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LM2904: Abnormal output of LM2904...

Part Number: LM2904

Dear Sirs,

My customer meet strange issue of LM2904 abnormal output voltage.

if customer touch the PCB or IC body, bend PCB a little, then the LM2904 output voltage will change.

Customer gain is set to 151, and the input signal is around 10mV, input voltage is +5V and -5V.

Customer also use typical non-inverting connection for testing, but issue still same.

Please give me some idea if there is anything need to pay attention or any other solutions that can help customer.

Thanks.

  • Hi Peter,

    this is pretty much normal as you couple hum and intereference into the circuit when touching the PCB or even the chip body. And as the gain is rather high the effect is appears rather pronounced.

    Use a solid ground plane, low ohmic feedback resistances, filtering and protection circuitry and, if necessary, shielding. Shielding means to cover the PCB by a metallic enclosure and to connect this enclosure to signal ground.

    Kai

  • Hello Peter,

    In addition to hum injection (50 or 60Hz) there is also package shift. Physical stress on the op amp package can affect the input offset error. This will be a DC shift with the stress. 

    How much change is observed and is the change DC or line frequency?

  • Hi Ron,

    the first time when I noticed that physical stress on the OPAmp package can affect the input offset voltage of an OPAmp was when I hand selected LF356s for "zero" input offset voltage many many years ago. After pulling them out of the test socket and pushing them into the circuit the offset voltage was longer "zero" :-)

    Kai