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OPA380: choose optical AMP

Tommy Tzeng
Genius 5895 points
Part Number: OPA380
Other Parts Discussed in Thread: OPA859

Tool/software:

Hi Sir,

may we know two quesiton, and learn from you,

1. According to the reference circuit on page 11 of the datasheet, with VBIAS set to 250 mV, the output signal swing is only about 200 mV (with an input optical signal strength of -21 dBm). However, the datasheet specifies that Vout can go up to 4.4 V.

2. Using the -3 dB bandwidth calculation formula on page 11 of the datasheet, with RF = 10 kΩ, the calculated bandwidth is about 29 MHz, but in practice the circuit fails at around 4 MHz.

3. With an input optical signal of 2 MHz, 50% duty cycle, and strength from -15 dBm to -27 dBm, after amplification by the OPA380, the output only works properly at around -18 dBm to -20 dBm, producing a square wave with an amplitude of about 200 mV.

we question as below:

1. Is the OPA380 suitable for this design requirement? If not, could you suggest an appropriate TIA solution?

2. How can I ensure stable reception of optical signal strengths in the range of -15 dBm to -27 dBm?

  • 0
    Genius 5895 points

    Hi Team,

    may i know any update or suggestion?

  • 0 in reply to Tommy Tzeng
    TI__Mastermind 29700 points

    Hello Tommy,

      I am sorry for the delay on your questions. Below includes feedback on your list of questions:

    1. The IN+ pin of the amplifier is set to VBIAS of 250mV because the amplifier is in single-supply configuration, and amplifiers, even if rail-to-rail have headroom from the supplies that need to be followed otherwise the amplifier will saturate and be operating in a non-linear region. Therefore, the headroom is accounted for by adding this voltage in case the input pulse is at 0V, then the pulse will be level shifted to 250mV, avoiding clipping at 0V. The Vout can still go up to 4.4V on a 5V supply as shown in specification table: 



      From above, if using a 5V single supply config, your output voltage range is typically 60mV to 4.6V, or max 100mV to 4.4V. 

    2. The formula on page 11 is an estimate for max bandwidth, but total bandwidth will be limited by feedback capacitance and Q factor which is not included in the equation. We recommend using this TIA calculator located here: https://dev.ti.com/gallery/view/TIA/TIA_GFN/ver/1.0.4/

    3. Do you happen to know the estimated input current range prior to the OPA380 amplification to dbM value range you listed? What is your load Z(Ohms) for these values?

      1) and 2) 
      For suggesting a suitable amplifier, what is you expected input range from #3, what is your photodiode capacitance, and what is your pulse rise time. We have other voltage feedback amplifiers that would work better, since you can use any regular VFB as a TIA and does not need to be advertised as transimpedance amplifier like OPA380.

    Thank you,
    Sima

  • 0 in reply to Sima Jalaleddine
    Prodigy 90 points

    Hello Sima:

    Thanks for your reply. This is my case and let me give you more information about this project.

    The key requirement about this project are

    1. DC~6MHz 50% duty cycle laser impulse, laser wave length 1310nm

    2. Laser Power between -15dBm~-27dBm

    I choose Photodiode Spec as below

       

    The photo current should be  between 28.4uA@-15dBm  to  1.8uA@-27dBm

    If OPA380 is not a good choice for this project, Would you can give me a better suggestion? 

  • 0 in reply to Gary Chiang
    TI__Mastermind 29700 points

    Hello Gary,

      Thank you for sharing the details. 

      I recommend using the OPA859 instead. The OPA859 is a FET input voltage-feedback amplifier with 900MHz GBW. FET-input amplifiers are best for high gain applications of transimpedance amplifier configurations.

      From your design requirements, you would need around 500MHz of GBW considering headroom of x2 on your bandwidth requirements, adding up expected input capacitance, and 60kOhm feedback resistor. 60kOhm feedback resistor was chosen due to your input range, at 1.8uA this will produce 100mV at the output and at 28.5uA this will produce 1.7V at the output. Therefore, you would need to 5V single supply with common-mode input (VIN+) biased to anywhere between 1.5V to 2.3V to adhere to the output voltage range of the device. If you want to bias directly at 2.5V (mid-supply), you could adjust feedback resistor down a bit to 50kOhms. 

    Thank you,

    Sima 

  • 0 in reply to Sima Jalaleddine
    Prodigy 90 points

    Hello Sima:

    Thanks for your recommendation, it very helpful  for this project. 

    The test PCB for OPA859 have been manufactured. It should be ready in few days.

    I will let you know the test result.

    Thank you 

    Gary

  • 0 in reply to Gary Chiang
    TI__Mastermind 29700 points

    Hello Gary,

      No problem! Thank you for the update, and yes please let us know how it goes.

    Thank you,

    Sima