Noise on lower end of DAC trace - can I fix this?

[Genesis]

i use tl084 as i remember and yes 103 is the chip code for 10K resistor.
the last stage is a comparator to create interruption trig on the microcontroller.
the photodiode is just a standard wide area use in barcode laser scanner.
this system detect the variations of light and the laser need to be modulated to be detected by this sensor.
this is why the ambiant light is rejected (but not fluorescent light due to her frequency and fast response)

about your problem can you explain more your scope connexion? you can add a 10K ohms resistor between the ground and the output of the dac to make a fake load.
peraps your 5v rail is noisy and add noise by the invert add wiring of the last stage.

[shobley]

Thanks - I will try to build your sensor at some point.

Tonight I will try to load the DAC output, and also inspect the 5v being added in the last stage.

I will post the results here.

Thank you for your help.

Steve

[shobley]

Quick question - I noticed that your circuit uses nF (nanoFarads) - I cannot find a vendor in the US that deals in nF capacitors - just pF and uF.

Even converting the values into pF/uF I can't find matching caps over here.

Is there somewhere I can get them from?

[Genesis]

yes from radiospares and farnell
but this is strange because they are standard values.

[shobley]

I goofed in my conversion...

They are common - you are right..

OK, just *three* more questions about your beam detector and I *promise* I'll leave you alone...

Is the comparator 2nd (-) input a copy of the laser blanking signal?
What is the collector of the NPN transistor tied to? +5v or -5V?
Is the diode in the comparator an LED, or there for another reason?

[Genesis]

don't worry about questions

i not understand the first one.

for the second and third the collector is connected to the +5v logic through the led for test but in my system the led is replaced by the INT0 input of the microcontroller

[shobley]

OK thanks!

My first question was about the inverting input to the final op-amp.

See the picture, what is connected to 'X'?
+5v, -5v, or the laser blanking signal.

One more question - do you know why Babelfish French->English keeps translating 'harpe' as 'toothing-stone'?

It did it with your pages, and it did it with Sebastien Pamart's Harpe Laser pages...

[Genesis]

the X point is at 5v
and the modulation is the signal expected at this point and not the laser modulation
about the translation i don't know this is strange ^ ^

[shobley]

Thanks again - I'm soooo happy that there's someone out there that's achieved what I want to do.

Finally what do you think - Jean Michel Jarre's laser harp - does it really work, or is it just a stage prop?

I was at the Manchester concert when the beam failed, yet the sounds continued...

Very disappointing for me.

[Genesis]

ah? the laser had a problem in concert?
about JMJ i think some concerts was in direct and other not.