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Thread: +/- 10v, or +/- 5v which is it?

  1. #21
    Join Date
    Apr 2007
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    Leicestershire, UK
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    I haven't built this myself but I'll have ago anyway.

    The caps don't inline as such but they go between + and GND and - and Gnd. The difference between - and GND is that you have a bi-polar or centre-tapped power supply which you'll need for the op-amps. (the TL074 IC is the quad op-amp.) The power connection for those isn't shown on the schematic but they would need to go to the + and - NOT the + and GND. I'm sure others will jump in if I'm wrong here.

    - Matt.
    Last edited by Lampy; 11-08-2007 at 09:28. Reason: spelling

  2. #22
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    Lampy is correct, the TL074 power supply pins are 4 and 11 and not shown, +15v on pin 4 and -15v on pin 11.

    Jim

  3. #23
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    Apr 2007
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    Also, make sure you supply the op-amps with +-15v - not from the output of the 7805 regulator.

  4. #24
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    Sep 2007
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    Cape Town, South Africa
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    Oh hell this is getting a little out of hand now guys, I'm going to redo the schematic this weekend and post it for your approval.

  5. #25
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    Aug 2007
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    Indianapolis, IN
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    I tried to build Norm's parallel DAC this weekend. I plugged in the +/- 15v PSU and the board burned out.

    In his instructions he says that the + and - terminals are reversed on the silk screen and should be wired up in reverse. That's what I did and it went "pop".

    I haven't heard anything from Norm, so I thought I would post here and ask if anyone has built a working board - and if so, what did they do regarding the differential power?

    I went on to make my own DAC out of a 595 register and an R-2R ladder. I mimicked the output stage from the schematic on this post, and got a fairly clean 10v p2p output under control from an Arduino board.

    So I don't think the PSU was to blame.

    Now before I connect this to my 20K scanner amps, is there anything else I should know? Any capacitors, resistors etc. I should use to smooth things out?

  6. #26
    Join Date
    May 2007
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    13

    Default Let's try to sort this out!

    Hi guys,

    I have a few points to make that may help us all to better understand the situation.

    1) I made an error when I said that my DAC kit outputs +10V to -10V. The output of X or Y swings from +5V to -5V relative to GND. That is single-ended indeed. The reasons: simplicity, cost and necessity. The idea was to offer the lowest cost possible kit with the least parts as possible. The simplifies the PCB creation and reduces its size too. Since single-ended can also connect to differential input drivers, my kit would be compatible with all equipment. And since most users don't absolutely need long cables for hobby use, single-ended was again a logical choice.

    2) There's definitely a widespread confusion about the voltage swing in the differential input amplifier versus the single-ended. I seem to be the only person in the world saying that:

    ILDA standard says that +5V to -5V should be sent at differential inputs.
    Now, I say that if that same amp is used with a single-ended DAC, the DAC's output should swing from +10V to -10V relative to GND (yes, 20Vp-p swing!) to get the same full deflection.

    The confusion comes from the fact that people consider only the +5 to -5, and not the subtraction that is done on a differential amp. In this case for an extreme deflection we have +5V - (-5V) = +10V. At the other extreme deflection we have -5V - (+5V) = -10V. That makes +10V to -10V don't you agree?

    Seen differently, isnt' +5V and -5V at one point in time, the same as +10V and GND? Of course.

    I will try to create a graphic to better illustrate the principle, and post it here.

    3) The small 0.1uF caps you see there are only for de-coupling. Standard practice.

    4) My next version of the DAC will output the full +10V to -10V, will have Low_Z (600 ohms) differential outputs for long cables, and some solder pads for two trim pots in case you want to reduce the level to match your amp.

    5) I admit that my schematic has been 90% copied from another one already on the web and that I don't have much credits for using it indeed. That's why I'm not too concerned about the fact that my work is being published here in this case. But please, in the future if you need to post my work for any reason and anywhere, at least send me an email before to ask about it. I'd appreciate that.

    Best regards,
    Norm

  7. #27
    Join Date
    Nov 2007
    Location
    Scottish Highlands
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    Hi, I just got one of Norm's DACs. Will be asembling in next few days (have to make PSU etc). I plan to use it with my little 10K Chinese scanners. Will let you know how I get on! Mind you, DACs, this is all new to me so don't expect miracles! Cheers, David

  8. #28
    Join Date
    Dec 2006
    Location
    Riverside, CA
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    Ok, I got bored at work and made a quick and dirty diagram. Hopefully it's understandable


    If your galvo has differential inputs and you drive it with a single-ended (±5v) signal, you'll only get half the deflection than if you drive it with a full differential (±10v) signal. It's also more susceptible to noise, which is the point of differential in the first place.

  9. #29
    Join Date
    Feb 2005
    Location
    Florida
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    802

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    Good for you aj
    I was wondering when someone was gunna do this..
    "My signature has been taken, so Insert another here"
    http://repairfaq.ece.drexel.edu/sam/laserfaq.htm
    *^_^* aka PhiloUHF

  10. #30
    Join Date
    May 2007
    Posts
    13

    Default Yess! You got it all right AJ - thanks!

    At last, someone seems to agree with me.
    That's exactly the kind of diagram I wanted to draw. Thanks for taking the time.

    Cheers,
    Norm

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