Electronics are still slow

Currently Waiting for New Parts

Yesterday I placed an order for the parts that I was missing. This includes the 100Mohm resistor and a breakoutboard for my dac ic. The dac ic is a DAC8534, which has a tssop-16 package. I bought a conversion board for a ssop-48 package thinking that I could just leave out the extra pins (the 16 pin version came in a pack of 6 and was more expensive). This was however not the case as the tssop-16 proved to be too small to reach the pads.

When I realized that I was gonna have to buy new parts one of my first ideas was to work on the dac while I waited for them. It was pretty annoying to be met by this sight. The lesson is that packages are not necessarily the same just because they have the same lead pitch.




Making a Custom PCB

The board to the left is what I planned to use for the preamp. It has solder pads in a repeated square pattern which gives you a lego-like freedom when placing components. This board in particular also has the same pitch as a SO-8 package so I can solder the opa129 op amp to it without any conversion board. For simple circuits like the preamp I like these prototyping boards since they allow you to be more hands on.

That being said these prototype boards are not without limitations. While they work well for most simpel things they are not great for sensitive stuff like RF or low current circuits. When dealing with very low currents you have to take a lot more things into account. For example the fact that small currents can leak through the pcb material and affect the signal. Since the preamp needs to measure low currents a custom PCB might be a better choice than this prototype board.

Since I was buying stuff i decided that I might as well design and order a custom pcb that will give me a better end result than the prototype board. It might require some extra patience but in the end I think it will be worth it.

So after spending the week researching low current pcb design and learning EasyEDA I have designed the circuit board bellow.

EasyEDA is the software used to create this. It can also be used for other stuff like making schematics and simulating circuits. It would definitely have been convenient earlier in the project. That being said being able to do things manually is probably also something I value. 

The yellow square is a guard ring that encloses the low current input. These are used in many circuits that handle very small currents. The guard ring is driven close to the same voltage as the input signal (in this case ground) and has a low impedence path to ground. This way very little current can leak to the input node. External currents simply leak into the guard trace instead. 

I also designed the tssop-16 conversion board for the DAC8534 with this program. These boards cost less that the conversion board readily available.

In the Meantime...

Since I'm waiting for these circuit boards I can't really experiment with the DAC or the preamp. Both use a surface mount IC that can't be placed in a solderless breadboard is hard to desolder. I did however find some opamps at school with relatively low input bias currents. They will be enough for me to experiment with for the time being.

Electronics are slow

Just a quick check in since I haven't posted in a while

As evidenced by the lack of updates the last week has been kind of slow. I wanted to start working on the electronics but it took me a while to get my equipment in order.

Tunnel Current and Preamp

In order to measure a tunneling current I have to complete the current-to-voltage preamp. I tried to work on that today but realized I don’t have all the parts necessary. Below is a simplified schematic of the current to voltage amplifier, or transimpedance amplifier as it’s called. The output voltage is simply the input current times the value of the feedback resistor but inverted. (-Vout = Rf * Iin)

The component that I am missing is the feedback resistor Rf. The tunnel current is usually very small which means that Rf needs to have a very high resistance. Dan Berard uses a 100Mohm resistor for this and I planned to do the same, but I don’t have anything higher than 10Mohm. I could off course string 10 of these together in series but I don’t want to create a big inductance loop in such a sensitive circuit. Any induced current would be added to the tunnel current and make the readings inaccurate. I’ll probably end up experimenting with one of the 10Mohm resistors, and later upgrade to a 100Mohm one when it becomes available. Only problem is that I don’t want to do to much resoldering since im dealing with small components that can be overheated.

Rail Splitter

I did however complete the power rail splitter circuit. Off course it’s only three components but it will an important part for the rest of the project. 

The circuit is based on the TLE2426 ic.

The rail splitter takes an input like 24V and splits it to 12V. The output can be used as a virtual ground on other components so that the original 24V acts like +/- 12V. This will be needed for all future op amp testing.

Hopefully next post will be about acheiving a tunnel current. In the worst case scenario I'm going to have to buy and wait for a 100Mohm resistor, but hopefully I'll get around that.

Extra

In my first (official) post I said that I might post some pictures of my notebook. Well, here are a few! 

These images document my ongoing research and planning. Some information is probably wrong or left out. A lot of these plans are off course different from how they actually ended up. As I build and solve problems the plan always changes.

These are pages from a few weeks before I started this blog. The one on the left is my first overview picture of the mechanics. The idea for this blog existed even back then so I wrote the notes in english. I did not end up staying true to that rule, as seen by the next picture on the right. The picture on the right is from when I was researching the analog feedback loop. I originally had plans for a much more advanced feedback, but ended up simplifying it so that I could begin building. This research did however lead to me learning about PID control, which is interesting and definitely something I'd like to use in some future project. The information is in Swedish, but it's all from the electronics section in this paper which is in english.

As we move further forward in the book we can see that my original ambition for clean and easy to understand notes gradually withers away. These are some notes and schematics of the different systems that will be used in the project. They are newer and more accurate to my current plan than the previous images. Here I wanted to plan everything down to the component level so that I could start ordering parts.

These schematics aim was to show all the components I needed to buy. The one on the left depicts all the electronics while the one on the next page to the right expands on the piezo driver since it didn't fit with the rest. The arrangement of the driver is directly taken from Dan Berards site. I didn't write some resistor values that I wasn't sure of. Instead I ordered many different resistors to be able to experiment and work it out as I go along, using the calculations in the previous notes.

These are not all of the pages in my notebook but the ones I feel give the most information. They will be something to ponder over until my next post.

Scanhead almost done

It has been a few days and I’ve made some progress.

Scanhead

The scan head is almost done! The only thing left now is making some sort of post to hold the tip and maybe some tension solution to keep the whole thing rigid.

Top half

I found a drill bit of suitable diameter for my fine adjustment screw. It does leave some play between the scan head and the fine threaded bushing, but it’s small enough to prevent the bushing from falling out. I have however had problems with the bushing turning with the adjustment screw preventing me from adjusting it’s height. I’ll probably solve this with a small piece of tape or something similar. I do not want to glue the bushing in with any super strong, everlasting ultra epoxy as I want to be able to change or reuse it later.

As seen in the picture the bushing does not leave much of the screw to be adjusted. This is in part due to the removable adjustment knob also taking up some thread space. I might just solve this by simply not using the knob. The screw does have a hole for a hex key so that it still can be adjusted. 

I’ve also thought about modifying the adjustment knob to make it fit over the bushing. If successful, this could give me a few extra millimeters of screw to play with.

Bottom half

I have glued on a copper “scene” on which the sample will be attached. It is glued on a piece of glass in order to isolate it from the scan head body which is grounded. The tip mount will also have some glass part incorporated for the same reason. These are the most electronically sensitive areas of the build.

I have also put a similar piece of glass on the other end of the bottom plate. This one is just to get less friction between this plate and the adjustment screw.

I have also constructed small standoffs which almost looks like tiny rocks in the picture. These are made of a harder metal and have a point to them. They will act as a pivot point when adjusting the tip height with the adjustment screw. Currently they are just placed on the bottom plate, but I plan to solder them in place when I know exactly where they are going to be.

If i ever redo this part I'll need to find a good way to cut glass. I sacrificed a few micriscope slides trying to get shards of a nice shape and size.

Until Next Time

After getting the tip mount in place I can really start experimenting with tunneling current and aproach. The first thing that I'll have to do then is to make the preamp. I plan on using the same circuit as Dan Berard but with a different op amp. It will however have to wait, since I'll be traveling this weekend.

Piezo Mount Finished!

Late Update Today

I know that I said that I would come with updates yesterday but school finished late and I ended up not getting the opportunity to work on the project. Today was however, a different story. I spent some time after school doing some drilling, cutting and a lot of filing. 

I have now completed the piezo mount for the scan head. The aluminium bar in this picture will become the top half of the scan head. On it is a plate held on by two screws. The piezo buzzer is mounted underneath this plate, so that it can be easily changed whenever necessary. 

I planned to have a wider bar for the scan head but it seems as though I missread the dimensions when ordering materials. It will still work though, since all the necessary parts still fit on it. I will however have to change out the three adjustment screws to two rigid standoffs and a a single adjustment screw. this design should retain pretty much the same functionality as the 3 screw one so it’s not a big compromise. 

For the scan head the only things left now is making a tip mount on the piezo and mounting the fine adjustment screw. The later requires a hole with a very specific diameter. Luckily my schools workshop has a small cnc router that could do the job. I have some tinkering time tomorrow, so if I get that router to obey my commands I can probably finnish the mechanical parts of the scan head then.

First post!

Ever since I came across Dan Berard’s site the idea of building a homemade STM has been in the back of my mind. After a lot of research and planning I was tired of just thinking about this project and decided to actually start building a first version.

The plan

Scan head

My scan head will probably be quite similar to Dans. I plan to have two pieces of aluminum separated by three screws acting as coarse approach. One of the pieces will hold the sample while the other one holds the screws and the piezo scanner. The piezo scanner will be made by a simple piezo buzzer with one of it’s electrodes cut into 4 quadrants. This method was invented by John Alexander who also used it in a homemade STM.

Electronics

My plans for the electronics are quite different from Dan Berard’s. I have decided to opt for an analog feedback control rather than a digital one. That being said I have borrowed some designs such as his preamp.

This is a block diagram showing the different parts of the STM electronics.

The Arduino tells the 4ch DAC to produce a raster pattern with it’s X and Y outputs. This causes the piezo to change shape and the scanning tip to move across the sample surface. The resulting tunneling current signal is amplified by the preamp. The error amp and integration amp process this signal into the desired Z position which is then sent to the piezo control circuit. The arduino reads the Z signal and sends it to the computer which (if everything works) produces a nice image.

What is STM?

All this might sound very mysterious to you reading if you don’t know what STM even is. Well since I’m interested in this stuff, I am also happy to explain it. 

Image taken from Wikipedia

STM stands for scanning tunneling microscopy. It is a technology in which you bring a very sharp needle very close to a conductive sample. If the sample and the needle are given a small voltage potential, and they are close enough, a tunneling current will occur. This is because of a phenomenon known as “quantum tunneling” in which particles (electrons in our case) can go from one place to another, even though they have a zero probability to be anywhere in between. This tunneling current is very sensitive to distance, so when the tip is moved across the sample the tunneling current changes with sample height. If the tip is scanned across the sample an image of the sample topography can be produced. Given that the microscope is built well enough, it can image individual atoms! Often the height of the needle is adjusted so that the tunneling current stays the same. In this case the height of the needle is what is used to produce the image.

For The Future

I have received the parts I have ordered and I am ready to start building! Tomorrow at school I will start working on the scanning head. (I dont have a drill press at home). My first goal is to achieve a tunneling current. After that I will start trying to produce a constant height image, after which I will get into the feedback electronics. This method of doing one thing at a time will hopefully save me some trouble shooting. I will probably also post some pictures of my notebook. I have used it for most of the planning of this project. It probably gives a more detailed (although harder to read) picture of my planning.

Updates are comming...

I am currently practising for SAT tests.

Expect more information about the project after November 3rd