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Original question:

LMH32404: TIA Circuit Compatibility w/ VEMD2023SLX01 Photodiode

LMH32404: Design Tradeoffs for AC-Coupling

Ezira Yimer Wolle
Prodigy 20 points
Part Number: LMH32404
Other Parts Discussed in Thread: OPA4820, LMH32401, TLV3544

Tool/software:

If using photodiodes as current sources, would the simplest way to AC-couple for ambient light filtering be via in-series capacitors prior to the input pin? What would the tradeoffs be to implement this versus setting up another power line to connect the PDs with a negative reverse voltage to make use of the integrated ambient light filtering of the LMH32404? 

Thank you!

  • 0
    TI__Mastermind 23385 points

    Hello Ezira,

     This is a very good question, and we have been experimenting this in the lab ourselves. Let me collect some resources and get back to you.

    Thank you!

    Sima

  • 0 in reply to Ezira Yimer Wolle
    Prodigy 20 points

    Currently re-thinking system performance (which may simplify the circuit design) and wanted to see what you think how suitable the LMH32404 is. My photodiode of interest needs to detect a proportional light input, which it will be exposed to between 0.25-0.5s every 15-20s (max current of 14uA). The photodiode is coated with an ambient light filter (light of interest is IR), which makes me think that AC coupling is not important since most of what im interested is a relatively static light input signal. Is my intuition correct here? 

    Additionally, how would this impact my GBWP calculations since my target frequency is low? I still need high gain but I can probably sacrifice on the necessary bandwidth. My main interest for the LMH32404 is the integrated TIA/mux functionality to save space/cost since I'm building a photodiode array and will have trouble fitting separate multiple multi-channel op-amp/mux ICs on my board so maybe their are other chips that are better suited for my system requirements.

  • 0
    TI__Mastermind 23385 points

    Hello Ezira,

      Sorry for the delay, we are still experimenting in the lab. This type of application probably would not be as simple as in-series capacitors at the input pin without affecting the signal fidelity of the input current. However, this e2e thread seems to have worked for some applications if you take in consideration the voltage bias of PD varying: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/225353/ac-coupled-photodiode-amplifier-differences-on-2-different-circuits. If it does work, the difference would be the increase in noise.

      If the photodiode has a filter that does indeed remove ambient light, then this might remove the concern of the ambient light saturating the amplifier. and your intuition is then correct. You would then not need to worry about ambient light cancellation on the amplifier end. 

      The device has an integrated gain of 20kOhm which with your PD's max current specified, this would be around 100 to 300mV. If you need more gain, I would suggest adding a second stage. Or you could try out the TLV3544 or OPA4820 and create a discrete TIA. This device has 4 channels, and is single-ended regular voltage feedback amplifier. From your previous e2e thread, I believe you are looking at around 3.5MHz of closed-loop bandwidth. Here would be the calculations of the discrete TIA which would need a feedback capacitance for stability (in LMH32401, this is internal to the device which is why the LMH32401 has a certain Cin range or it would be unstable). 

     You can find the TIA calculator here: https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/956151/faq-transimpedance-amplifier-calculator

     For layout, try to place the PD as close as possible to the input of the amplifiers. 

    Thank you,
    Sima