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Thread: Howto: simple Differential Amp + TTL ?

  1. #1
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    Default Howto: simple Differential Amp + TTL ?

    Hello Folks!

    I have a 6ch CMedia ready and want to build an quick & dirty correction board.

    Could you help me?

    I drawed a plan with the patient help of Nicolas (weartronics) and his great Differential Amp Design. As i cant get the ±9V DC Converter here, only 5 to ±12V, which wont get enought power from usb; i thought of taking one of the 12V wallwarts i have colleting dust, and creating ±5V from it by splitting regulated 10V. the dropout of the IC is 2V so it fits, or not?

    Also, i have a only TTL modulated Laser (Pf-114) so i dont need real linear amplifying by now, it should just be able to blank the Laser with Spaghetti and HE. Maybe someone could give me a solution that uses only Transistors for the RGB chan or even raw unamped Soundcard Output.

    Sorry im a total noob in Electronics. i can solder even SMD without problems but have no clue how the stuff works and how to design a circuit from scratch and what parts i need etc...

    Thanks
    TC
    Attached Thumbnails Attached Thumbnails 300mw_rgb_animation_lao40c.jpg  

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  2. #2
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    Hint #1: Swap Voffset and your DAC inputs. Otherwise, you'd be amplifying the offset and doing something strange with the DAC input alongside it... not what you want, probably. You'll want the offset voltage to act as a ground for the op-amps... for a single amplifier stage in the chain only.

    As for the power supply: Try to get some 1N4001/1N4002's and build a bridge rectifier with a few capacitors, and an LM7805/7905 combo. That way, you will get some nice stable +/-5V supply lines with the ground isolated from the adaptor's negative supply, and, more importantly, you can use a simple 9VAC stomp box adapter to power it.

    If you want to do everything 'just right', add a fuse and power switch, and make sure the op-amp's power supplies are filtered near the chips with 1uF or so of capacitance to ground.

    The TTL output will work fine with the correction amp, you can set the laser to TTL in HE or Spaghetti. The laser's intensity control should be 0-5V anyway, the only difference is that your laser only has on/off control instead of a continuous curve. You do not need to change the circuit because of that.

  3. #3
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    Quote Originally Posted by Stoney3K View Post
    Hint #1: Swap Voffset and your DAC inputs. Otherwise, you'd be amplifying the offset and doing something strange with the DAC input alongside it... not what you want, probably.
    I disagree. For a buffer, you apply the incoming signal to the non-inverting (+) inputs as in that drawing..

    @Xero: I posted this on LPF also: A +-12V DC-DC converter should easily be able to be powered by USB. I'm using one on my board right now..

    Here's the beginning of a parts list for you:

    TL072 op-amp (dual op-amp, so you'll need a total of 4 according to that schematic)
    +-12V dc-dc converter

    (you can google search for datasheets for these, or get them from your supplier. They will show you how to hook each one up (which inputs/outputs are which).

    those are the main parts. The rest are standard resistors, some capacitors on the op-amp power rails (as stoney3K mentioned), and a DB-25 connector.

    I don't think you'll need the +-5V supply if you just power the DC-DC converter straight from the USB connections on your soundcard as shown in the tutorial.
    Last edited by ElektroFreak; 02-04-2010 at 15:44.

  4. #4
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    Quote Originally Posted by ElektroFreak View Post
    I disagree. For a buffer, you apply the incoming signal to the non-inverting (+) inputs as in that drawing..
    whats the use of a "buffer" and why it have to be a OpAmp with ±5V supply and voltage half splitting?
    just wonder why you cant use a negative constant voltage regulator set to -2.25V giving the Vref...

    Quote Originally Posted by Stoney3K View Post
    The TTL output will work fine with the correction amp, you can set the laser to TTL in HE or Spaghetti. The laser's intensity control should be 0-5V anyway, the only difference is that your laser only has on/off control instead of a continuous curve. You do not need to change the circuit because of that.
    my intent was to reduce the board size for the TTL color channels, by using SMD Transistors or some small parts, or even the raw dac output but kompensated with the negative offest vref. would it work?

    @ElektroFreak: About the ±12V DC-DC ... i can get those hre in Germany:
    http://www.reichelt.de/?;ACTION=3;LA...;ARTICLE=35064
    Are they ok (it's german but the important tech fact are written in english on the bottom of the site)

  5. #5
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    Quote Originally Posted by Xer0 View Post
    whats the use of a "buffer" and why it have to be a OpAmp with ±5V supply and voltage half splitting?
    just wonder why you cant use a negative constant voltage regulator set to -2.25V giving the Vref...

    The kind of circuit used in the correction amp is called a buffer. That's the only significance of the term.

    Op-amps require both positive and negative voltage to operate, so a negative-only regulator wouldn't do (if I understand your question correctly). The TL072 will take up to +/-18V on its power inputs, and the DC-DC converter supplies +/-12V. This is just fine for the op amp, and will give you more voltage on the outputs and thus a wider scan angle and larger image.

    You'll also need to add offsets to your modulation channels since the +/-12V will cause them to deliver more than the necessary +5V to the lasers. For the modulation channels only, use the schematic in the middle of the drawing found in this post: http://www.photonlexicon.com/forums/...5371#post55371
    For the galvo channels use the circuit in your drawing. This circuit includes the necessary adjustments to set the voltage and offset. The DAC inputs from your drawing correspond to the DC audio inputs in this circuit. Then you just feed the modulation op-amps the +/-12V from the DC-DC converter just like in your drawing.

  6. #6
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    Quote Originally Posted by ElektroFreak View Post
    I don't think you'll need the +-5V supply if you just power the DC-DC converter straight from the USB connections on your soundcard as shown in the tutorial.
    Thx for the partlist. I updatet my Sheme now:

    But i dont think it will work so. what should i feed to the Buffer? will it still hold the around -2.25V it shoud provide when i set it with the voltage divider?

  7. #7
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    Quote Originally Posted by ElektroFreak View Post
    Op-amps require both positive and negative voltage to operate, so a negative-only regulator wouldn't do (if I understand your question correctly)
    Not fully. im confused why the "Buffer" needs an OpAmp. It's purpose is to deliver a negative Voltage offset to all Amplifier stages, right? So could i use a negative regulator to provide a voltage in the range of 2.25V?

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    that's not what I was referring to actually. Each amplifier stage is a buffer circuit.

    For Vref you could simply branch a 7905 regulator off of the -12V from the DC-DC converter to get -5V and apply that to the circuit pictured in the post I linked to for all 6 channels. You would lose your differential signaling, but you gain simplicity since you can then power everything from one source. You can see the offset adjustment part of the circuit (R3). Otherwise you'd need to provide vref separately as in your circuit. I can't see a reason why you can't use a neg. regulator for vref, myself. If you refer back to the post I linked to, you can see that they accomplish a vref source using a resistor and a 5.1V zener diode. Then a pot is used to adjust the -5.1V as necessary.

    EDIT: lol.. I just took a closer look at the high-res diagram you posted. Based on a comparison with the other correction amp schematic, I think Stoney3K is right. It seems that the DAC input should be placed on the (-) input of the op-amp. My fault for not realizing that earlier. It doesn't change the power requirements, though. I think you could get away with providing vref directly like you suggested using a regulator circuit, and you should be able to supply voltage to the regulator circuit from the DC-DC converter. If you add R2 and R3 from the post I linked to previously to your circuit in the same fashion and apply Vref there you should be functional.

    I'm waiting for drlava to show up here and set everything 100% straight. He's the expert..
    Last edited by ElektroFreak; 02-04-2010 at 19:53. Reason: accuracy

  9. #9
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    lazy option - use a Burr Brown DRV134

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  10. #10
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    Quote Originally Posted by ElektroFreak View Post
    I disagree. For a buffer, you apply the incoming signal to the non-inverting (+) inputs as in that drawing..
    True, but the left-hand circuit in his drawing is wrong: Voffset is applied at the negative input with 50k of forward and 150k of feedback resistance, where the DAC input is at the positive. That's an inverting amplifier with the DAC input acting as a virtual earth, and the offset being amplified 3 times.

    I expect he wanted the result to be the other way round, nulling the offset and amplfiying the DAC input.

    BTW, if you apply the Voffset straight to the inputs of an op-amp, it doesn't need buffering. The op-amps inputs are high-impedance already. When you use the offset voltage in a summing circuit, it's a different story.

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