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LMV852: Anyone notice any difference in microphonics with these EMI hardened op-amps?

Robert Eaton
Prodigy 10 points
Part Number: LMV852
Other Parts Discussed in Thread: LME49726

I tried using the above part in an existing microphone design and during frequency response testing (on an artificial mouth) there were anomalies in the response that varied depending on the mechanical coupling to the test equipment. The frequencies of the anomalies vary some but are always there if the microphone has this opamp installed. The same equipment has been used for years and consists of a B&K artificial mouth an Audio Precision System Two analyzer, and a Crown power amp. (If you're familiar with the AP, the curve times out at several places during the sweep.) Never saw this issue before and it repeats with more than one device.

I'm wondering if the added RC components (specifically the caps) used to improve EMI resistance could be making these chips somewhat microphonic?

Has anyone run into this problem?

  • 0
    TI__Guru*** 140316 points

    Robert,

    The RC cutoff is way above the op amps bandwidth, so I would not expect any issue. The RC does increase the input capacitance a little, but for audio, the Xc is still very high impedance.

  • 0 in reply to Ron Michallick
    Prodigy 10 points

    Hi Ron,

    You're correct but there is something weird going on. Two units that tested fine had this issue when the op amps were changed. Other units with the old op amps (LME49726) still test OK. This is actually in a headset with a boom mic (the boom only has an electret element inside). This isn't a production test, so no fixture is holding the headset. The boom is clamped over the artificial voice, and the headset sits on the table next to it. Holding the headset off the table with my hand gives an obvious improvement. Changing the sound pressure also has an effect. We tried 84, 94, and 104 dB SPL. The worst result is at 94 dB SPL. I assume something is resonant (maybe the table). Now the old opamp has a solder pad on the bottom which bonds it to the pc board. Maybe that is a factor, however I have built prototype boards in the past without using that pad. More testing to do.....