help with voltage conversion

Discussion in 'The Projects Forum' started by evengravy, Oct 3, 2013.

  1. evengravy

    Thread Starter New Member

    Oct 6, 2012
    6
    0
    Hi,

    In a thread (linked below) a user wanted to produce a +/-5v output signal plus ground which was converted from a 0 to 5v input signal linearly.

    http://forum.allaboutcircuits.com/showthread.php?t=61002

    The thread is locked, hence why I am posting here, but I would like to modify the circuitry for the same exact functionality but with 0 to 3.3v input signal,

    Could someone possibly help me?

    Thanks in advance,

    John
     
    Last edited: Oct 3, 2013
  2. evengravy

    Thread Starter New Member

    Oct 6, 2012
    6
    0
    Essentially there are two inputs -In gnd +In. The galvanometer specification society ILDA have this to say about controlling galvanometers of this type.
    A bipolar analog signal whose voltage range is 10V peak-to-peak differential.
    A differential voltage level of -10V shall deflect the laser beam to the left side of the projection area. A differential voltage level of +10V shall deflect the signal to the right side. A differential voltage of +10V occurs if the normal signal line has a voltage level of +10V with respect to the voltage level of the inverted signal line. When compared to the ground level of the differential driver, the voltage level of the normal signal line shall be +5V and the voltage level of the inverted signal shall be -5V.

    I assume I would need both an inverted and non-inverted signal feeding the -in and +in pins?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Ahh, you finally DID post.
    I don't reply to E-mails, and your PM's (private messages) are disabled.
    I posted to the thread you mentioned in your E-mail:
    http://forum.allaboutcircuits.com/showthread.php?p=653006
    Specifically:
    http://forum.allaboutcircuits.com/showthread.php?p=653006#post653006

    I have not shown bypass capacitors across the V+/V- supplies of the TL07x opamp, nor have I shown a 0.33uF cap on the input of the regulator; but they are NOT optional.

    If you use a multichannel opamp and there are unused channels, you should connect the inverting (-) input of those channels to their respective outputs, and the non-inverting (+) input to ground to prevent them from randomly oscillating and causing odd problems.

    The values of R3, R4 & R5 aren't particularly critical; it's the ratio of R3 to R4+R5 that's important. With that being said, R4 + R5 should be at least 2K, or the loading on the output of the opamp will become excessive. If you use high values (over, say, 100k) for R4+R5, you risk having noisy signals.
     
    Last edited: Oct 4, 2013
  4. evengravy

    Thread Starter New Member

    Oct 6, 2012
    6
    0
    Thank you for this, I noticed only after the pm to you your tag line stating not to do so, apologies.

    Concerning the negative input of the galvanometer could I just set up an inverting voltage follower using an opamp such that when input is 3.3v +in is at +5v and -in is at -5v for example.

    The ilda spec sheet mentions that these inputs are 10v differential,

    Thanks again,

    John
     
  5. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    Yes; basically just follow the example inverter I had in the previous post for R4, R5t and U1b.
     
  6. evengravy

    Thread Starter New Member

    Oct 6, 2012
    6
    0
    You are a gentleman, thanks a lot
     
  7. evengravy

    Thread Starter New Member

    Oct 6, 2012
    6
    0
    Sgt. Wookie,

    I simulated your circuit today and it worked exactly as I expected it would thanks again. However, after testing the micro-controller (arduino due) it seems the DAC does not output 0-3.3v as I had read online but rather 0.55v to 2.75v



    I really should have checked this prior to posting, but arduino did not detail the manufacturer from which to source the data sheet and I did not have it in my hand until today, you live and learn,

    Could you tell me the calculation(s) for altering the circuit? I presume the voltage regulator to the opamp input should offer sufficient adjustment but I am not too sure how to calculate the resistor values,

    Thanks again,
     
Loading...