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Original question:

LMH6554: DC Coupled driver for ADS42LB69

LMH6554: Fast overdrive recovery info for LMH6554, and LMH6881?

Poh Kok Keng
Genius 5352 points
Part Number: LMH6554
Other Parts Discussed in Thread: LMH6881, , LMH32404, LMH32401, LMH34400, LMH6654

Dear Support,

I need some advice on this TIA design for OTDR. 

For the following design block, i am using LTC6268 -> LMH6554 -> LMH6881 -> LMH6881.

One problem i faced is when there is an optical pulse arrived at 5V @3ns pulse. it will overdrive the circuit and go into saturation.

I am not able to find what is the fast overdrive recovery parameter for LMH6554, and LMH6881? Can you tell me what is the overdrive recovery value for LMH6554 and LMH6881 ?

The above circuit take microsecond to saturate before it can return to normal operation. Do you have recommendation how I can avoid the circuit enter into saturation? or any suggestion how to avoid that?

One of the competitor part i found is LT5554 which stated 5ns fast overdrive recovery which i think might be good to test out. Does TI have similar devices which fast overdrive recovery in ns? 

Best Regards,
Felix

  • 0
    TI__Genius 17536 points

    Hello Felix,

    I can find and confirm the overdrive recovery specifications for these two devices, the LMA6654 & LMH6881.  Could you clarify why your design would necessitate these two devices to undergo overdrive recovery?  Is there not an option to design-in circuitry to mitigate the overdriven signals?

    Best,
    Alec

  • 0 in reply to Alec Saebeler
    Genius 5352 points

    hi Alec,

    Yes, the input signal is a optical signal in ns, with the gain of 60 to 80dB, any small input signal will saturate the input circuit. Due to this saturation, the circuit will enter saturation for a few ms before it return to normal. That is the reason why we need a fast overdrive recovery to make sure the circuit return to normal operation in ns range. 

    Do you have any suggestion on signal conditioning to make sure the circuit dont saturate? what device or any reference design we can reference to?

  • 0 in reply to Poh Kok Keng
    TI__Genius 17536 points

    Hello Felix,

    Have you also considered our TIA devices which have particular features well-suited to optical signalling?  Such devices include the LMH34400, LMH32401, and the LMH32404.  If integrated devices are not well-suited, our OPA85x family of devices may also be performant here.

    I can look into whether there is a way to avoid saturating the LMH6654 or LMH6881 as well.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    Genius 5352 points

    hi Alec, 

    Have you manage to find the overdrive recovery spec for  LMA6654 & LMH6881 to share? 

    Best regards,Felix

  • 0 in reply to Poh Kok Keng
    TI__Genius 17536 points

    Hello Felix,

    Both parts are older and I cannot find the overdrive recovery spec among data I have access to and can locate.  My concern is the test was not performed when these devices were characterized, which is why the overdrive recovery specification is not present on the LMH6881 and LMH6654 datasheets.  I am asking my colleagues to take a look as well.

    Best,

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 17536 points

    Hello Felix,

    I was able to discuss with my colleagues; I do think the overdrive recovery and optical TIA application needs are met by our OPA85X family of parts.  If you do still require overdrive recovery data on these LMH parts, do know that with a high speed pulse generator and an EVM, you or your customer can investigate and verify performance data.

    If you and your customer are not familiar with the OPA85X family of devices, they are highly capable and effective TIAs designed to be operable in LiDAR and other optical front end designs, among other TIA uses.  All parts in this family have overdrive recovery specified in the device datasheet.

    Best,

    Alec