Need help with op-amp circuit

Discussion in 'The Projects Forum' started by bodenste, Nov 17, 2012.

  1. bodenste

    Thread Starter New Member

    Sep 24, 2010
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    I am working on a project and need some help. I am controlling a laser with an analog signal from 0V to 1V. If the voltage is greater than 0, the laser is on and if the voltage is 0 the laser is off. The laser requires at least 30 mA and the DAC I use does not provide sufficient current. So, I use a simple op-amp to buffer the signal.

    There are two issues: first, the op-amp adds about 50 mV to the incoming signal. That's prevents the laser from turning off. To solve this, I added a switch after the op-amp to select between either "op amp to laser" or "ground to laser."

    This works great, but unfortunately disconnecting the op-amp output and then re-connecting it to the load causes some ringing (and time) to stabilize at the appropriate output voltage.

    I have tried simply adding a 1k resistor between the op-amp output and ground. This provides a minor improvement, but only minor.

    Does anyone have any ideas? I have not tried a small resistor. It was suggested by the vendor that I use a more complex op-amp so I can switch the input to the op-amp and remove the 50 mV bias, but doing so would require a dual power supply (right now I have a single +12V line in).

    Any ideas? I now know that leaving an op-amp floating when "off" was probably a bad idea. But I'm not sure how to be handle this situation.
     
  2. wayneh

    Expert

    Sep 9, 2010
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    Very simple - get a different op-amp, one with an output that includes the negative rail of its power supply. For instance TLV272IP which goes to both rails, but there are many others depending on your other needs.

    You know, a single transistor would also work for you. 0V would turn it off, 1V would turn it on. Assuming a current gain of 10X, you need a resistor on the base of the transistor to limit the base current to about 3mA when there is about 0.35V on it (1V signal minus the 0.65V drop from base to emitter). 120Ω ought to do it. I assume the DAC can handle a 3mA load? There are transistors with higher current gain if you need to reduce that farther.
     
    Last edited: Nov 17, 2012
  3. crutschow

    Expert

    Mar 14, 2008
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    A comparator may also work for you.
     
  4. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Can you provide documentation, like a datasheet, for the laser?
     
  5. bodenste

    Thread Starter New Member

    Sep 24, 2010
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    Hi All,

    Thanks for the help! I was a little unclear but I think I see the solution. The laser control is actually a device called an AOM - it goes post-laser and modulates the power of the laser. The device takes between 0V and 1V and the laser power increases with V. To get complete extinction requires 0V (somewhere between -5 mV and 5 mV) and maximum power requires +1V.

    I currently use a TI OPA140 with a single voltage supply (+5V). In my original design when I input 0V/GND into the OPA140 I got 60 mV out. I am sure this is because of the single supply.

    So, I think the solution is: add a negative supply to the OPA140. So, now my question is, how do I do this? I have +12V input, and that's it. I use a LM317 to produce the +5V. What is the best way to produce the -5V?

    Thanks!
     
  6. bodenste

    Thread Starter New Member

    Sep 24, 2010
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    Oh and before I forget, the input voltage is 0 to 1V and I need the entire range (it's calibrated to laser power so I need a fairly accurate DAC - it's also noise-sentive).
     
  7. wayneh

    Expert

    Sep 9, 2010
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    Just thinking out loud here: You might be able to put a diode between the AOM's negative pole and ground, raising the voltage there about 0.6V. Use that same elevated ground for the DAC and on the non-inverting pin of your op-amp instead of the op-amp ground, which is now -0.6V relative to everything else. Now your op-amp can easily hit "0" from the AOM's perspective since it's 0.6V above the negative rail. And you'll still get a full 1V range of operation.

    If that isn't an option, maybe you just need an old computer PSU. It'll have +5, +12 and -12 supplies in it. Cheap, easy to find, self contained. Or you could just use a single AA battery to lower the negative rail of the op-amp relative to everything else.
     
  8. KrisBlueNZ

    Member

    Oct 17, 2012
    111
    14
    It would really help if you posted a schematic. Even something incomplete.

    If the op-amp only needs to source current, and you don't need high performance, you can put an NPN emitter follower on the output of the op-amp. Take the circuit output, AND the feedback path, from the emitter of the transistor, and put a load resistor from the emitter to ground. The transistor will drop about 0.7V so the op-amp doesn't need to be able to drive its output to ground. This will also increase the drive current available.

    You might want to consider an op-amp with a low offset voltage too. I've used the TI TLE2021 and found it's pretty good.

    Of course you can also generate a negative supply. The ICL7660 is the traditional part for doing this, but you could check out maxim-ic.com and linear.com (Linear Technology), and/or do a search on Digikey or Mouser for a negative supply generator. You wouldn't need much current so a switched capacitor one like the ICL7660 would be fine.

    I would probably go with the emitter follower solution though. But show us a schematic!
     
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