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LMH6629: cable driver

xyz xyz
Intellectual 1000 points
Part Number: LMH6629
Other Parts Discussed in Thread: LMH6552

Hello

I have an ultra low noise amplifier with a +10V/V gain based on the LMH6629. I want to connect a 1meter 50 Ohm cable at the output of the this stage.

1-now I want to know whether the IC is capable to drive the cable? I should be careful of the output current of the IC? 

2-if I'm gonna use a FDA such LMH6552 after the coax cable, the FDA can eliminate the even order distortions of the LMH6629 stage? the second harmonic of the LMH6629 is not very good. in my application I should keep the distortion better than 90dBc :)

I mean the below architecture :

signal (10KHz to 5~10MHz)---unity gain Buffer(x1)---LMH6629(x10)----50 ohm----1meter 50 ohm coax cable----filter----FDA===ADC

please guide me in a certain way :) bcoz I'm gonna order from overseas :) I mean after ordering I can not buy some ICs again!

Regards

  • 0
    TI__Genius 15975 points

    Hi,

    To answer your questions:
    1. The LMH6629 should be able to drive the 1m long cable. The +/-250mA output current of the LMH6629 should be sufficient to drive the cable.
    2. The FDA cannot completely eliminate the even order distortions of the LMH6629 stage, because the FDA will only buffer or gain the signal it is fed at the cable input. You would certainly need to implement a low pass filter in-between the FDA and the ADC to reduce the even-order harmonics.

    Also, there should not be any restrictions on ordering ICs again, no matter where you are ordering from.

    Best Regards,
    Rohit

  • 0 in reply to Rohit Bhat
    Intellectual 1000 points
    I have an idea.
    I'm thinking of translating a signal to differential with a FDA like below:
    signal--FDA(x10)==two 50 ohm coax cables===FDA(x1)==LPF==ADC
    bcoz in the LMH6552 I've found a line driver but I don't have SFTP cable

    is it a good idea?
    I mean in the real world which one is better?
    the first architecture with one coax cable or the second architecture with two coax cables?

    thanks
  • 0 in reply to xyz xyz
    TI__Genius 15975 points
    Hi,

    If you could drive the signal differentially across the coax cable, that should provide immunity from any common mode noise or improve the even order distortion. So, the architecture with two coax cables is beneficial while driving long length cables. Having said that, however, 1m cable is not too long for 5 to 10MHz frequencies and you should be able to easily accomplish this using a single-ended architecture.

    Best Regards,
    Rohit