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Walkie-talkie signal affect the performance of op-amp LMC6484

Dan Li62
Intellectual 420 points
Other Parts Discussed in Thread: LMC6484, LMV831, LMV851, OPA4192, OPA4171, OPA4180, LMV834

Our group has designed a wireless strain gage interface board with op-amp LMC6484. But we found a problem when using walkie-talkie. The walkie-talkie signal will affect the signal of the strain gage. Has anyone met the similar problems? And is there any solution on it?

  • 0
    TI__Guru 52296 points
    Hello Dan,

    Yes. This is called EMI. RF conducted into the amplifier at frequencies beyond the passband of the amplifier (>VHF) can cause rectification internal to the amplifier, (like an AM radio) causing offset shifts.

    See AN-1868 and the LMV831 and LMV851 datasheet applications section for more information.

    Also see the following:

    www.ti.com/.../snoa531a
    e2e.ti.com/.../how-to-use-emi-hardened-op-amps-to-reduce-your-errors
    www.ti.com/.../getliterature.tsp

    Many Op Amps now contain EMI Filtering to reduce (but not eliminate) the effects of applied RF. The first devices to have this were the LMV83x and LMV85x series and introduced the "EMIRR" (EMI Rejection Ratio) concept due to the effects of cell phones on circuits. Now the EMI filters are incorporated in many recent devices.

    These EMI hardened amps can be found by going to the parametric search page and checking "EMI Hardened" under the "Additional Features" category.

    www.ti.com/.../op-amps-products.page

    If your supply voltage is under 5V, there are many choices, including the original LMV834. If over 5V, then try the OPA4171, OPA4180 or OPA4192.

    A common "fix" is to place small value capacitors (15pF to 100pF) in a "delta" configuration across the inputs. This means a capacitor from each input to ground, and a capacitor between the inputs. The ground should be a solid RF ground, and not a signal ground. The capacitors should be small (usually SMT 0805 or smaller) and placed as close to the inputs as possible. Though caution must be used as adding capacitance to the inputs can cause peaking or oscillations depending on the feedback and source resistances. Ferrite beads and small capacitors *at* the point where the signals enter the board are also recommended.

    Note that the internal filters are designed to be effective at UHF and above frequencies, so lower frequencies still require external filtering. The best hardening is when a combination of external filters and EMI hardened devices are used.

    Regards,