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OPA129: Eliminate noise on floating op amp input

Aditya Teja Damera
Prodigy 185 points

Part Number: OPA129

Hi,

I am using OPA129 as an non inverting amplifier for an in put range -0.5V to +0.5V , but whenever I dont give an input to the non inverting pin( leaving it floating), It picks up a very small noise( its freq is ~50 Hz ..guessing the AC hum) and amplifies it . I would like to eliminate this noise. Can you please help?

Thank you,

Aditya Teja Damera

  • 0
    TI__Mastermind 23050 points
    Hello Aditya,

    It is generally not recommended to leave the inputs of an op-amp floating. If you need to disconnect your input, then you could put a resistor to ground on the non-inverting terminal, depending on the input impedance you need.

    Other alternatives become a lot more involved and would entail shielding, or using a difference amplifier. You could build a discrete instrumentation amplifier to maintain the high input impedance. Let me know if either of these sound like viable alternatives.
  • 0
    Guru 140200 points
    Hi Aditya,

    tell more about your application. Do have a schematic for us?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 185 points

    Hi Kai, 

    Its just an amplifier interfaced with pH sensor. So when i removed the sensor, it picks up noise and amplifies it.

    Thanks,

    Aditya Teja

  • 0 in reply to Aditya Teja Damera
    Guru 140200 points
    Hi Aditya,

    ok, and because the ph sensor is very high-ohmic there cannot be a simple resistor mounted from input to ground, because the signal would drop, right?

    Hhm, you could short circuit the input of OPA129 to ground by the help of a switch, whenever you have removed the ph sensor. But if you plug-in the ph sensor while the switch is closed you would short circuit also the ph sensor and by this destroy the ph sensor! So, this would not be a good solution. For the same reason I wouldn't take a connector with a switch contact, like can be seen with these 3.5mm audio jacks. The danger is too great, that you short circuit the ph sensor unwantedly.

    Why removing the ph sensor, at all? For cost reasons? The best solution would be, not to remove the ph sensor from the OPA129.

    If you need to remove it, I would surround the OPA129 circuit with a metal housing and connect it to 0V of your circuit. Mount a connector for the ph sensor into the metal housing, so that the whole input line to the OPA129 is sourounded by the housing. If you still have annoying hum, you could additionally use a low pass filter which you put behind the OPA129. Then hum should no longer be a problem. At the same time you would have provided a protection for the OPA129 input, because with a housing you can no longer touch the input line.

    Kai
  • 0 in reply to Zak Kaye
    Prodigy 185 points
    Thanks for the reply Zak, We are hoping shielding might resolve the issue, but we don't have the new PCB yet to test it out.
  • 0 in reply to kai klaas69
    Prodigy 185 points
    Hi Kai,
    Thanks for the reply, I have a doubt..wont the Low Pass Filter produce some drop in the voltage from pH sensor due to series resistance when added in front of OPA129 ? which would disturb the design right?

    Thanks,
    Aditya Teja
  • 0 in reply to Aditya Teja Damera
    Guru 140200 points
    Hi Aditya,

    I dind't mean in front of, but behind the OPA129. :-) You could add an active low pass filter using another operational amplifier. Or may be even a passive low pass filter would be sufficient?

    Kai
  • 0
    TI__Genius 11135 points
    Aditya
    We haven't heard back from you so we assume you were able to resolve your issue. If not, just post a reply below or create a new thread if this one has locked due to time-out.
    Thanks
    Dennis