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Amplifier and Ph electrode

I am trying to implement an Alkalinity measurement device.

  1. I am using reference electrode[16-702] and ph electrode[16-705] by MicroElectrodes, INC
    1. The output value from electrode will be between range of plus/minus 410 mV
    2. Response time for ph electrode[16-705] is 5 - 15 seconds

  1. Now we need a amplifier that can take input from electrode will be in range of plus/minus 410 mV.
    1. Also the amplifier needs to be able to take input impedance greater then 1 * 10^12 [1 Tera]
    2. Also the amplifier needs to have bias current under 1 * 10^-12[1 Pico]
    3. The output gain of plus/minus 20 V
  2. The output of the amplifier will be input to A/D converter[USB-1408FS]  by Measurement Computing.

At the moment I have ph electrode connected to AD8222 amplifier and something in the system is causing a intereference

which is causing a incorrect output signal or noise in the signal.

Is AD8222 amplifier I am using correct or Should I use some other Amplifier?

Message was edited by: ambar rathie
  • Hi, topgun303.

    I have few concerns:

    1. What is the supply voltage of AD8222? It should be a dual supply of more than ±2.3 V because you have an input voltage of -410mV or this should be within the required supply range of 36V.

    2. Since I'm confused with your statement of "The output gain of plus/minus 20 V" here are some points.

         a. If you want the AD8222 to produce an output of 20V, sorry but this part can't do that because it can only have an output voltage of -16.8V to +16.6V for a supply of ±18V. However, this output voltage range of AD8222 is very acceptable by USB-1408FS so this should not be a problem.

         b. If you are referring to 20V/V gain, then this part can do that but be sure that your supply must be greater than ±10V.

    3. What are the connections of REF1 and REF2? These pins should not be left floating for these pins affect your output voltage. Please refer to page 16 of the AD8222 datasheet for the proper connections of these pins.

    4. If you could provide the schematic, that would be much easier to help you.



  • Hi, topgun303.

    Yes, an RC filter before AD8222 would really help. Based on the prototype note you sent, I think you have not placed any capacitor on the footprints with "SPARE" labels in the evaluation board you're using. Please refer to page 18 of the AD8222 datasheet on what values to use on these capacitors. Also, you may refer to this article about designing RFI filters -

    We do have a SPICE model for AD8222 which you can find in this link - - under Design Tools, Models, Drivers and Software section. Please use Multisim to simulate this.



  • Ms. Anna, I will get back to you on the info per you comment. I am attaching some notes and a pic. I don't have a schematic but I will work on it, post it here in couple of days.

    Also I have talked to ph electrode manufacture and he stated that we would need RC filter with time constant of 1 or 2 on the output to take care of noise. I was wondering if analog devices have library for AD8222 to simulate the circuit using pspice or multisim software

    Attached two pics

    Attached prototype notes

  • Ms. Anna,

    1.    I downloaded the spice model of AD8222 and tryed to open it in multisim [Ver 13.0] but got a following error:

    File-open-<select file>

    Loading design ....

    Ignored: Title line '* AD8222 SPICE Macro-model'

    Error: Main circuit not found in SPICE netlist

    Finished loading design .....

    2.   Anyhow, so I went into my parts list and found these op-amps. I am not sure whats the difference between following models..

    I am using AD8222 EVALZ board without thermal pad. But as stated on page 23 of spec sheet standard grade via high performance grade. My guess to use the last two op-amp in bold.





    3.   I would also like your recommendation on an op-amp amplifier beside AD8222 that is suitable with the application such as pH electrode and reference electrode.

  • Hi, topgun303.

    For your concerns:

    1. I think you don't need to download the AD8222 SPICE Model from the link I gave you. This is already included in the components in Multisim ver.13. The reason, maybe, of the error you had was that Multisim can't identify which AD8222 to use, the model you added or the built-in model. Just select the AD8222 from the components in Multisim.

    2. Your guess is correct! The parts with "H" have no exposed pad and so these are recommended for your evaluation board without thermal pad. The only difference between the parts with "A" and "B" is their grades. "B" grade has higher specs than "A" grade. You may refer to Specs table in the datasheet for more details on this.

    3. I would recommend AD8220 because it has 10pA input bias current which is within your requirement. It is also a Rail-to-rail In amp but has lower input and output headroom to consider. You may look at this link for its datasheet -

    I've seen in your attachment, CN0326. This is a very good reference for your application but the parts used here does not fit your requirements. Let me know if you will be referring to this circuit note so that I may recommend parts suited to your needs.



  • Ms. Anna,

    I have attached my ckt for AD8222 using MAX406 as a voltage follower. The only problem I see is that I don't know why there is a high resistance on AD8222 [+ pin].

  • Hi, topgun303.

    Does the two +pins (pin 4 and pin 9) of AD8222 have high resistances? How high is it? Have you fabricated the board? The problem you're seeing might be in your PCB trace (parasitic resistance). Can you provide your PCB layout?



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    EZ Admin