Clarification on ILDA voltages

[harrissanford]

I have, but it confused me. As written, it just screamed a 20v p-p setup to me. With your clarification, it makes more sense.. but I still feel they should have use the +10V and 0V terminology. Guess I'm a little slow... Hopefully it helps someone else who reads it later.

So this quote bothers me too: "must have a true differential input stage!" Again, I've searched around, but I don't grasp this fully either. Sorry! I'm a software man who only dabbles in electronics enough to gain the occasional shock!

[taggalucci]

I'm a software man who only dabbles in electronics enough to gain the occasional shock!

Ditto!

I'll try and explain in terms of observable output...

 +     G     -      WRTG   WRTOP   Outcome
 0v    0v    0v      0v     0v     galvo centered
+5v    0v   -5v     +5v    +10v    galvo far extent 1
-5v    0v   +5v     -5v    -10v    galvo far extent 2

WRTG = With respect to ground
WRTOP = With respect to opposing pole

If you follow Bill Benner's projector wiring guide, the input signal ground connector of the galvo amp is not connected at all. This is how I have my projectors wired.

[harrissanford]

Ok, something clicked, and that is the fact the + line can go +/- 5v ... and the - line can go +/- 5v from gnd. I thought the + line ranged between 0 to +5v and the - line ranged between 0 to -5v before that table you typed taggalucci.. this would result in the following diagram which is NOT correct:



With that said, my diagram #1 design would not work.. well it would (and does!) but I'm only getting 1/2 the range, right? My chart for that would look like:

  +      G     -      WRTG   WRTOP   Outcome
 0v      0v    0v       0v    0v     galvo centered
+5v      0v    0v      +5v    +5v    galvo far extent 1
 0v      0v   +5v     -5v    -5v    galvo far extent 2

Am I on the right path?

[taggalucci]

Yeah, I think so, though I'm not sure what you're trying to achieve with driving via Arduino outputs. I think you need a DAC in between.

Someone with better electronics knowledge than I could get your design on the right track.

[harrissanford]

I do intend to use a dac, but I don't have one on-hand (well, I guess you could count pwm and a filter.. but! we won't go there)

Before I built one, I just wanted to make sure I was doing it right... which I wasn't!

Now I need to re-think how to combine two 8-bit dacs to get me 16 bits, then (without gain or offset pots) properly modulate it! I guess I could use gain and offset stuff.. but I want to see if I can do it without these.

[taggalucci]

I do intend to use a dac

Yes, I missed that in your earlier post. Good luck with your project, keep us updated Hopefully someone else will chip in. J4cbo would be a great source of knowledge.

[Stoney3K]

I do intend to use a dac, but I don't have one on-hand (well, I guess you could count pwm and a filter.. but! we won't go there)

Before I built one, I just wanted to make sure I was doing it right... which I wasn't!

Now I need to re-think how to combine two 8-bit dacs to get me 16 bits, then (without gain or offset pots) properly modulate it! I guess I could use gain and offset stuff.. but I want to see if I can do it without these.

If you only intend to use it for beam projection (since you're using an Arduino, a laser harp comes to mind), using the internal PWM with a low-pass filter rolling off at 10-15kHz will do fine. You may want to stick a buffer op-amp in between to take care of the galvo's input impedance.

You can drive galvos in a single-ended way (- signal and ground connected together) but it will give you half the possible angle, as well as offset the projected image to the top/right. If you have a projector that is dedicated to your project, it's not a big problem, but if you intend to plug your project into different laser projectors, you may need to do some correction, which is basically the same as the Laserboy Correction Amp, applying a -2,5v offset and a gain of 6dB.

A more elegant solution is to use a DAC chip connected to SPI, such as the Microchip MCP4922 used in the Arduino Wave Shield. It's a 12-bit DAC, but you can combine two 4921/4922 chips together to form a 24-bit DAC (using the output of one DAC as the reference voltage of the other).

[mixedgas]

I'll scan in the ILDA reccomended LINE DRIVER AND LINE RECEIVERcircuits tonight, all will become clear. Yes, they do exist, some older ILDA standards got, um, deleted, with the change to the DB25 connector from the 37 pin connector many years ago.

You need each line to swing +/5 volts, and the one line is a mirrored copy of the other. Costs a whopping 4$ to do with opamps.

Steve

[dsli_jon]

Garbage, taken-out..

[mixedgas]

Clueless by Four inbound?

Too bad, his company is advertising some nice direct copies of American made scientific and industrial lasers.

That one is a Verdi clone, would drop right in on my Verdi coldplate at work...

Steve