New preamp!

I have completed the new preamp with the custom PCB and high quality op-amp. It is a lot more sensitive than the one I used for my previous tests. Previously I used a 10Mohm feedback resistor. What that means is that the output voltage in an ideal world is always 10 million times the input current. (as long as it's within the supply voltage). It sounds like a lot, and would probably be usable, but not ideal for this stuff.The new preamp uses a 100Mohm meaning that it's 10 times more sensitive. Additionally, the new op amp is higher quality. We don't live in an ideal world, so the output voltage will have some noise, and some offset. The op amp that I'm using now, the OPA 129, will stay closer to that "ideal world" output. The most important difference, is that it has a much lower input bias current (Iib). Any input current below the Iib rating will be entirely undetectable for an op amp. This means that the OPA can detect much smaller currents than the tl072 that I used before.


I tested this preamp with a feedback circuit similar to the one I used previously. After some adjustments I managed to get this result which looks like it could be a tunneling current. I talked about this in a previous post I will not go into very much detail here.

Like before The green trace here shows the output height signal going to the piezo and the red trace shows the amplified tunnel current from the tip.

Next goals

Now that I have an ok working feedback loop it is time to start looking into the digital side of things. The X, Y, set-current and bias voltages vill be controlled by the DAC 8534 chip. DAC stands for Digital-to-Analog-Converter. As the name implies the chip takes in digital instructiones (ones and zeroes), and outputs an analog voltage. I plan to send the digital instructions with an arduino, which will also read the data to produce a picture in the end. I don't have a lot of experience with this stuff so it should be interesting. The DAC datasheet describes the digital interface and what standards it's compatible with. I think that there are some Arduino libraries to help me out with this as well, so it should be doable.

I still have some analog stuff left however. When I get the DAC to follow my command I will need to build a piezo driver circuit. I also plan to make a better feedback circuit when I have the other systems in place.