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Original question:

LMH6551: Validity in using a fully differential amplifier to amplify a photodiode signal

LMH6551: Question Regarding Using FDA as TIA – LMH6551 vs THS4567 vs THS4509

문 찬희
Prodigy 50 points
Part Number: LMH6551
Other Parts Discussed in Thread: THS4567,

Tool/software:

Dear TI engineers,

Thank you for the helpful response on the TI forum. I’ve carefully read the following post:
https://e2e.ti.com/support/amplifiers-group/amplifiers/f/amplifiers-forum/1337724/lmh6551-validity-in-using-a-fully-differential-amplifier-to-amplify-a-photodiode-signal

However, I still have some questions and would greatly appreciate further clarification.

According to the discussion, it seems that using an FDA as a TIA is not practical in real applications. Is the main reason due to:

  • The transformer-based internal architecture of the FDA?

  • The difficulty in maintaining proper photodiode biasing when connecting the cathode and anode to the differential inputs of the FDA?

One point I’m especially curious about is that the THS4567 datasheet (page 1) shows a differential TIA configuration.
Does this mean that FDA-based TIA implementations are possible with certain devices like the THS4567, and that it is rather the internal structural differences between the LMH6551 and THS4567 that make it less feasible in the case of LMH6551?

I’m still learning system-level analog design, so I’m asking to better understand the design trade-offs.
For reference, the paper I’m currently studying directly connects the cathode and anode of the photodiode to the FDA differential inputs. The title is:

“A wide dynamic range CMOS laser radar receiver with a time-domain walk error compensation scheme”

I would be grateful for your insight whenever you have time.

Best regards,
Chanhui Moon

  • 0
    TI__Mastermind 24289 points

    Hello Chanhui,

    I apologize; I wrote a response but the webpage did not capture and submit the response to you.

    The THS4567 has additional design elements which allow for setting the input common mode externally; this enables the THS4567 to act as a TIA and provide differential amplification to the next stage in the signal chain without a separate TIA.  While a typical FDA such as LMH6551 can do FDA tasks well, the device lacks the consideration for TIA function that was enabled in the THS4567.

    In short, the THS4567 was designed to be able to have additional functions over a traditional FDA, with the trade-offs and functional modes listed in the datasheet.

    Best,

    Alec