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OPA862: VFB is about 0.5V when VIN is connected to ground

Dyon Buitenkamp
Prodigy 60 points
Part Number: OPA862

Hi,

I am working on a design using a transimpedence amplifier configuration using the OPA862.

I am using single supply (3.3V and GND) and wanted to connect the noninverting input (VIN) to ground, but no matter what I try, the inverting input (VFB) is at about 480mV. Because of this, the circuit does not detect any optical input to the photodiode I am using. I tried setting VIN to 0.5V, and it seemed to work. 

Is there a reason I measure 480mV at VFB? How would I be able to make it work with VIN set to ground?

Thank you,

Dyon

  • +1
    TI__Mastermind 24965 points

    Hello Dyon,

      The reason is because setting the noninverting input to ground at single supply violates the input common-mode range of the device:

           

      This follows why the amplifier functions properly once VIN+ is set to a voltage to 500mV. For single supply applications, it is best to bias the amplifier at the midsupply of the voltage supplies for maximum output voltage range. For 3.3V to GNd, this would be 1.65V.

    Thank you,

    Sima