• State
  • Locked Locked
  • Replies 3 replies
  • Subscribers 47 subscribers
  • Views 212 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

OPA858: OPA858 Near field radiation issue

Gabriel Xu
Intellectual 2145 points
Part Number: OPA858

Tool/software:

Hi Team

Customer using OPA858 in their optical module project. When they use near field probe and spectrum analyzer found 20dBm radiation in 2Ghz frequency point.

This circuit is used in PD light power detection, the IN- connect to PD light detector. However, the test is execute without optical input signal and also have issue.

Customer also found if we remove 0.75P capacitor (in parallel with 1K resistor connect to FB pin) the radiation disappear.

They also try to adjust the capacitance of this cap and found larger the capacitance the radiation energy will be much lower.

The schematic have shown below:

 

Customer want to know are this amplifier have such high frequency range in internal circuit?
What's your suggestion for possible root cause?
  • 0
    TI__Mastermind 28865 points

    Hello Gabriel,

       Does 20dBm radiation in 2GHz frequency refer to the power level of oscillation at 2GHz? If so, it is standard for this to occur in all TIA configuration. The 0.75p capacitor is used to stabilize the circuit when additional capacitance from the photodiode is seen at the input of the amplifier. These two capacitor values create a pole/zero pair which depending on where is placed in frequency region will impact the stability of the entire system. The input PD might be small, which needs a very small feedback capacitor which can be contributed by the board parasitic. These two values are important especially for a decompensated part such as this device that is stable at gains greater than 7V/V, which in TIA configuration, noise gain is calculated to be 1+ (CTOT/CF) where CTOT is the input capacitance of the PD and estimated board parasitic + OPA858 common-mode and differential capacitance. 

       What is the customer's input capacitance contributed from their PD?

       Edit: I read that input is not used, this is the reason. With no input capacitor, then there is no pole/zero pair with feedback capacitor.

    Thank you,
    Sima 

  • 0 in reply to Sima Jalaleddine
    TI__Intellectual 2145 points

    Hi Sima

    For the PD details capacitance value I need to further check with customer.

    Base on your feedback this radiation phenomenon is a known issue? Do we have workaround to fix it?

    Are the radiation frequency always to be 2Ghz or depend on external compoents?

    So you mean that the source of the radiation is coming from the device inside? 

  • 0 in reply to Gabriel Xu
    TI__Mastermind 28865 points

    Hello Gabriel,

      By radiation, I have been assuming oscillation at 2GHz. Is this correct assumption? If so, this is caused by instability that can happen to any amplifier, and especially common in TIA configurations.

    Thank you,
    Sima