a high impedance buffer that appears to be connected to the output of U4 (transimpedance amplifier TIA).  Is the buffer U7 intended to sense to the working sense probe or the TIA output?

    Device U8 is labeled a 1x differential amplifier.  Which device is intended to implement the difference amplifier U8? Or do you plan to implement a discrete difference amplifier using an op-amp and 4 resistors?

    Thank you and Regards,

    Luis

      

  • The last schematic shows a circuit where U7 is a high impedance buffer that appears to be connected to the output of U4 (transimpedance amplifier TIA).  Is the buffer U7 intended to sense to the working sense probe or the TIA output?

    Yes, U7 intended to sense the working electrode potential to compare it with reference electrode potential. So it use for working sense.

    Device U8 is labeled a 1x differential amplifier.  Which device is intended to implement the difference amplifier U8? Or do you plan to implement a discrete difference amplifier using an op-amp and 4 resistors?

    U8 is used to measure the potential difference between reference electrode and working electrode so it is unity gain differential amplifier with 4 resistors. 

    Your suggestion is appreciated on given schematic, if any.

    Thanks,

    Regards,

    Nirav 

  • HI Nirav,

    Attached is generic application note on how to implement a simple difference amplifier using an op-amp and 4 resistors.  An amplifier like the OPA197 with 4 resistors could work in the application:  https://www.ti.com/lit/an/sboa274a/sboa274a.pdf  Another approach is to replace the two buffers and difference amplifier with a high impedance instrumentation amplifier with a Gain of 1 V/V.

    On the counter electrode amplifier circuit, you need to ensure that the complete circuit loop that forms between the REF and CTR probes forms a negative feedback loop, and apply the potentiostatic set point signal (or +/-1V control signal) on the terminal of the amplifier that is outside of the feedback loop:

    If you need assistance, please provide a clear and detailed/properly labeled schematic; and specify the current output range, the required potentiostat voltage, cell capacitance/impedance characteristics, any timing requirements, etc.

    Regards,

    Luis 

  • Hello Luis,

    I am trying to upload schematic image but it may be technical issue so I could not upload the schematic.

    Once I show you schematic then you can judge design.

  • Hi Nirav,

    Thank you. I received your message.  I am out of office through Tuesday 11/30th due to US Holiday.  I am happy to review here or via private conversation next week.

    Thank you and Regards,

    Luis

  • Hil Luis,

    Please find schematic, as per your suggestion. I have tested the circuit but I dont get the output. 

    Please review it and feedback me. 

  • HI Nirav,

    Please see figure below.

    - The control voltage +/-1V needs to be connected to the non-inverting (IN+) terminal of the control amplifier (U3). The loop that forms between the CE, RE, and the buffered difference amplifier (U8), and control amplifier (U3) has negative feedback, where the difference amplifier with G=+1 senses the voltage between RE and WE_Sense.  The output of the difference amplifier U8 connects to the inverting (IN-) terminal of control amplifier U3.  Added Ccomp capacitor on the the feedback of the control amplifier U3.

    - Buffer U7 connects to the working electrode sense probe (WE_sense), where the working sense lead is connected on the element to the working electrode lead.  The buffer output U7 connects to the negative input of the difference amplifier U8. (Notice, the WE_Sense lead is not connected to the I2V output.)

    Hope this helps,

    Thank you and Regards,

    Luis

  • Hi Luis,

    This is nice illustration. Still I want to know that why Ccomp and Rcomp connected and what is its value.

    (Notice, the WE_Sense lead is not connected to the I2V output.)

    I know, WE_Sense lead is connected to Working electrode only, not its I2V output. 

  • Hi Nirav,

    Your schematic above shows the I2V output connected to the working sense probe; this is the reason I commented the I2V output should not be connected to the working sense probe:

    The Ccomp and RComp will set the time constant of the control amplifier, and affect the loop stability.  These values need to be selected according to the the capacitive load of the cell at the control amplifier output, and the reference probe impedance in the cell since these impedances affect the stability of the circuit. You also need to consider the timing requirements on the application.  Stability analysis of the circuit is necessary according to the cell capacitance/impedance characteristics; while accounting for your bandwidth or timing requirements specific to your application. Since this application specific information is not available in the post above, I did not include the compensation component values.

    Thank you and Regards,

    Luis

  • Hi Luis,

    I have prepared circuit given in schematic for testing purpose. I have not used Rcomp and Ccomp.

    POTENTIOSTAT.TSC

    As a output, I am getting this type of chart.

    It should be look like this.

    Currently, I am using below specific cell.

    WE: Platinum Electrode

    RE: Ag/AgCl in saturated KCL

    CE: Platinum Electrode

    Solution: 0.1M Fe+2/Fe+3 / 0.1 M KCL

    We are using for CV (Cyclic Voltammetry) application. We are using scan rate 50mV/Sec. We are using OPA228P as control amplifier.

    How can we find the loop stability and Ccomp and RComp accordingly.

    Thanks & Regards,

    Nirav

  • Hello Nirav,

    As we have discussed, an estimate of the equivalent load capacitance on the CE probe, and the equivalent RE probe impedance is required to perform stability analysis, since these impedances will affect the control amplifier and the stability of the potentiostat circuit loop.

    Op-amp stability analysis is an extensive subject.  If you wish to learn about op-amp stability, you can start by reviewing the following TI Precision Lab tutorial sessions 10.1 to 10.6  below that cover some basics on op-amp stability analysis,

       https://training.ti.com/ti-precision-labs-op-amps-stability-introduction

       https://training.ti.com/ti-precision-labs-op-amps-stability-phase-margin

       https://training.ti.com/ti-precision-labs-op-amps-stability-spice-simulation

       https://training.ti.com/ti-precision-labs-op-amps-stability-measuring-system-stability

    For advanced topics on op-amp stability, you can also refer to the power-point presentation that cover op-amp stability analysis in more detail.

       https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/735210/faq-how-do-i-make-an-op-amp-circuit-stable

    In order to perform the analysis, you need to have an estimate of the equivalent load capacitance on the CE probe, and the reference electrode impedance/resistance.  You also need to define what is he highest current expected on the cell; and the capacitance at the WE probe at the transimpedance amplifier input.

    Regards,

    Luis

  • Hello Luis,

    I have already worked on below given circuit and find out below given output.

    As per above output according to given circuit, I think, there may be no any issue of stability. 

    I have tried to improve the circuit for better outcome. The circuit, with your suggestion, attached below. The output of below given circuit is also shown in attached photo. 

    2330.POTENTIOSTAT.TSC

    Chart (1).docx

    As per circuit, I think, there is still missing but I cant judge the missing part. 

    Please suggest. 

    Thanks,

    Nirav

  • HI Nirav,

    The conservative approach is to perform detailed stability analysis of the circuit using simulation.  The stability analysis allow us to ensure the circuit has enough margin to guarantee stability.  However, in order to perform this stability analysis in simulation, as we have discussed, you will need to provide a detailed equivalent circuit model of the electrodes specific to your cell that incorporates the expected resistance/capacitances seen by the circuit.  Unfortunately, without this information, I am unable to determine stability and unable to comment.

    A quick test you can perform on the hardware is to monitor the output of the control amplifier U3 with an oscilloscope probe, and check for any oscillations, and /or for excessive overshoot/undershoot when changing the control voltage.   

    Regards,

    Luis

  • Hello Luis,

    Would you think that above shown problem is related to stability of opamp??

    There is no any over-shooting or under shooting signal observed.

    I think there may be issue of design.

    Thanks,

    Nirav 

  • Hello Nirav,

    I will go ahead and compensate the circuit using another common equivalent impedance circuit model example readily available on the web, and provide you an update with a compensated circuit. Although this may not be the exact compensation for your application, this will serve as a procedure example. I will get back to you tomorrow.

    Thank you,

    Luis

  • Hello Luis,

    To maintain the constant voltage between RE & WE, the circuit must be close loop. As (+) of U4 is virtual ground. so Voltage at (-) of U4 is also zero. It means that voltage difference between RE & WE is equal to voltage at RE. So I have reduce the circuit as per image "Potentiostat1".

    Still I have get output as per image "result of potentiostat1".

    So have modified the control amplifier U3 as per circuit given in image "Potentiostat2".

  • HI Nirav,

     A similar circuit can be implemented with the OPA2197. The OPA2197 was developed in 2016 and the OPAx197 TINA model accurately models the amplifier's open-loop gain (AOL), and open-loop output impedance (Zo), which allows accurate stability analysis using simulation.

    The example circuit includes an "equivalent circuit model" to represent the electrochemical impedance of the cell.
    This electrochemical impedance model can vary greatly depending on the specific cell characteristics, and therefore, this impedance model needs to be accurate per your specific cell application. The circuit below is only provided as an example, and is compensated for the specific electrochemical cell impedance model assumed in the simulation. 

    I also adjusted the RC filter between the ADS1115 and output of OPA197 to ensure the circuit was stable, using a 1kOhm and 1nF prior driving the ADS1115.

    As I have mentioned before, I can't provide assistance on how to model your chemical cell, since this is specific to the voltammetry application, beyond the scope of  support with amplifier circuits.  Here is a link to an external application note that provides information on common equivalent circuits used to model the impedance of electrochemical cells that may be helpful.  Hope this helps and good luck with your project.

    Thank you and Regards,

    Luis

    TINA file:

    Potentiostat_OPA2197_12-20-21.TSC