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Thread: Current on the ILDA cable pins

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    Default Current on the ILDA cable pins

    Just a quick question, is there a minimum amount of current/power that a DAC must be able to deliver to the data pins on the ILDA cable? I couldn't find anything about that in the ilda standards docs, only voltage information. I mean, can you use a hypothetical opamp per channel that can only handle like half a milliamp at max signal, or will that end up going up in smoke?

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    Quote Originally Posted by Grix View Post
    Just a quick question, is there a minimum amount of current/power that a DAC must be able to deliver to the data pins on the ILDA cable? I couldn't find anything about that in the ilda standards docs, only voltage information. I mean, can you use a hypothetical opamp per channel that can only handle like half a milliamp at max signal, or will that end up going up in smoke?

    The ILDA standard is designed to prevent the issue using 150 ohm series resistors and the right op-amp.
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    The ILDA standards were not written like a IEEE standard, but if your a good technician and read between the lines, that data is there.
    They specify the impedance of the cable, the ability to drive a certain amount of cable capacitance, and how the line receiver works. They specify that the cable conductors be fed through a 100 to 150 ohm termination resistor to match the cable impedance and to partially protect the system from shorts. They specify a current limit on the driver.
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    If built properly, an ILDA line driver is thus "doubly terminated" by the series resistor or is current limited.
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    OK, so lets do a worst case, a shorted failure of a an op-amp to ground, on a graphics output line... ie the Direct Current Worse Case with a shorted cable with the specified ILDA source termination. E= IR, thus E/R= I. So 12 volt supply rail divided by 150 ohms = .080 Ampere, = 80 milliamps. The specified TL084 opamps self limit to about +/- 20 ma of output current anyways.
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    They specify the ability to drive 0.1 microfarads of cable capacitance , which is around 350 meters of cable. So at 3 KHz AC, which is a reasonable number for an ILDA signal, the capacitive reactance of 0.1 uF is ~ 5200 ohms. No problem there as a load for an opamp. At 30 Hz, the low end, that calculates to 50,000 ohms, no problem there. So the AC signal loading with the specified cable, running differential is reliably low, if you have a proper line receiver, which will have an input impedance of 1K ohms or more.
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    So if you design your ILDA differential transmitter to have the specified 100-150 ohm terminating resistors in series with the load, worse case is 80 mA with a 12V supply rail. Only problem is vendors who do not know the standard and who do not put limiting/matching resistors on their boards.
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    I know Pangolin goes out of their way to protect their driver part of their board from shorts, I suspect Ihor does the same.
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    So worse case analysis, 80 mA short circuit current if you have the specified terminating resistors in your circuit. The technical committee team back in the day suggested the TL08x family because they had enough bandwidth and they had the soft limiting.
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    I have asked ILDA to allow PL to maintain a truncated copy of the ILDA Cable recommended circuits and standards, that PDF is here:
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    http://www.photonlexicon.com/forums/...8&d=1390537157
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    Where people get into trouble is poorly designed commercial or home made cards that do not have output limiting or source terminating resistors. . A certain popular sound card level shifter circuit comes to mind as having this issue.
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    While it is non-standard practice, it would not be a problem to make a short ILDA jumper cable with the series terminating resistors at one end, if you live in fear. Your image size will decrease about 5% worse case if you do this.
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    The color lines get interesting if you do not use analog color:
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    ILDA has a note in the standard specifies the TTL/ON-OFF color signal receiver shall not be a TTL chip, because they cannot take a +/- 10V signal from a mis-wired cable. They specify a CMOS chip or a line receiver chip for the color lines if they are on/off.
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    QUOTE:
    "The driver circuitry shall be current-limited to 100 mA or less and shall tolerate a direct short to ground or any potential within +/- 10 volts of ground."

    End Quote.
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    So their 100 mA lines up quite nicely with my 80 mA estimate.

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    What this comes down to is a cheap 25 cent un-limited Opamp will work unless the line is shorted or too long. Its up to the circuit designer to ensure their driver can drive 1000 feet of cable and not put more then +/- 10V on the line. If the OP-Amp can't drive the cable impedance it is unlikely to be damaged, but the signal will be distorted. If the cable is shorted, a unprotected cheap op-amp will die.
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    See attached snippit from the ILDA Standard for the suggested circuits with the protection resistors circled.
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    Where this gets sticky is many cheap diode drivers do not follow the standard on their line inputs. Ouch...
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    Steve
    Attached Thumbnails Attached Thumbnails illdadrivers.png  

    Last edited by mixedgas; 09-04-2015 at 08:45.
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    Thank you very much for the detailed response!

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    Quote Originally Posted by Grix View Post
    Thank you very much for the detailed response!
    Your welcome. This is one of the most confusing aspects of the ILDA cable. It is widely miss-understood. I am hoping this will open a dialog or help others.

    Steve
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