Laser output conditioning

[controllogix]

I would like to condition a signal generated by a laser that profiles the product we run. The output is a +/- 5vdc but the type of profile the laser generates actually produces a 50 - 200mv ac signal @ around 850Hz. I would like to look at only the ac signal and produce a 0-10 vdc output. The ac peak to peak signal is a representation of the profile I need. As the peak to peak signal gets larger, I want my output to get larger. I appreciate any help, Thanks in advance.

 

[Lewish]

If you take the output and couple it thru a capacitor to an amplifier, this will remove the DC component of the signal. As the frequency of the signal, 850Hz is quite low, the capacitor will need to be quite large. If I know the input resistance of your amplifier, I can calculate the capacitor for you.
Then you just need enough gain in the amplifier to bring your signal up to the desired output level. For a 200mv signal in to get a 10V signal out requires a gain of 50.

 

[controllogix]

Thanks for the input. I'm not sure what the input resistance would be or what type of amplifier to use. The signal I need to amplify is limited to a RL of 1kohm.Can I rectify the output from the amp to get the signal to a DC level?

 

[Lewish]

Yes, you can rectify the output of the amp to get DC, but you probably want to then run it into an integrator to get an accurate level over time. The output of the rectifier will be varying from zero to your peak voltage and back to zero with each cycle of the AC. The integrator will convert this to a solid DC level.
If you use an Opamp for the amplifier, the front end resistance will be at least 100Kohm for run of the mill parts up to several Gigohms for FET input parts. If you use a part with a 100Kohm input, then your capacitor can be 10uF or larger. Use a tantalum part, if possible, to get the best noise performance.
Good luck with the project.

 

[controllogix]

Thanks alot Lewish, you have been very helpful. Would it be too much to ask for a drawing of what you speak of?

 

[Lewish]

I'm not to good at drawings, but may I suggest you find a copy of "The Art of Electronics" by Horowitz and Hill. It was written for physics students and explains the circuits in easy to understand ways. You should be able to find it at any major library, or pick up a used copy from some vendor on the web. It is a great reference book for all kinds of useful circuits.