OPA703: Potentiostat Clarification needed

Hi HARI, 

HARI T O1 said:

May I know how this circuit maintains the Potential at Reference Electrode and Working Electrode same.

I am enclosing the potentiostat application note for your reference. 

https://www.ti.com/lit/ta/sszt075/sszt075.pdf?ts=1746545157823&ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252FLMP91000

HARI T O1 said:

Then how this circuit maintains same potential at working electrode and reference electrode.

Inside of the 3-electrode system, it has ionic electrolyte that is conducting ions or current through the electrode and liquid interface. The reference electrode is placed very close to the Working electrode and reference potential between Working and Reference is maintained through the ionic conduction inside of the 3-electrode cell. 

Please take a look at video clips over youtube.com, here I listed one. There are many video clips on the topics. This involves electrochemistry, electronics where redox reaction is taken place inside of the 3-electrode system. I am not going to explain how it works over here.  

https://www.youtube.com/watch?v=pzB122dTij8

Regarding op amps, OPA391 or LMP7721 or several other low Ib current op amps are the best options for the potentiostat application. 

https://www.ti.com/lit/ug/snou004/snou004.pdf

If you have other questions, please let me know. 

Best,

Raymond

Hi Raymond,

Thank you very much.

I am interfacing this Chlorine Sensor from Alphasense . This is a zero bias sensor.

I am following the refrence design provided by Alphasense with minimum changes.In the reference design Alphasense suggest to use dual supply for opamp.But I will be using a single supply of 3.3V

Below is the circuit from the reference design.

My modified circuit is given below.The supply of the system is 3.3V.The only change I made is ,you can see that in the reference design non inverting input of the both opamps are grounded.In my design I provided a voltage of Vdd/2 at the non inverting input of both the opamps.May I know the below circuit is fine or not.

Hi Raymond,

Thank you very much.

may I know is it possible to implement this circuit in TINA-TI.

I know I can implement opamp sections.

Any way to model the above sensor.

Regards

HARI

Hi HARI, 

Yes, it can be done. If you put together the general schematic in Tina-TI, I will review it. Please select an op amp for your need. OPA391 is reasonably priced and it works well for the sensing application. You may use others, but you need to aware the pros and cons when selecting op amps.  

The output of Cl2 sensor is in current, not in voltage. So the output stage needs to be configured as TIA via op amp. Please let me know what is the Vbias voltage you are using the the Cl2 sensor. Since the atmosphere does not present in Cl2 gas, the output stage can only go up as the concentration of Cl2 gas is increased. The Vbias voltage at TIA is the 0ppm in Cl2 gas.  And you will plot output voltage vs. Iin, which is the calibration plot. Please make sure that this is repeatable over temperature - that is why the T sensor measurement is very important (That is the major error source, if T is not accurate). 

It is possible that the sensor's output has compensated with Temperature (check with manufacture), where you do not need to use Nernst equation (the cell has done or linearized for you and you have paid for the feature). 

Best,

Raymond

Hi Raymond,

I modeled the CL2 sensor for my simulation.Please see the circuit diagram below.I am also attaching the simulation file also.

May I know your thoughts.

CL2_POTSTAT.TSC

Regards

HARI 

Hi HARI, 

From the working electrode, you would need this TIA circuit. I did not optimize this circuit since I do not know the capacitance of the Cl2 cell at WE (working) electrode.

Since Cl2 gas's output is a current source, you need to implement TIA circuit. Other sensors from the manufacture likely outputs in voltage. 

BTW, My understanding is based on the datasheet. It is possible that the Cl2 sensor is converted from current to voltage at cell internally. If that is the case, your voltage gain circuit is correct.  However, you need to lower your Vbias voltage to 200mV range, otherwise output voltage will be saturated, and you only have 1.65V useful range for the 3.3V rail, if you biased to 1.65V. 

Cl2 TIA sensor.TSC

At CE (counter) electrode, I do not believe that you can apply 1.65V to the electrode. It is a bit high for such small cell, and you need to check with the manufacturer. The datasheet said that is should be +/-150mV or 300mV. And I think that 300mV is more reasonable for single supply rail. Otherwise, you may damage the gas cell.  

The Ag/AgCl reference is only in 0.2V range, and the non-inverting input is tied to GND in dual supply rails. 

If you have other questions, please let me know. 

Best,

Raymond 

Raymond Zhang1 said:

However, you need to lower your Vbias voltage to 200mV range, otherwise output voltage will be saturated, and you only have 1.65V useful range for the 3.3V rail, if you biased to 1.65V. 

May I know You mean the output will go more than 3.3V?.

I did the calculation as shown below.Assuming negative current.

The current source is -6uA for 10 ppm.The TIA resistor is 22k.

Vout = 1.65 - (22k*6uA)
= 1.52V

The voltage developed in the TIA resistor is always substracted from 1.65V.

Raymond Zhang1 said:

The datasheet said that is should be +/-150mV or 300mV. And I think that 300mV is more reasonable for single supply rail. Otherwise, you may damage the gas cell

I failed to see this information in the datasheet.If you don't mind could you please tell me where you find this information

I chcked with Alphasense support.They told zero bias and also the person told me that he is not from electronics background!!!

Regards

HARI

Hi HARI, 

My interpretation of the specification is that the current is flowing into the WE as I simulated. GND node here is connected to WE. Please ask the manufacturer to verify this. 

The Cl2 gas is reduced at WE, meaning electrons are flowing out of WE, and current by convention is flowing into the WE. So I think that the negative current is specified by this convention. So the TIA voltage should increase as [CL2] concentration is increased. 

I am not sue if the cell is sourcing voltage at output. If the sensor outputs voltage, then the following circuit is correct.  But I think that this is bare 3-electrochemical cell. 

BTW, 30ppm-50ppm Cl2 gas is fatal to human. So I think that the gas cell can only sense up to 10ppm in Cl2 concentration. Again, check with the supplier, because I do not believe that it can do -6uA in current (that is a lot of ppm Cl2 gas). 

HARI T O1 said:

The voltage developed in the TIA resistor is always substracted from 1.65V.

The output voltage depends on how the current is flowing through the feedback resistor. Check the current direction at the WE. If it is flowing opposite from that I simulated, then your calculation is correct.

Best,

Raymond

Hi Raymond,

Thank you very much.

May I know how you designed C2(100p).The compensation capacitor

Regards

HARI

Hi HARI, 

The electrochemical reaction is slow, the time constant is likely up to 10-60 seconds. So TIA circuit's BW is limited to 1/(2*pi*2.2MHz*100pF) = 723 Hz. 

I do not have information about the input capacitance of the current source. If it is in a few pF or slightly more, the circuit is likely ok. In any case, I need that piece of input capacitance information from you or supplier - to verify the TIA loop stability. 

Best,

Raymond