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Odd INA333 Behavior.

Patrick Hamilton
Prodigy 40 points
Other Parts Discussed in Thread: INA333

I am using an INA333 for a thermocouple amplifier and have observed some nonlinear behavior that I don't understand.  I am hoping someone can give me some insight into what is going on.  For most of the input signal range the amplifier works great, but for a small voltage input range the output becomes noisy (looks like random low frequency around 400 Hz, but it could be higher frequencies aliased by my digital scope) and the output stops tracking the input.  I lower the input voltage and the output stays at the same noisy output level until I reach an input point where the output suddenly drops and begins cleanly tracking the input again.  The input range that seems to present problems is from 1.37 to 1.19 mV.

The INA333 has +-2.5V supplies.  Both inputs have 1M resistors in series with the thermocouple inputs and a 1M resistors in parallel with a 0.1 uF cap  connected to GND (I know that these 1M resistors are a problem with the INA333 offset current, but I have been calibrating the output in software).  There is also a 1 uF cap across the inputs, and I am using a 3K resistor to set the gain at 34.33.  The reference input is grounded (I do the cold junction compensation at a later stage).

I have not observed this in all my boards, but that may be because I didn't know exactly what I was looking for.  I will try to reproduce it with other boards.

Any help would be appreciated.

-Pat

 

  • 0
    Prodigy 40 points

    Well I tested another board.  I couldn't reproduce the behavior in the second board I tested, but as I varied the temperature (and thus the input voltage to the INA333) the frequency of the noise riding on the thermocouple signal (a manageable level of noise) varied with the level of the thermocouple signal.  Does the DC voltage level at the input some how tune the high frequency characteristics at the input?

  • 0
    TI__Expert 7185 points

    Hi Pat,

    I do not have a quick explanation for what you are observing.  Are you experiencing this with your INA333 circuit connected to the thermocouple or an independent source?  Would you mind posting your schematic to the forum so that we put a picture to your description?  Also, if you can post a scope shot of the "noisy" output signal and the output transition from noisy to clean input signal this might be helpful as well.

    Thanks,
    Matt

  • 0 in reply to Matthew Hann
    Prodigy 40 points

    Matt,

    Thanks for the response.  Do you have a suggestion for a utility that would make easy to do a circuit sketch I can cut and paste.  I am not sure how I could pull it out of PADS.

    Pat

  • 0 in reply to Patrick Hamilton
    Prodigy 40 points

    Matt,

    And, I am experiencing the problem when connected to a the thermocouple.  I vary the thermocouple temperature by controlling the temperature of a water bath.

    -Pat

     

  • 0 in reply to Patrick Hamilton
    TI__Expert 7185 points

    Patrick,

    I will contact you offline and you can send me your PADS .ddB file directly.

    Thanks,

    Matt

  • 0 in reply to Matthew Hann
    Prodigy 15 points

    Would there be any conclusion to share on this issue?

    Thanks

    Dan

  • 0 in reply to Daniel Rudelle
    Guru 21975 points

    Pat,

    Matt is traveling and unable to follow up.

    Looking at your schematic, I agree with Matt that there is no obvious flaw in your circuit. I share his concern or suspicion about the 1M source and common-mode biasing resistors. These seem unnecessarily high and may invite problems. Is the thermocouple connected directly to the inputs that are shown? Can you provide a photo or describe the 300Hz noise that you see?

    There should be nothing going on in the INA333 in the 300Hz region and nothing related to chopping frequency (which is much higher) is input voltage-dependent. The gain-set resistor nodes can be a bit sensitive. If there is excessive capacitance associated with these pins it could lead to problems. Perhaps there could be some coupling of the high-Z inputs to these nodes that would cause a problem.

    I recommend reducing the input and biasing resistors to 10k and see if this changes the behavior.

    Regards,  Bruce.