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OPA818: Voltage Rail Leakage and Input Noise

Part Number: OPA818

I recently designed a trans-impedance amplifier based on the designs laid out in the datasheet for the OPA818, utilizing the exact capacitor values specified over the power rails and resistor value over the feedback pin designed for a 100-kΩ gain. After testing the resulting circuit I noticed a strange output when I applied 0A, seeing that there was around 50mV being output with no input current.

After checking everything on my circuit I set up one of the OPA818 amplifiers on its own board with nothing attached and see a 10MΩ resistance over the negative input to both the positive and negative input pins. While this is extremely high for the sake of most applications, I am working with input signals on the scale of pA and this is causing a wild amount of input noise on the front end of my amplifying circuit, rendering the amplifier that I designed useless. The OPA818 boasts an extremely low input noise of 2.2 nV/√Hz and 3 fA/√Hz which was my reason for purchasing, however I am seeing input noise on the order of 100pA-1µA.

Is there any way to improve this input noise level? Am I missing something extremely important?

  • Hello Kenneth,

      Which Figure in the datasheet are you referring to when you were explaining your design? Would you be able to send a schematic of your circuit? Would you also be able to share your setup/board?

    Thank you,

    Sima 

  • Well Kenneth

    The equivalent input current noise for a transimpedance design is a lot more than just the current noise term - often the output noise is dominated by the input voltage noise getting a peaking gain to the output. 

    Transimpedance design and full noise expression is in this presentation, the noise expression here is simplified assuming a post filter at < than the feedback pole frequency which is  good idea. 

    5050.Transimpedance design flow using high speed op amps.pptx

  • Sima,

    First, I accidentally clicked that the issue was resolved instead of reply and it does not seem like I can turn that off so here we are.

    The schematic shown here from the data sheet is what I based my original circuit from. I utilized the proper supply decoupling as specified in the datasheet as well. The only difference in my circuit design is the ground between the feedback capacitors. My circuit does not have this ground and it only has an feedback capacitor placed in parallel with the resistor. Would you expect this ground to be beneficial with regard to input noise? Additionally, due to the current storm in the northeast I am unable to take a picture of the schematic as I cannot get to my laboratory but I trust that you can take my word that the actual handywork is sound.

  • Hi Kenneth,

    the reason why the T-network is used here is throroughly explained in section 9.2.1 of datasheet.

    I guess that your circuit is oscillating because of the improper wiring and that this is the cause for the increased noise.

    What is your detector capacitance? And what is your desired bandwidth?

    Kai