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Original question:

LMH6553: Single power supply and single-end input

LMH6553: Single power supply and single-end input

na na78
Guru 16770 points
Part Number: LMH6553

Hi

I'm sorry for my late reply.

In the related thread, it is indicated that high clamp over drive (like 1600%) is too high.

Is the limitation of overdrive is up to 100%?

Our customer examined 4V/V( Rf=270ohm, Rg=62ohm), but output long setting issue is not resolved.

BestRegards

  • 0
    TI__Genius 15975 points
    Hi,

    I don't think the LMH6553 overdrive is limited up to 100%. However, as you keep increasing the overdrive it takes more time for the outputs to recover from the clamp.

    I can probably take a look at an LMH6553EVM on the bench next week, and see if I can replicate as to what the customer sees. However, would it be possible for you to post the updated schematic, so that I can take a look and review it?

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Guru 16770 points

    Hi Rohit

    Thank you for your reply.

    I appreciate if you could check that it would take time for output with over 100% overdrive situation in reference board.

    I'm sorry that I can not send complete design but it shows around LMH6553.

    BestRegards

  • 0 in reply to na na78
    TI__Genius 15975 points

    Hi,

    I measured the clamp performance using the LMH6553 evaluation board, and I cannot replicate the issue which the customer sees for >100% clamp overdrive performance. However, it is important to note that my circuit (shown below) is different than the customer's circuit; but it should be achieving the 16V/V gain setting which is the customer's application use case. So, would it be possible for the customer to replicate their setup as shown below?

    The measurement setup have been kept the same such as the +/-5V supply, Vcm = 2.5V, Vclamp = 3.5V and the input voltage is pulsed from 0 to 2.2V. As you can see from the single-ended output voltage waveform (Vout+), it does not show the slow low-to-high rising edge which is otherwise seen in the customer's setup.

    One thing I noticed is that the LMH6553 being a current feedback amplifier, it is not recommended to add the 2pF parallel cap across the feedback resistor due to stability issues. I am not sure that the slow rising edge could be due to this feedback cap, but it is worth trying whether removing the cap helps address this issue. The other thing is that there is no proper 50-ohm termination at the input/output looking at the customer's schematic which could certainly cause the slow rising edge issue. I would certainly recommend the customer to try out the below circuit and see if the issue is resolved.

    Best Regards,

    Rohit

  • 0 in reply to Rohit Bhat
    Guru 16770 points
    Hi Rohit

    I'm sorry for late reply.
    I appreciate your help! Thank you for your confirmation.

    Did you apply V+=5V and V-= -5V?

    In customer design, supply volage is 5V single supply. (V+=5V and V- = GND)

    Is it possible to check with 5V single supply operation?

    BestRegareds
  • 0 in reply to na na78
    TI__Genius 15975 points
    Hi,

    My setup uses +/-5V supply, and not V+ = 5V and V- = GND. Yes, I should be able to check 5V single supply operation clamp performance and report back.

    Best Regards,
    Rohit
  • 0 in reply to Rohit Bhat
    Guru 16770 points
    Hi Rohit

    Thank you for your reply.

    I'm looking forward to hearing from you.

    BestRegards
  • 0 in reply to na na78
    TI__Genius 15975 points

    Hi,

    I checked the clamp performance for Vs = +5V supply, and the output waveform looks very similar to the +/-5V supply scenario. I certainly don't see the slow rising edge as seen in the customer's setup. I would highly recommend the customer to switch to the circuit I attached above that gives 16V/V gain.

    The current customer's circuit does not seem to have 50-ohm input impedance looking into the LMH6553 input. So, it is possible that the slow rising edge is caused because of it, where the input source and load (LMH6553 input) are not terminated properly.

    Best Regards,

    Rohit