I now have a PCB for the data converters and analog electronics! Full schematics and gerber files are available on my website. I’m now using digital feedback and linearizing the tunneling current data in software using a logarithmic lookup table. This really stabilized the feedback loop and I’m now able to get atomic resolution in constant current mode, something which I just couldn’t get to work with the previous analog feedback loop. I’m also using sigma-delta modulation to increase the resolution of the scanner drive signals from 16-bit to 20-bit.
Here's a picture of the analog PCB:
I'm still using a Teensy 3.1 but now that It's running a PI loop and sigma-delta, it's slowing things down a bit. I've put the Teensy on a separate piece of prototype board and connected it to the main analog PCB with a ribbon cable. This way, I can easily upgrade the digital electronics later on. I'm also working on an AFM, which will use an FPGA, and may integrate the STM digital electronics into the same FPGA. I've left plenty of room in the box for it :)
Here are some images acquired with the new system:
Take a close look at this image of gold. If you zoom in on the image, you can see the atomic layers!