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LMH6629: Ultra small photodiode AC current amplification (2 MHz) using TIA

Simon Francois
Prodigy 70 points
Part Number: LMH6629
Other Parts Discussed in Thread: OPA855, OPA858

Dear Specialists and TI users, 

In past when implementing Photodiode amplification stage for VLC I usually used a transimpedance amplifier such LMH6629 and then use a high speed comparator set at Vref. 

However here I need to amplify very small current at the order of nA or tens of nA from S1223 Photodiode (Cp = 10pF) and remove DC bias. For such small current the voltage variation at the output of the TIA is too small in order to connect directly the comparator. 

I tought to use Voltage amplifier in cascade but I am afraid of the noise figure and also would tought how removing DC bias regarding ultra small AC variation (f=2MHz). 

If possible for you of course, I would really appreciate to have your advice on these question. 

I wish you a nice day, 

Yours, 

Simon 

  • 0
    TI__Genius 17671 points

    Hello Simon,

    As I understand it you require help with the following topics:

    - Selecting a gain-stage amplifier to go in-between the TIA and comparator stage

    - Removing DC bias from TIA -> Gain -> Comparator chain

    I can work on a simulation and recommend to you a circuit setup for this application.  When you use the term VLC, are you referring to visible light communication?

    Please allow me time to build a circuit and check the stability of my design.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 70 points

    Hello Alec, 

    Firstly let me thanks you a lot for your reply. You really not need to provide a solution it was more for asking a global advise if it is a good idea to go trough this solution by using Tia and then to amplify the output of the tia using different opamp stage while before I was directly connecting the TIA to the comparator.

    For the DC bias I tough to use simply a high pass filter between the TIA and amplification stage or a DC servo to remove dc current. What do you think about this ? 

    Thanks you for time and help for my question.

    have a nice day,

    Simon 

  • 0 in reply to Simon Francois
    TI__Genius 17671 points

    Hello Simon,

    Yes, your solution with a gain stage between the TIA and comparator is a valid one to approach the design.

    I have supported customers before using TIAs such as OPA855, OPA858, and LMH6629 as both a TIA and gain stage in signal chains.

    I will check with my team on your question about removing the DC current.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 70 points

    Hello Alec, 

    Thanks for your reply. I will try to design using OPA 855. For DC removal I ll really appreciate some advice on my proposal. 

    Again thanks you, have a nice day, 

    Simon 

  • 0 in reply to Simon Francois
    TI__Genius 17671 points

    Hello Simon,

    I will take a look at an LMH6629 + OPA855 design.  Did you have a comparator selected?

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Prodigy 70 points

    Hello Alec, 

    Firstly sorry for my late answer, I was abroad with limited access to TI forum. 

    Actually I do not have. I tought to take a recent TI high speed comparator but didn't looked yet on it. 

    Thanks you, 

    Simon 

  • 0 in reply to Simon Francois
    TI__Genius 17671 points

    Hello Simon,

    Thank you for the update.  Please let me know about questions or support you need as you proceed with you analysis and design.

    Best,

    Alec