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ADS1148 noise immunity issue

Aung Thet
Prodigy 195 points
Other Parts Discussed in Thread: ADS1148, INA117

We are using ADS1148 in our Break Maker Product. Attached is the schematic of the ADS1148 circuit.

As we have a big press motor (We supplied 230V pulsating DC through rectifier  and as it keeps running forward and reverse, the generated noises are causing the errors in readings. If we let it happen for an hour, the chip stops measuring the correct temperature and starts sending out random values.

I have tried putting 0.1uF caps as in attached schematic, and also increased analog supply to 5V with separate digital supply of 3.3V but this does not solve the problem.

Please suggest me possible solutions.

Regards

Aung3162.ADS1148 circuit.pdf

  • 0
    TI__Guru** 113765 points

    Aung,

    There are a number of potential issues.  On the inputs you have differential and common mode caps of the same value.  The differential caps (like C703) must be at least 10 times larger than the common mode caps (like C701 and C702) or you may see problems with offset and drift.  Another thing, caps by themselves really don't do much other than store charge.  You need to design an input filter by adding some series resistance.  Depending on the frequency of noise, you may need to add some additional EMI/RFI filters like a ferrite.

    This may not be the end of the problem.  Noise may be getting into your PCB, in particular your ground.  If you do not have a clean ground plane you most likely will see noise related to ground disturbance.  You may also need to add additional shielding, especially if you have cable runs that can act as antenna.  If this is a prototype and you have a number of wires that are generating or picking up induced noise, these can also be problematic.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 195 points

    Hello Bob,

    Thanks for your quick answer.

     "The differential caps (like C703) must be at least 10 times larger than the common mode caps (like C701 and C702) or you may see problems with offset and drift."

    Could you please give me some links on this topic? I would like to read and learn more about it.

    Regards

    Aung

  • 0 in reply to Aung Thet
    TI__Guru** 113765 points

    Hi Aung,

    Basically you don't want common mode errors to create a differential voltage.  This is primarily due to mismatch of components that may become worse as temperature drifts.  I've attached a link to an app note that was specifically written for the INA117, but the principles still apply.

    http://www.ti.com/analog/docs/litabsmultiplefilelist.tsp?literatureNumber=sboa016&docCategoryId=1&familyId=505

    Best regards,

    Bob B