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Powering multiple optical front-ends

Pavel Yermolenko
Intellectual 280 points

Other Parts Discussed in Thread: OPA857, LMH6628, OPA657, THS4631, TPS7A21, TPS7A20, TPS7A57, TPS7A47, TPS7A96, TPS7A94

Hello,

There are 8 optical front-ends on the board ... the front-ends based on OPA657 (or LMH6628, OPA857, THS4631).

Is it important to use one LDO per front-end  or  powering all 8 frontends by a single LDO will be also Ok ?

Thanks.

  • 0
    TI__Mastermind 22860 points

    Hi Pavel,

    There are no big concerns from an amplifier perspective if you use one LDO to power multiple devices. The concern is an LDO device that has trouble providing current to all the devices at any given point. This will be specific to the design and the current the amplifiers need to provide which will be coming from the LDO output. However, from a device perspective as long as the LDO is providing the required current the device should behave as expected.

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    Intellectual 280 points

    Hi Ignacio,

    Thanks. For pure analog front ends, a single LDO for multiple front ends is probably fine, but what about mixed-signal front ends, like the LT1328? It generates a TTL output, so probably to noisy-isolate different front-ends it would be more prudent to use one LDO per front-end, or, at least isolate the different front-ends by ferrite beads... given that LT1328 has a low pass internal filter ? Or does this caution seem exaggerated?

  • 0 in reply to Pavel Yermolenko
    TI__Mastermind 22860 points

    Hi Pavel,

    There should be no real concern for having one LDO power multiple devices if proper decoupling is done for the supply pins. However, I will transfer this thread to our LDO team as they can give you the best advice for proper implementation of their device and offer low noise LDO options for your design. 

    Best Regards,

    Ignacio

  • 0 in reply to Ignacio Vazquez Lam1
    TI__Mastermind 37945 points

    Hi Pavel,

    The LDO will operate just fine with this load as long as the load is not too high to trip current limit protection, or the power dissipation is not too great to trip thermal protection.  At a system level you will want to assess if transient conditions induced by one front end will interfere with the operation of another front end.  The performance of the LDO will depend on which one you select but even our state of the art performance LDOs (TPS7A57, TPS7A94, TPS7A96, TPS7A47, or perhaps the TPS7A20 or TPS7A21) still have some small transient response.  Filtering using ferrite beads may help but please make sure the LC filter is damped (we usually recommend Q<2).

    Thanks,

    Stephen