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
hanks--Fritz
  • 0
    •  Analog Employees 
    on Mar 26, 2019 10:45 AM over 1 year ago

    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.

    Regards,

    Nikko

  • Nikko--here is the product page for the IST-AG LFS1505 four pole probe that we are using. We have an independent RTD connected to the temperature sense lines. (PT1000).

    www.ist-ag.com/.../DCLFS1505_E.pdf

  • 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.

  • 0
    •  Analog Employees 
    on Mar 28, 2019 12:58 AM over 1 year ago in reply to fsonnichsen389

    Hi Fritz,

    I was looking at the datasheet of your conductivity cell you have sent. The CN0359 board had come out from the factory with the following settings: 

    1. Set EVAL-CN0359-EB1Z to following setting:
    2. Excitation voltage: 1V
    3. Excitation frequency: 100Hz
    4. T-H setup time: 10%
    5. T-H hold time: 1%
    6. Temperature coefficient: +2.00%
    7. Cell constant: 1.0000/cm

    I was looking at the datasheet of your conductivity cell you have sent and it seems that it is not matched.


    Could you try changing the settings and let us know if this made any difference?

    Thanks,

    Glenn

  • Glenn
    Thank you for your reply and sorry for the late response. We have been deliberating on this and I wanted to try to thoroughly understand the problem. Here is some more focused information:

    We are running the system within spec for our probe. The IST sensor mandates less than V=0.35 (e.g. 0.7 Vpp). We set f=1kHz and leave the timing parameters alone. We set the cell constant to that in the IST datasheet (e.g 0.68)

    We are measuring the output of diff. amp. U15 with respect to the circuit ground.

    With the above setup we see the down drift shown in the scope shoots that we posted.

    If we crank up the voltage beyond the recommendation by IST, to 2Vpp, the drift seems to go away. This however ultimately damages the sensor and we have seen this.

    The only other clue is that adding a 1M resistor to pin2 of the conductivity cell connector seems to slow but not stop the drift.