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OPA846: Frequency fluctuations in TIA

Abhinaba Roy
Prodigy 10 points
Part Number: OPA846

HI All ,
I have designed one Transimpedance amplifier  so that it can detect visible light  emitted from LED. 
The LED is being driven using  square wave of high frequency (like 1 MHz, 5 MHz) etc .

The circuit  designed is able to detect light and give a corresponding voltage . But the issue with the circuit is that  the frequency is constantly varying (ie going from  KHz to MHz ). 
The LED is placed very close to the photodiode and we  are testing the circuit in dark room .  Also  the noise in the circuit is also more.
Can anyone help in identifying what is the issue that is causing fluctuations in the output frequency and how to reduce it ?
Also i want to know how we can eliminate the noise (used a LPF, HPF ) to filter out the undesired frequencies but the issue still persists >

Below attached is the schematic diagram and results on  a oscilloscope .

  • 0
    TI__Mastermind 28525 points

    Hello Abhinaba,

      Thank you for sharing the schematic and scope shot! It looks like the amplifier/circuit could be unstable and oscillating which could be the reason why you are seeing the undesired frequencies. Would you be able to also share a snippet of your layout (PD + amplifier)

    Best Regards,

    Sima

  • 0
    Prodigy 10 points

    hi Sima ,
    Thanks for the response. Please find below the snippet of layout .
    According to the circuit diagram, we have used OPA846 IC and other related components of SMD type and soldered the whole circuit in a PCB and we are looking into the output.

  • 0
    Prodigy 10 points

    one query: Since this circuit is designed for high frequency, do we need to use any special kind of PCBs and any particular  components .?
      In the circuit we have used normal wires to see the output 

  • 0 in reply to Abhinaba Roy
    TI__Mastermind 28525 points

    Hello Abhinaba,

       Yes, that is correct. For high speed applications, especially for an amplifier such as the OPA846, it is highly recommended to use a PCB to prototype/test your design. Breadboarding your design with very high speed amplifiers cause instability due to the high parasitics. We recommend following the board layout section of the amplifier's datasheet, which for the OPA846 is located on pages 18 and 19. The datasheet even recommends not using a socket for this device:

       "Socketing a high-speed part like the OPA846 is not recommended. The additional lead length and pin-to-pin capacitance introduced by the socket can create an extremely troublesome parasitic network, which can make it almost impossible to achieve a smooth, stable frequency response. Best results are obtained by soldering the OPA846 onto the board"

       I looked over your design requirements and schematic, and your TIA will be unstable with the selected feedback capacitance. For a 5MHz, 10kOhm transimpedance gain, and around 20pF of input capacitance with an added 5pF from PCB + internal amplifier's common-mode and differential input capacitance: 

       Calculator II outputs a required feedback capacitance of around 4.5pF. The frequency response would follow a Butterworth response with a quality factor of 0.707 and a phase margin of 65 degrees. Calculator I outputs the maximum closed-loop TIA bandwidth you can achieve to obtain a Butterworth frequency response when using the OPA846's GBW of 1750MHz. 

      You can find the calculator at this High Speed Amplifier FAQ: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1270018/faq-h-frequently-asked-questions---high-speed-amplifiers.

       And, I would recommend looking over this e2e on layout techniques for high speed TIAs: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1089964/opa855-q1-output-layout-instruction. The most important takeaways are inner ground/power plane cutouts directly under the input/output pins of the amplifier and placing the photodiode as close to the inverting input of the amplifier.

      Also, here is a reference for general high-speed PCB layout techniques: https://www.ti.com/lit/ml/slyp173/slyp173.pdf?

      You can try replacing the feedback capacitor which could result in an improvement. But, eventually, it is highly recommended to design using a PCB. Let me know if you have any further questions.

    Thank you,
    Sima