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50 ohm double ended 0-5V rail-to-rail, high speed line driver

C P
Prodigy 50 points

Hello, 

I am somewhat new to RF signal work. I have a device that requires a 0-5V TTL pulsed signal input. The input load is 50 ohms and the pulses are 5 to 15 ns in width with rise/fall times of ~1-3 ns. I have a pulse generator circuit that produces sharp 5 - 15 ns TTL pulses and I would like to use it to trigger my 50 ohm device through a 12" SMA coaxial cable that is either singly or doubly terminated. As such, I need a 0-5V rail-to-rail line driver capable of high speed switching and sourcing currents of 100mA. I am also not sure if I will have to use a doubly terminated line to reduce reflections, in which case I would need a driver with a 2V/V output.
Another option is to replace the AND gate that is currently outputting my TTL pulse signal with an AND gate capable of sourcing sufficient current. 

From my understanding, many of the differential ADC drivers can be used as 50ohm line drivers, but this seems like overkill. Would you be able to help me select an appropriate component that can meet my requirements? Thank you,

Angus

  • 0
    TI__Mastermind 20820 points

    Hi Angus,

    I have to dig around in the portfolio to see if we have something that is going to meet your design needs. I will follow up with you on Monday when I have some answers.To clarify, you are looking for a TTL driver capable of sourcing 100mA? Single channel?

    Regards,

    Mike

  • 0 in reply to Michael Peffers
    Prodigy 50 points

    Hi Mike, 

    Apologies for the slow reply. I am looking for any line driver (single channel is fine) capable of sourcing 100mA and switching 0-5V at at least 200MHz. This could be a digital device, or an op-amp. I would have guessed driving high speed TTL levels through 50ohm coax would be a common problem for anyone developing products to interface with lab equipment, but it has been very difficult to find a line driver for the job. Thanks, 

    Angus

  • 0 in reply to C P
    TI__Genius 13475 points

    Angus

    You are saying that you want to switch 0-5V, but also state TTL level signals - going 0-5V would imply CMOS level signals, which if you are restricted to a singe 5V supply will make things difficult.   If the signals are truly TTL level, then we could use a buffer which was not completely rail-to-rail.   Otherwise, to meet the impedance matching requirements implied, you will need to use power supplies outside of the 0-5V range.

    Mark Sauerwald

     

  • 0 in reply to Mark Sauerwald88474
    Prodigy 50 points

    Hi Mark, 

    The output signal at the termination end of the coax must be LV-TTL levels (high level input threshold 2.0V, low level threshold 0.8V). Ideally I would be using doubly terminated coax (50ohms terminated at each end), in which case I need to drive ~5V at the output of the driver to safely achieve 2.5V at the termination of the coax. I was thinking a high speed op-amp might be capable of this, but the 5V output is pushing things and it has been difficult for me to find a suitable device.

    As an alternative, it would be great to find a solution with more flexibility to the voltage driven at the device output. For instance a device capable of driving 0-10V to achieve 0-5V at the termination end of the coax. I do not think this is possible with any op-amps you offer, but perhaps there is a high speed FET driver that could accomplish this. If there is a way to use a fast switch this would be an ideal solution because of its simplicity and flexibility. Regards, 

    Angus

  • 0 in reply to C P
    TI__Genius 13475 points

    Angus

    If it were me, I would probably look to a driver consisting of a high speed Op-amp and an external transistor inside the loop.    this would allow you to have an op-amp which runs off of the 5V rail, and have a second, higher voltage rail (8 to 10V?) which would supply the current to the drive transistor.   If you used a 50Ohm resistor the the high rail, then this would provide your back termination.

    Mark