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TLV2333: EMI Susceptibility in magnetic field environment

Wayne Wang20
Expert 4360 points
Part Number: TLV2333
Other Parts Discussed in Thread: OPA2379,

Hi team,

My customer is developing a GI product and the product needs to be tested in the magnetic field environment. At first, customer used OPA2379 on the board to do amplifier and buffer. But they found that when the board is under magnetic field environment, the output of OPA2379 drifted away until saturation. please see below picture. And after a while the output returns back to the normal range. This phenomenon repeats periodically. Then customer changed OPA2379 to TLV2333 and found that this issue still happens but it happens less times in a period time than OPA2379. I noticed that in the TLV2333 datasheet, it describes the EMI susceptibility of the input stage filter. Do you think this feature is helpful to this case?

What's more, if we want to totally solve this drift issue in the magnetic field environment, do we need to select the device with robust EMI susceptibility or we still need to use the physical method like adding the shielding case? Thanks.

Best regards,

Wayne

  • 0
    Guru 140200 points
    Hi Wayne,

    what frequency range of magnetic field interference are we talking about?

    The internal EMI filter of TLV2333 begins to work above 8MHz. So, if the magnet field interference is of much lower frequency, this filter will not help much.

    Shielding is a very powerfull tool! Equally which OPAmp you choose, you should add some shielding.

    Kai
  • 0 in reply to kai klaas69
    TI__Expert 4360 points

    Hi Kai,

    The  magnet field interference is from 80MHz to 2GHz. So TLV2333 internal EMI filter should work at this range. In terms of the shielding case, as this product is a Fault Indicator product and it is hanging to the power line in mid-air, so the product cannot connect to the earth ground. If so, how should we deal with the shielding case grounding? Leave it floating or connect it to the board system GND?

    What's more, customer said many other users put a cap between the two input pins of TLV2333 and this cap works as a balance cap. So I just want to check with you if this balance cap is helpful and how it works? Thanks.

    Best regards,

    Wayne

  • 0 in reply to Wayne Wang20
    Guru 140200 points
    Hi Wayne,

    I would connect the shield to the local board system GND.

    A cap between the input pins of TLV2333 can help to reduce the influence of EMI. But it can also result in an erosion of phase margin and decrease stability. Then the introduce of a phase lead capacitance in the feedback path can help.

    Kai
  • 0 in reply to kai klaas69
    TI__Expert 4360 points
    Hi Kai，

    Thanks for your help. But do you have a recommended value for the balance cap? And could you help explain why it can reduce the influence of EMI? Thanks.

    Best regards,
    Wayne
  • 0 in reply to Wayne Wang20
    Guru 140200 points

    Hi Wayne,

    usually, about 100pF is used. Its purpose is to short-circuit EMI between the two inputs of OPAmp. But as it has no influence on the common mode EMI, it's benefit is a bit questionable. I would use this sort of filtering very carefully, because it can considerably erode the phase margin and cause stability issues. Using this cap between the inputs of an OPAmp is a bit a thing of the past. Today, with the fast OPAmps it's no longer seen.

    Kai

  • 0
    TI__Guru 63770 points

    Wayne,

    I agree with Kai that placing any large capacitor between the input terminals is not advisable because of potential stability issues it may cause.  Furthermore, commenting on the issue, EMI filter in TLV2333 only helps with the magnetic field interference feeding through the input pins. However, it is also possible that EMI in your application enters the signal path of the op amp through the supply pin, which do NOT have EMI filter. For that reason, it may be worth trying to add a ferrite bid (or an inductor) in series with supply pin, which together with the supply bypass cap (>0.1uF) may be able to filter out EMI interference causing the issue you see.