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LMH6553: How to use the LMH6553SD/NOPB

Part Number: LMH6553

to whom it may concern

We apologize for the sudden contact. My name is Daisuke Suzuki from Olympus DEM Technology 2.
Please tell me about the LMH6553 (differential amplifier).

・Regarding the output signal, is it prohibited to use Pch open and Nch 50Ω termination?
We would like to use this differential amplifier within our company, but on top of that, we would like to consider stopping the signal on only one channel.
Originally, I think that a single amplifier should be used, but it would be helpful if you could teach me.

(example)
Should the Nch side be open if the Pch side is open?
If the Pch side is 50Ω, should the Nch side also be 50Ω?
Is the Pch side open and only the Nch side 50Ω unusable?
Such an image.

Thank you very much.

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DEM技術2
daisuke.suzuki13@olympus.com
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  • FDA's like the LMH6553 prefer a balanced load to get the best 2nd harmonic suppression. You could physically operate with just one output as the feedback R will provide some load, but there are no specifications in that mode. 

    You should probably use a dual op amp instead - perhaps with disable on each channel seperately. 

  • Thank you for your reply.
    I'm sorry, I seem to have made a mistake. . .
    Could you tell me how to fix it, if necessary?

    Please let me know one more point.
    In the case of the attached circuit, if the - side is open and the + side is connected to 1 MΩ (assumed to be open, R242 not installed, R249 installed), is it correct that the balanced state is maintained? I understand that there is no problem because the signal itself on the + side is not stopped.

  • Hello Suzuki,

      Thank you for sharing the schematic. A few questions:

    1. Are you looking for a differential input to single ended signal conversion?
    2. Will this be AC coupled at the input? 
    3. What is your bandwidth and input signal characteristics (signal shape, voltage input range, etc.) 
    4. Would you be able to share the specifics of the application or end equipment?

       In the case you have shown, I agree with Michael that the FDA (fully differential amplifier) will be unbalanced. The LMH6553 or most of our FDAs will work as a single-ended output if the FDA drives the same matched load at the positive and negative; however, the THD will be degraded as Michael explained with the 2nd harmonics. If your signal is AC coupled, then a transformer may be used at the output; but this will depend on the answer to 4th question where we might suggest another amplifier that works best for your application. 

    Thank you,
    Sima 

  • Thank you for your reply.
    1. I wanted to ask about how to use this device when using differential as a single, and the purpose was to know if this usage would make it unusable. I didn't want to change the differential input to single ended. If there is a single-ended device version of the LMH6553, I would like to consider it.
    2. Assumes input AC coupling.
    It is a clock (square wave) of about 1V at 3.100MHz.
    4. It will be the next generation endoscope. It is input through a long cable of about 4m.
    I would like to consider actually checking the waveform with this configuration and evaluating the influence of the second harmonic.

  • Hello Suzuki,

      1. You may use it as a single-ended output if impedances are matched, but would affect harmonics. Since input AC coupled, you could use a transformer at output for single-ended use while retaining FDA distortion. 

      3. Is this for both inputs, are they out of phase from one another? This is important for above conversion and understanding if another type of amplifier would be needed.

      4. Thank your for end application. What is the fully differential amplifier driving? As in what is the load or next stage after this amplifier?

    Best Regards,

    Sima