Difference amplifier for Maxyz and CNI lasers..?

[Stuka]

I'm getting close to wiring everything up on the RGB projector, and plan on trying to stick with Bill Benner's "...Making the Right Connections for Perfect Laser Images" article as much as possible.
(By the way - if you're building / overhauling / troubleshooting a projector's electrical system, I HIGHLY recommend this article! Here's a link to the latest published version: http://www.laserist.org/laserist2008...onnections.htm)

Here's my question:

Bill recommends the use of difference amplifier circuit for the scanner amps AND the laser modulation input, using a TL084 or similar op-amp. I have two CNI LP-series lasers (532nm @ 400mw, and 473nm @ 100mw), and a pair of Maxyz 660nm combined through a PBS cube.

Has anyone here utilized a difference amplifier for the modulation of any of these particular lasers, and if so, any pros /cons / recommendations you would be willing to share?

Thanks in advance!

Randy

[DZ]

Hey Randy, adding a difference amp to pass the single ended modulation to the laser driver in your projector is certainly the best way to eliminate the potential for noise. The one problem is that in some or most LD2K's the -R, -G and -B lines are internally grounded. So there is no differential signal coming from the QM2K for color. I seem to recall my first LD2K having differential color lines and the need to externally ground them to get the full 0 to 5V swing since I only had single ended AOM's at the time. My current LD2K IS internally grounded. I wonder though, if the LD2K's being sold today are back to not being internally grounded and ARE delivering a differential signal. It seems that the documentation has changed, again, to reflect that the -R, -G and -B lines swing from -2.5V to 0, whereas the documentation in the last release of the software indicates 0V for the color lines. Perhaps Bill could chime in as I also am curious to know. If they are sending differential again for the color lines, is there a way my board can be modified to do the same?

[Pangolin]

What David wrote above is true -- there are some QM2000 that have -RGB tied to ground and some that don't. Actually, we can say that it is user-selectable. The one's that have -RGB tied to ground are easy to change if you want to. Just unsolder a few resistors on the back of the board and away you go.

BUT, this has absolutely nothing to do with whether or not you would want to use differential within the projector itself. Generally you always want to use differential within the projector, regardless of the disposition of the signal source (computer or whatever).

I will have to write an article about differential some day so that I can simply give people a download link and won't have to keep writing that... the onus of differential is on the receiver not the transmitter. I did write a little bit about differential signaling and why (and where) it should be used in my Projector Connections article that Randy pointed to above.

Bill

[Stuka]

Works for me!

Guess I'll be building some differential amps for the modulation - and hope nothing expensive goes "poof" in the process!!

[drlava]

I agree, for long 100ft+ wire runs that are susceptible to interference, for DAC-to-projetor runs where they are on different ground circuits, and for runs that are on ungrounded circuits or have 'leaky' power supplies, differential receivers can help.
However nothing, not even differential circuits are a substitute to proper grounding internally and solid, low impedance ground connections between the units. With these two grounding basics, in most cases single ended connections can operate well without distortion. In the cases where differential inputs are needed, they are needed for each laser, and given that most lasers don't have differential modulation inputs a converterter circuit would be needed for R,G,and B lasers.

[Pangolin]

Yes, differential isn't the only answer, but for a lot of people it could be a big part of the answer. Every time I receive a scanning system for re-tuning I also wind up re-wiring it differential before sending it back.

At the risk of repeating a link above here it is again:
http://www.laserist.org/laserist2008...onnections.htm

Bill

[Stuka]

Great info - thanks to everyone who chimed in!!

One more question (which goes back to my "poof" comment earlier!) -

When using a differential amp for the analog inputs to the lasers, any additional concerns / precautions that should be taken to prevent overdriving the modulation circuits on the lasers, or will the differential circuits pretty much take care of everything?

[mixedgas]

Great info - thanks to everyone who chimed in!!

One more question (which goes back to my "poof" comment earlier!) -

When using a differential amp for the analog inputs to the lasers, any additional concerns / precautions that should be taken to prevent overdriving the modulation circuits on the lasers, or will the differential circuits pretty much take care of everything?

hang a 5.2 V zener diode on the input of the laser via a 500 ohm resistor and that would protect both TTL and Analog from OPPSes

signal from diff amp--------500R------+-------- to Laser input


ASCII art didnt survive PL's font, so see attached

Steve

[Stuka]

Thanks for the assist!!