Page 2 of 2 FirstFirst 12
Results 11 to 14 of 14

Thread: Question about X and Y differential ILDA signals

  1. #11
    Join Date
    Mar 2012
    Location
    Akron, Ohio USA
    Posts
    2,197

    Default

    Quote Originally Posted by HankLloydRight View Post
    I do have a bipolar power supply.
    That's good. Then the design is probably straight forward.

    Quote Originally Posted by HankLloydRight View Post
    I realized in my dreams...
    That's an awesome thing! I've figured out how to do some really neat stuff in code that way.




    .
    Last edited by james; 10-17-2022 at 07:14.
    Creator of LaserBoy!
    LaserBoy is free and runs in Windows, MacOS and Linux (including Raspberry Pi!).
    Download LaserBoy!
    YouTube Tutorials
    Ask me about my LaserBoy Correction Amp Kit for sale!
    All software has a learning curve usually proportional to its capabilities and unique features. Pointing with a mouse is in no way easier than tapping a key.

  2. #12
    Join Date
    Sep 2014
    Location
    Colorado USA
    Posts
    793

    Default Allow me to offer some insight on this.

    Quote Originally Posted by HankLloydRight View Post
    Hello folks.. I've got a question about how to handle the X/Y differential signals.

    From everything I (think I) understand, the X-/Y- signals are just the X+/Y+ signals inverted. And X+/Y+ signals are 0v to +5v and X-/Y- are 0v to -5v.

    Say I'm building an ADC for ILDA signals (input), I know I can run the X+/Y+signals into two ADC channels, and then invert the X-/Y- signals and run them into tow more ADC channels -- but that uses up four ADC channels. But is that necessary?

    Consider these two other options instead:

    1. Invert the incoming X-/Y- signals with op-amps, and then average them with the X+/Y+ signals (in theory they should be the same, right?) -- and then feed that average X/Y signal into the ADC using two ADC channels

    2. Only sample the X+ and Y+ lines (using two ADC channels) and just ignore the X-/Y- signals, and just mimic the X-/Y- signals in software by inverting the X+/Y+ data stream before writing out the digital data stream.

    Any flaws in these approaches? Thanks.

    Sorry for this late response but I only just saw this post.

    This relates to processing only X,Y digital signals and assumes binary values from 0 to whatever the upper bit range is (256, 1024, 4096, 65536, etc.).

    1. Use a dual, 4 quadrant (aka bi-polar), multiplying DAC whether 8-bit, 10-bit, 12-bit, 16-bit or whatever. Dual means "2 Channel", one for the X signals and one for the Y signals.

    2. Multiplying DACs have a Reference input for each channel that controls the maximum peak-to-peak analog voltage swing above and below analog zero volts, the Analog Devices AD7528 is a good example of an 8-bit, dual, multiplying DAC.

    3. For DACS that have unipolar analog outputs of say, 0-5v, their outputs can be shifted by using a simple unity-gain op-amp summing amplifier with two inverting inputs. One input connects to the o-5v analog output from the DAC and the second input connects to a stable -2.5vdc reference on the second input. The result is that the summing amp shifts the DAC output by -2.5v making it have a bi-polar voltage swing of 2.5v. If the summing amp has a gain of 2 then the final summing output would have a + and - 5v signal swing. We refer to this type of bi-polar output signal as an unbalanced output since the active signal is measured with respect to analog ground.

    4. Once the unipolar DAC output conversion to bi-polar analog signal is achieved then the bipolar output is feed into an op-amp circuit that converts an unbalance output to a balanced output that the standard ILDA projector input connector uses for XY signals.

    An example circuit might look like this (this is a correction to my earlier schematic post):

    Click image for larger version. 

Name:	ExampleSum&DiffCircuits.jpg 
Views:	2 
Size:	126.2 KB 
ID:	59525


    The 50K potentiometer should be a 10-turn, precision trim pot and set to be a negative voltage equal to half of the peak value of the DAC analog output voltage, or about -2.5v. The final differential output voltage level signal swings should be close to +10V and -10V.

    Let me know if this helps or if you have questions.
    Last edited by lasermaster1977; 10-29-2022 at 16:12.
    ________________________________
    Everything depends on everything else

  3. #13
    Join Date
    May 2014
    Location
    Connecticut
    Posts
    884

    Default

    Thanks. That's pretty much the circuit that I'm using now for ILDAC-WAV. VPOS is +-10v and VNEG is -10v. DACX is 0 to +5v (12-bit DAC chip output)

    Click image for larger version. 

Name:	Annotation 2022-11-04 123028.png 
Views:	1 
Size:	72.2 KB 
ID:	59621

    Although I'm using two 10kΩ pots for gain and offset which seem to be working fine.

    I'm curious why you suggest 50kΩ pots instead?

  4. #14
    Join Date
    Sep 2014
    Location
    Colorado USA
    Posts
    793

    Default

    Quote Originally Posted by HankLloydRight View Post
    Thanks. That's pretty much the circuit that I'm using now for ILDAC-WAV. VPOS is +-10v and VNEG is -10v. DACX is 0 to +5v (12-bit DAC chip output)

    Click image for larger version. 

Name:	Annotation 2022-11-04 123028.png 
Views:	1 
Size:	72.2 KB 
ID:	59621

    Although I'm using two 10kΩ pots for gain and offset which seem to be working fine.

    I'm curious why you suggest 50kΩ pots instead?

    That's a good question. The simple answer is just to lightly load the 5v reference voltage source, which may or may not be a concern in your case.

    With the inverting opamp's 10K input resistor tied to the wiper of the 50K variable resistor, this makes the trim pot behave in a non-linear manner since the 10K resistor is in parallel to the wiper lead and ground. But, since the only purpose of the trim pot is to be adjusted to a set voltage offset value that yields the desired DC offset on the opamp's output, this is a minor inconvenience. A 5K trim pot would be more appropriate since the 10K input resistor is in parallel between the wiper's position and the grounded end of the pot. This makes the trim pot's adjustment behavior far closer to a linear voltage change as the wiper position is moved.

    Also there is nothing wrong with your using 10K trim pots, the same considerations apply regarding the non-linear voltage behavior as the wiper position is changed.
    ________________________________
    Everything depends on everything else

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •