DB-25 Signal Buffer w/ op-amp

Discussion in 'The Projects Forum' started by schwactopus, Feb 22, 2011.

  1. schwactopus

    Thread Starter New Member

    Feb 17, 2011
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    I am designing a signal buffer/splitter that takes analog signals (+/- 5V) (<< 30kHz) from a DB-25 cable and splits them among several outputs, also DB-25. Essentially each output is an exact copy of the input signals. At the moment, I am using TL074 chips for the outputs with one op-amp per output signal. The opamps are set up as "voltage followers" (inverting input tied to output) and the output is tied directly to the output pin with no extra circuitry.

    My question is whether or not I need any more circuitry to drive the signals down the DB-25 or if the op-amps will suffice. This is the first time I've dealt with signal buffers and unfortunately they don't teach you anything practical in school (at least in my experience...)

    If it's at all confusing I can try to upload schematics, but it's straight forward so I figured I could get away without any schematics.

    Owen
     
  2. CDRIVE

    Senior Member

    Jul 1, 2008
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    The answer to your question could be better answered if we know what the length of your cable is and what your load impedances are.
     
  3. schwactopus

    Thread Starter New Member

    Feb 17, 2011
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    Cable length is anywhere up to 300' (this project is going to be used in many different circumstances). I do not know the load impedance but I'm pretty sure it draws very little current. (I can find out more tomorrow). I set up a prototype on a breadboard using some cheapo quad op-amp chips (don't know the p/n off the top of my head, but can find out tomorrow) and it seemed to work fine, I was just wondering if there is some issue that may arise that I haven't considered.

    Thanks a lot
     
  4. CDRIVE

    Senior Member

    Jul 1, 2008
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    That's a long run. Is the signal audio?
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    For a long run, you may want to consider a differential pair per channel.
     
  6. CDRIVE

    Senior Member

    Jul 1, 2008
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    Yes, and CAT5 is a heck of a lot cheaper.
     
  7. schwactopus

    Thread Starter New Member

    Feb 17, 2011
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    Unfortunately it's not my call which cable to use. I'm just complying with an already established standard. The signals are differential and each channel terminates at an op-amp differential amplifier like the one in the picture: http://www.electronics-tutorials.ws/opamp/opamp18.gif R1=R3=5k R2=R4=10k. V1 is the negative signal and V2 is the positive signal. The op-amp being used at the terminating end is LM324 (I think...). (I'm only designing the input stage, before the DB-25 cable. The other side of the DB-25 cable has already been designed and I'm trying to interface with it.)

    The prototype I have set up right now seems to work fine I'm just trying to figure out if there's something I'm not thinking about in the design.

    BTW the signal is NOT audio. It has several channels corresponding to different aspects of an image (such as X and Y coordinates, color values, etc) and as a result the signal is somewhat periodic, but has a very large period. I haven't looked at any of the signals with a scope but if I have to guess, I would put a maximum frequency at a few kilohertz.

    Thanks for the response so far.
     
  8. CDRIVE

    Senior Member

    Jul 1, 2008
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    Your link was bad.

    Typically video is transmitted over 75 Ohm Coax. When transmitting video over any cable VSWR needs to be considered and handled very similar to RF over a wire medium. I'm not saying that a 75 Ohm coax is required for video. What I am saying is that all conductors running parallel to each other will have a characteristic impedance, and the output impedance of the source, and the input impedance of the receiver should match the characteristic impedance of the line. Not doing so will cause video degradation, ghosting, and if severe enough ... sync problems. If the video is color things get even worse.

    If we can see what the receiver opamp looks like we can determine what the input Z is. Try posting it again.
     
  9. schwactopus

    Thread Starter New Member

    Feb 17, 2011
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  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    Unfortunately your link depicts a typical (generic) Differential OpAmp. It doesn't show us what you have at the other end of your cable. We don't even know if it's AC or DC coupled, no less the input Z.

    Normally, I would expect the manufacturer to supply the specs, including, drive requirements.

    Perhaps we have a member who's familiar with what you have.
     
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