CN-0359 testing

Hi!

After reading the CN-0359 application note, I am purchasing at Digikey online shop and testing the EVAL-CN-0359 board.

There were a few problems in the process of testing.

The questions are in the attachment file.

Please give me good advice.

Thank you.

Best regards.

CN-0359 test and Q_.zip

Is it caused by contact potential???

Parents
  • Hi ksw280,

    Apologies for the delayed response. This sounds like it might be similar to an issue we saw with a physically small conductivity cell. CN0359 works well with certain cells, but after it turns out that the op-amp bias current (and maybe the small but nonzero DC component of the excitation waveform) are enough to cause issues with smaller probes.

    What worked for the probe in question was to simply capacitively couple everything, except the cell's return/ virtual ground connection. These are some notes from the engineer that tested this theory:

    I used a breadboard to make the following connections shown in the attached circuit.

    • A 10 µF tantalum capacitor in series between the conductivity cell excitation and J5P1
    • A 1 µF polycarbonate capacitor in series between conductivity cell sense output and and J5P2
    • A 1 µF polycarbonate capacitor in series between conductivity cell sense output and J5P3
    • A 1 MΩ resistor to ground on J5P2s
    • A 1 MΩ resistor to ground on J5P3

    Could you give this a try?

    -Mark

  • Hi Mark,

    Thank you for answer.
    I've seen this in "CN0359: Float of signal from Instrumentation Amplifier."
    However, the excitation current leaks through the 1M Ohm resistor connected to J5P3. This causes a measurement error and eliminates the effect of using the U14 buffer.
    Before asking, I tested by connecting a 1M Ohm resistor to J5P2 without a capacitor, but the DC drift didn't go away.
    What do you think about my opinion?

    Thank you again for your kindness.

    Kwon

  • Hi Kwon,

    Indeed, the 1M (or 10M) resistor will introduce an error, but it's worth trying out in order to understand whether the bias current and it's long term effect on your sensor is the root cause of the drift you're seeing. You could start out by just capacitively coupling the excitation (pin 1) - this would remove DC content from the excitation signal. Also note that since you know the value of this resistor, you could compensate in software (if it solves the electrical problem.)

    IF you trace the problem back to bias current from the AD8253, you could buffer it separately with an external ADA4000-1 (unity gain buffer.)

    And a side question - have you tested the circuit with fixed resistors to emulate your actual sensor? This is always a good reality check to rule out other problems with the board.

    -Mark

  • Hi Mark,

    Now I will understand your will.
    "The error caused by the 1M Ohm resistor connected to J5P3 is known and can be compensated in software."
    I will summarize it like this.

    Thank you in the meantime.

    Kwon

  • We'd be interested in your results, it appears there's some subtle issues with certain probes. On that note, can you provide the part number for the probe you're using, or is it your own proprietary design?

    -Mark

  • It is our own proprietary design, looks like bull's eye. Please refer to the attached photo for the shape.

    Kwon

  • Understood - just speculating, but if your sense electrodes are very small, then you may indeed need to take special precautions and use picoamp bias current op-amps on both.

    -Mark

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