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Question about DS30BA101 cable Driver and output impedance ...

Jesse Chamberlain
Prodigy 40 points
Other Parts Discussed in Thread: DS30BA101, DS30EA101, DS15EA101, DS15BA101, DS32ELX0124, DS32EL0421, DS32EL0124, DS32ELX0421

Hello All,

I was looking at using the DS30BA101 Cable driver and corresponding DS30EA101 cable equalizer in a system I am designing.  In the datasheet on page 7, it shows a configuration with these chips using a 75 ohm unbalanced co-ax transmission line.  Here is the question I have, instead of driving a 75 ohm load, can this device drive a 50 ohm unbalanced co-ax cable?  Can this part do that by simply changing the resistor configurations to be 50ohm/25Ohm values?  For my application, I can't use a 75Ohm coax connector due to constraints, and 50 ohm is required. 

These parts seem very similar to the DS15BA101/DS15EA101 chip pair, but I need to go up to 3.125Gb/s.   If this chip pair can't do it, are their other chips that TI has that can drive an unbalanced 50 ohm co-ax from DC upto 3.125Gb/s? 

Thanks for any help.

Jesse C.

  • 0
    TI__Expert 4400 points

    This application is outside of the DS30BA101 / DS30EA101 datasheets, but it should still work with the following caveats:

    • The cable length you can equalize will be shorter than what is advertised in the DS30EA101 datasheet
    • The equalizer output jitter will be slightly higher than what is advertised in the DS30EA101 datasheet

    The DS30BA101 is designed to drive 75 ohm coaxial cable (single ended) or 100 ohm differential cable.  The datasheet shows the proper cable driver configurations so that, when paired with the DS30EA101 adaptive equalizer, the equalizer can properly recover the signal on the other end of the cable.

    For the DS30EA101 to adaptively equalize and properly recover the signal, the DS30BA101 output amplitude (going into the cable), should be 800 mVP-P +/-10% (single ended).  The datasheet specifies the settings to drive this amplitude into 75 ohm coax or 100 ohm differential cable.

    The DS30BA101 can technically be used to drive 50 ohm coax, but it cannot drive 800 mVP-P into 50 ohm coax due to current limitations.  Note that the DS30BA101 output amplitude is inversely proportional to the value of the RVO resistor.  The absolute lowest value of RVO that should be used is 665 ohms, and with this value (and 50 ohm output pullup resistors, driving into 50 ohm coax cable), the DS30BA101 output amplitude will be about 600 mVP-P.  While this is not the ideal value for driving coax cable to be equalized by the DS30EA101, the DS30EA101 equalizer is fairly robust to launch amplitude variations and will still work (with the above mentioned caveats).

    Refer to Figure 2 (Cable Extender Chipset Application Circuit for 75 ohm Coaxial Cable) in the DS30BA101 datasheet.  To configure the devices for your application with 50 ohm coax, replace the RVO resistor with 665 ohms, replace the 75 ohm resistors with 49.9 ohms, and replace the 37.4 ohm resistor with 24.9 ohms.

  • 0 in reply to Gary Melchior
    Prodigy 40 points

    Thank you for that answer.  The desired length that we wish to drive this signal is about 20ish meters.  I plan on prototyping this interface to verify the signal integrity for our configuration. 

    Are their any other TI chips that are designed for, or can be configured to drive a 50ohm co-ax cable, that goes up to 3.125Gb/s?

    Thanks!

    Jesse C.

  • 0 in reply to Jesse Chamberlain
    TI__Guru* 93670 points

    Hi Jesse,

    FPGA Link SerDes can support this rate(3.125G). Please take a look at DS32EL0421 or DS32ELX0421 and DS32EL0124/DS32ELX0124 respectively. 

    You mentioned 50 ohm coax, can we be more specific and let us know the type and etc. Also, what is the potential volume for this application.

    Regards,,nasser

  • 0 in reply to Nasser Mohammadi
    Prodigy 40 points

    Hi Nasser,

    We actually are using that particular chip set for an existing application, but we are using then in a differential (balanced) SMA configuration.  Can these parts be used in a single-ended (unbalanced) config, ie driving only one SMA connector?   What length of cable does the integrated cable driver and equalizer support?  (approx, would they be able to go 20-30 meters?).

    We would be using RG-316 co-ax for our application.

    Thanks.

    Jesse C.

  • 0 in reply to Jesse Chamberlain
    TI__Expert 3050 points

    Hi Jesse,

    I support the DS32EL(X) products. The DS32EL0421 and the DS32ELX0421 are capable of driving 100 ohm differential, 50 ohm single ended and 75 ohm single ended interconnects. By default the devices are configured to drive 100 ohm differential or 50 ohm single ended signals. To properly drive one 50 ohm single ended interconnect you will need to properly terminate the unused high speed output to balance the CML driver. Connect the unused output to a capacitor, either 0.1uF or 0.05uF, and then connect the capacitor to a 50 ohm resistor to ground.

    The length of cable supported varies greatly depending on the cable's loss characteristics. We have not tested RG-316 type coax cable, so I cannot give you are definitive maximum cable length. 

    If you have any questions, please let me know.

    Mike Wolfe

    DPS APPS / SVA