Signal processing of AFM (DC) signals using labview

[AFMuser]

I am trying to conduct FFT analysis on DC signals from the thermal fluctuations of a cantilever (liquid operation) from a Nanoscope III multimode AFM. Our DAQ board is PC6110E. I am using one of the available signal processing examples to process the signal, but we find that there is a lot of noise in our resolved spectra. Similar work using a SRS spectrum analyzer yields very good and clean spectra. Is there any amplification/filtering etc. that can be implemented before or after the DAQ board that can eliminate/reduce this noise? The frequencies of interest of our signal is between 5-100 kHz. Thanks in advance.

[Ben]

Hi AFMuser,

I used to work for Jeremy Levy (at the University of Pittsburgh) the inventor of ANSOM (Appertureless Near-Field Scanning Optical Microscopy).

He was able to get very good results using a Lock-in Amplifier.

The unit was very price-y but gave good results.

If the cost is prohibitive you could put to gether a PLL and monitor the feed-back.

Ben

[Filipe A.]

Hello;

If the range of frequency would not so extense, you could use one of the NI signal conditioning modules. However, as that is not the case, you will probably need to implement a filter, in Software, to get rid of the harmonics with higher frequency than 100KHz.

Furthermore, I'm attaching a good Application Note that talks about noise on DAQ systems, so you can get some usefull information, and maybe apply some of the techniques on your system to avoid noise.
Hope this helps.
Filipe

[membruce]

It?s interesting that you are getting good results from another spectrum analyzer. The FFT should do a pretty good job of filtering for you. You could be splattering the signal with harmonic artifacts of your digitization. Is your sample rate high enough?

Have you played with the FFT parameters (Window size, etc.)?

Do you have a function generator that you can test your DAQ system with? If so, you might want to put it on your ADC and ramp the frequency up. Watch the spectrum and see if you see artifacts. Another test is to attenuate the signal until you lose it. That will give you an idea of the sensitivity of your system.

I have worked with a Nanoscope II before. If it?s not confidential, what are you doing that the controller can?t?

Bruce