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OPA132: Extreme cable length driver

Jesse Meyer
Prodigy 20 points
Part Number: OPA132
Other Parts Discussed in Thread: OPA211, , MAX3221

Tool/software:

Hello everyone,
I have an interesting project where I need to create a line driver that can signal across thousands of feet of cable. Cable will be between 75 and 110 ohm and between 1.2 and 1.7 uF.

Our target is between 40 and 200 kbaud.

For reference we are currently do 50kbaud by feeding our CMOS level outputs into a OPA211 that then buffers to a push pull BJT to signal 30 VP2P.  Simple but effective:
Current Design

I am wondering if switching to OPA132 or possibly MAX3221 (we are open to sending RS232) would yield better results or should we take steps to improve the circuit design from another IC.

Min and max baud rates will be 50k and 200k so I am calculating a minimum slew of 4.7V/us and to reach full speed 18.8V/us.

Also, to lower ISI we only pulse on transition instead of entire clock cycle. For example:

I'd like to be able apply pre-emphasis to the pulses but don't see any solutions that build that into an IC beyond the MAX3221.

If anyone has experience with extreme cable length signaling, open to try new ideas.

  • 0
    TI__Mastermind 23588 points

    Jesse,

    Op amps will become unstable when they drive heavy capacitive loads.  This drawback can be eliminated by compensating the amplifier.  Generally, this amounts to adding a resistor between the amplifier output and the capacitor (called Riso method).  There are many other methods, but the end result is generally a bandwidth limitation significantly below the expected bandwidth for the circuit.  When you are trying to buffer a digital signal, I think a better solution would probably be to use a comparator or a digital line driver chip.  These kinds of devices are only driving to a logic-high or logic-low state and do not have the issues that amplifiers with stability. They may still have limitations in what capacitance they can drive.  I think your idea for the MAX3221 is probably a better approach.  You should post on the forum associated with the MAX3221.  They may have other suggestions or advice.  You could ask the comparator forum as well.

    Best regards, Art

  • 0 in reply to Art Kay
    Prodigy 20 points

    Appreciate the help Art. I will cross post.