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LMH32401: Looking for the best TIA

Art Mecina
Genius 16655 points
Part Number: LMH32401
Other Parts Discussed in Thread: OPA858, OPA657

Hello Guys,

Good day.

Need your assistance on selecting the best Trans-Impedance Amplifier.

Our customer is designing LIDAR which can measure up to 100 meter. Right now, they are developing prototype board. So, is LMH32401 the best IC for this? Although they prefer modules. Do we have modules for this? And is it possible for sample modules be sent to customer?

They came across sample modules that uses OPA657 and OPA858. These TIA seems to be good for APD diode. But they need PIN diodes, so is LMH32401 more appropriate? Or any better TIA to use?

Thanks in advance!

Art

  • 0
    TI__Genius 15565 points

    Hello Art,

    This will depend on a few factors, a few questions for clarification:

    1. What is the input capacitance of their photodiode?

    2. What is the desired transimpedance gain? What bandwidth are they trying to achieve?

    3. The LMH32401 will show best performance when used in a photodiode sinking current configuration (Negative bias voltage). Are you using a sourcing or sinking configuration for the photodiode?

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    TI__Genius 16655 points

    Hello Hasan,

    Apologies for the delayed response.

    Below are the answer to your inquiries:

    1. What is the input capacitance of their photodiode?

    Answer - It can be chanced between 9pF and 15pF

    2. What is the desired transimpedance gain? What bandwidth are they trying to achieve?

    Answer - It will- be 20K or 50K.


    3. The LMH32401 will show best performance when used in a photodiode sinking current configuration (Negative bias voltage). Are you using a sourcing or sinking configuration for the photodiode?

    Answer - Customer will design as positive bias voltage. As transimpedance amplifier they can use one of the LMH32401, OPA858 or OPA657 components. What is the advantages or disadvantages of these three components ?

    Thanks!

    Art

  • 0 in reply to Art Mecina
    TI__Genius 15565 points

    Hello Art,

    No mention of the desired bandwidth mentioned in your response, please provide when you can. Also can you provide supplies that will be used here? In regards to the three TIA choices you provided, the LMH32401 will differ the most from the other two and may not be suited for your design. 

    To begin, using the LMH32401 will limit your maximum transimpedance gain to 20k-ohms. Also if using the LMH32401 with a positive bias, device performance will be limited (reduced output swing & longer overload recovery times). Some potential benefits are the you will be able to easily switch between two transimpedance gains, you have a differential output with Vocm control & you have ambient light cancellation feature. 

    Now between the OPA858 & OPA657 I will list a few advantages/disadvantages, however I'll hold off on recommending a specific device until desired bandwidth is known. By using an unnecessarily high GBW amplifier for your TIA without additional filtering, you may end up with greater overall noise in your output signal. 

    Advantages for OPA858:

    • Larger GBW
    • Lower broadband noise
    • Better slew rate
    • Greater output swing assuming same supplies
    • Greater output drive
    • Lower input capacitance
    • Power-down feature

    Advantages for OPA657:

    • Larger supply voltage range
    • Lower power consumption

     Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    TI__Genius 16655 points

    Thanks Hasan!

    I will let you know as soon as I got the confirmation from the customer regarding the bandwidth and supplies.

    Art

  • 0 in reply to Hasan Babiker31
    TI__Genius 16655 points

    Hello Hasan,

    The customer will use between 80 KHz and 150 KHz frequencies. GBP of the TIA will be 1.6 GHz.

    The duty cycle rate will be between 0.1% and 1%. Also could you explain what you mean “additional filtering” for TIA?

    Thanks!

    Art

  • 0 in reply to Art Mecina
    TI__Genius 15565 points

    Hello Art,

    The devices you are looking at are way above the requirements needed to build this trans-impedance circuit. Note that the transimpedance gain doesn't determine the GBP of the amplifier but rather the noise gain of the circuit (which is mostly shaped by your input and feedback capacitors). You can use this calculator to figure out your amplifier requirements. 

    Note that this calculator uses the assumption that Cin >> CF, so results may seem a little off for this specific use case. This can be fixed if we increase your TIA gain further as i've done below. But the main thing to note here is that these requirements can be achieved with an amplifier that has a GBP < 1MHz. 

    By additional filtering I just meant adding an additional low pass filter following your TIA circuit. 

    Best,

    Hasan Babiker

  • 0 in reply to Hasan Babiker31
    TI__Genius 16655 points

    Thanks Hasan!

    Art