hi Groover - I was just checking out your homepage, looks like you're doing some cool stuff over there!
Welcome aboard and happy new year-
-Josh
p.s. Thanks for the link (photondream)!
hi Groover - I was just checking out your homepage, looks like you're doing some cool stuff over there!
Welcome aboard and happy new year-
-Josh
p.s. Thanks for the link (photondream)!
Thanks josh.
yaddatrance:
Maybe you can help me with a problem. I get a terrible interference so I traced it back with the scope and finally found the source. It's coming from the DAC itself. Most irritating and I can’t figure out why I get it at the DAC output. I first thought it might be dataerror but it's off sync with the data transfer.
I'm all out of ideas here.
For clarification, this is the schedule:
http://www.slackersdelight.com/laser/images/dac2.jpg
You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!
Hey Groover, great to hear from you...
Assuming you are testing your TI chip after disconnecting the
opamps completely...
First thing I would do is buy 2 - 10uF tantalum capacistors
and attach them as close to your DAC as possible (one between
Vss and ground and the second between Vdd and ground)...
Your schematic only shows the caps on the power supply itself...
the "decoupling" caps on a DAC, especially a high-power DAC
like yours isn't optional... This may in fact fix your problem
completely... The "tantalum" part is important due to the ESR
and low leakage. If you don't have one handy, throw an electrolytic
one in, to see if it helps a bit...
Another problem I see is that you have tied your analog ground to
your digtal ground... Pins 5 & 6 should not be directly tied together,
put a resistor between it... The digital ground should run to the parallel
port, the analog ground should be run to the opamps...
The second problem you may run into is possibly "railing" your voltages
on the DAC itself... Your Vss is max -7V, but if you feed it from a 7905
with -5V, then you'll see performance degradation or undefined behavior
near the rails...
You should follow the data sheet as shown above when running it bipolar
as you seem to be doing...
The way I personally like to do it is make the DAC run in unipolar
configuration and then use opamps to correct the p-p voltage and offset
before feeding to the inverting and unity gain opamps for the differential
output.
Hope some of this helps... I'm running out to Vegas to do a show, but
I'll be back on the 2nd or so to see any replies/questions/etc...
P.S. scope the ground lines on your PC parallel port itself... You may
want an RC circuit between the two to filter noise... the crap noise may
be coming from your PC and exhibiting as a ground loop. If you feel
like doing it the correct way, use optoisolators on the inputs from
the PC.
Your work is just not of this world. I'm very impressed. After I had a look at your 8 bit dacs I thought I might do a quick n dirty dac but still a little better than the first one.
Just remember I am a amateur
http://www.slackersdelight.com/laser/images/dac3.jpg
I know the values for the amplifying op are a bit low but I plan to use what I can get a hold of on a short notice.
You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!
Thanks! At this analog stuff I'm quite new... I'm still learning as I go,
and I tend to find things out the hard way Your first circuit is far
more impressive than my first shot out the door, which used DAC0800s.
Heh... here's a trick on the resistors... I just bought a metric assload of
milspec precision 10K resistors, and when I need 2*R, I just use two
of them in series
That way I don't have to measure and do lots of work...
Oh and it looks like you're doing an adjust to bump it to +/-10V (20Vp-p),
which is not the differential spec. Differential is just 10Vp-p...
A long time ago, we laserists ran differential at 20Vp-p, but we ran
into big problems with the opamps railing, so we cut it down to
a reasonable value... A side effect of the new standard was that it also
kept the multiplying resistors very sane.
Another tip (since 8-bit dacs aren't accurate) is to put in a voltage bridge
onto the reference line between +5V and ground with a few caps on
there to smooth it out. That way you can tune the projected images down
without losing resolution. Keeping any pots and noisy passive components
out of the analog AC side makes a huge difference in clarity.
Oh yeah, you can use the ILE to turn off the inputs on the DACs when you
want to start adding color and other channels too btw.
Thanks for the tip. I have a bad habit of leaving out things on my schematics. I've picked up some tantalum capacistors that are going in there too as decoupling.
You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!
Made another schematic and thisone is the actual schematic I'm using right now. Just got it working.
http://www.slackersdelight.com/laser/images/dac4.jpg
Well, it's time for bed. 7:30 AM here.
You know, I have one simple request. And that is to have sharks with frickin' laser beams attached to their heads!
Beautiful!
Optoisolation is definitely my more favorite fixes to problems
It always works!
You're missing the current limiting resistors off the top 3 optos, but seeing
as you used in on the rest, I'm sure you already know...
But great design!