Post Go back to editing

CN0359: Float of signal from Instrumentation Amplifier

We have set up EVAL-CN0359 and have the following problem. 
We are measuring the voltage from J5 pins 2 or 3.
These feed AD8253.
We see the expected distorted square wave on the scope but it is floating up and down between the rails.
Page 18 of the datasheet for AD8253 possibly implies that the inputs are not stabilized by a bias current path to ground in the CN0359 layout.
See also fig. 6 of your article by Kitchin concerning "Common Problems When Designing Amplifier Circuits".

Any advice here? T
  • Hi Fritz,

    Is there a conductivity probe connected on J5? If there is, can you send me the product page for it? Also, can you send a snapshot from the signal captured on your scope? I'll try to recreate the cause of the floating effect you're seeing

    It is correct that a current return path is needed, especially for AC coupled signals. The CN0359 does not use AC-coupling on the signal. The current driving the conductivity probe enters through one of the electrodes (the one connected to pin 1 of J5 and/or pin 2 if it is a 2-electrode probe) and has its return path via the trans-impedance amplifier U19.



  • Nikko-I am attaching pics of the scope shots.

    The first one is a time lapse showing the gradual drift of the overall signal. The other shots are of pins 1,2,and 3 w.r.t ground.

  • Glenn

    1- pin 9 shows 1.64

    2- The output of U15 with 2&3 at gnd shows 1.828V

    3-Our probe is 10M 11pf

    4-We did the above at 0.3V and 1kHz. At 5.1kHz it is roughly the same, the U15 output showing 1.838V

    We did all of this on a new board. Looking at the output of U15 at 5.1kHz, 0.3V and with the IST sensor, we still see the gradual drift to the rails.


  • Any further action on this? We are experiencing plating on our probes, very likely due to this phenomenon. Are you equipped to set up this circuit in your lab and duplicate our results?



  • Hi Fritz,

    Apologies on the delay.

    Based on the data that you sent. The system looks to be working fine. I am thinking if the probe that you have could be not functioning the way it is. This could be the overvoltage condition on the earlier part and the added plating on the probes. What kind of plating do you have on the probe?

    Do you happen to have an extra probe to try this out? We have the board but do not have your exact same probe.



  • Glenn-we had replaced the failed probe some time ago and are running 2 or 3 of you boards with fresh probes, all exhibiting the same behaviour. We also have run the CN0359 with a commercial TOPAC probe and it similarly shows this problem.

       I do not quite understand how the system can work with the common mode drifting between the rails and indeed when it does this an error condition is raised by your software.

    Where are you located? We can send you a few test probes.

    Meanwhile have you tested this with a probe of your own and watched the voltages with a o'scope? If you see the problem you can continue analyzing it. If not we can buy the same probe and do some tests with it here.

    Also is it possible to communicate via telephone? We lose a whole day between each exchange otherwise and we can quickly try the things that you suggest over the phone.

    Thanks you


  • Hi Fritz,

     We would be happy to take a look at your application.

    If you could send us a sample of your probe. We are located in our office here in the Philippines. Our address is below:

    Analog Devices Inc.

    Gateway Business Park

    Javalera Gen. Trias, Cavite


    Postal Code 4107

    Attention to: Glenn Amparo



  • Thanks Glenn-I shipped out two sensors today and I would expect them to arrive late in the week.

    Keep in mind that we see this same problem with 4-pole sensors from other vendors so if you have one on hand you may get started with that.


  • Hi Fritz,

    Thank you for sending the probes. We look forward to its arrival.



  • FormerMember
    0 FormerMember
on Apr 22, 2019 4:37 PM in reply to fsonnichsen389

Hi Fritz,

Do you have a tracking number you can share for the probes?  Also have you tried this setup using precision resistors instead of the conductivity probe?  Its a good exercise to go through to ensure any drift that you are seeing is in fact due to the probe/solution input or related to the hardware/board configuration.

  • Hi all,

    Just wanted to leave a note saying Fritz has also sent me a probe to do some testing at our Wilmington site. 

  • Reply Children
    • Hi Fritz,

      We have some preliminary testing done with a TOPAC probe that we have available here and we have seen no anomalies on the output signal. We have tested it with varying resistor values in place of the probe and was also able to get correct values with the readings and the output signal looks as expected concluding that the circuit works as expected with the given test cases.

      We have received the probes yesterday and have been able to start the testing of the board with your probe. We are able to see and replicate your test case where the output of the in-amp swings to the negative rail.

      We are now trying to explore the possible instances why this would happen. Lia and our team are sharing our data to gain better traction in solving this issue.

      Please stay tuned on the advancements of our testing.

      Thank you,