Using the AD5940 or ADuCM355 with 4-lead electrochemical gas sensors (reference, counter, sensing and aux electrodes)

Hello everyone,

I have read most of what I could find about the AD5940 or ADuCM355, including datasheets, other questions from this forum and the GitHub page of the library for this parts, but still couldn't fully understand this:

Can this parts be used to interface with one 4-lead electrochemical amperometric gas sensor with reference, counter, sensing (working) and aux electrodes?

As an example, the Alphasensens B4 line of sensors for CO, H2S, SO2, etc all have an auxiliary 4th electrode that shares the same RE and CE of the sensing electrode. The aux electrode is shielded from the target gas, and is only affected by ambient conditions such as temperature. So the purpose of the aux electrode is to help correct the influence of temperature in the sensing electrodes reading. This is an example of the circuit used for those sensors:

From what I could understand so far, the only way of using AD5940 in this configuration would be to use the low power TIA with the sensing electrode and use the high speed TIA with the aux electrode connected to the DE pin. Is that correct? With the HSTIA I wouldn't have the capacitor to help filter the noise from the sensor, right? Would it be possible to use the LPTIA for both aux and sensing? Would I have more options if I used ADuCM355 (one more channel) instead? 

Thank you in advance!

  • +1
    •  Analog Employees 
    on Apr 25, 2021 3:39 PM

    In AD5940:

     Capacitor to filter the noise can be selected from HSRTIACON[12:5]. To use external RTIA and CTIA with HSTIA, refer to Page 48 of AD5940 datasheet (External RTIA selection).

    In ADuCM355:

    Two channels are available. Both the LPTIAs can be used, one for SE0, another for the aux electrode, giving the same voltages to CE0 and CE1.

  • Hi Akila, thank you very much for your answer!

    I've read the AD5940 datasheet section about the HSTIA and understood the following:

    1 - The maximum internal capacitance selectable via HSRTIACON[12:5] is 16 pF. So if i need more than that, the only way to do it is doing what page 48 of the datasheet suggests: also using an external RTIA. I can't use the programmable internal RTIA and an external CTIA at the same time, is that correct?

    2- If I use the configuration suggested in page 48 of the datasheet, the circuit would look something like this:

    So I would need to connect the auxiliary electrode to the AIN1 pin, and also use an external RLOAD resistor if I need. Is that correct?

  • +1
    •  Analog Employees 
    on Apr 26, 2021 6:35 PM in reply to vsantolim


    1- Yes. Programmable internal RTIA and an external CTIA can't be used at the same time.

    2- yes. The circuit drawn is correct. Note that while measuring Aux electrode, SE0 must be disconnected from HSTIA negative input, via switch setting. Either SE0 or Aux can be measured at a time.

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