Computer Data Acquisition

Back when the Scanning Tunneling Microscope was invented in 1981, the CPU in most computers was very slow. The feedback loop needs a bandwidth of about 5 kHz and the CPU couldn’t deliver that speed. The only way to get the desired bandwidth was to build the feedback system using analog electronics. By the 1990s, Digital Signal Processors (DSPs) were available and allowed the most of the feedback loop to be built in software. This allowed much greater flexibility in customizing the feedback system, but was expensive since DSP-based systems cost over $2500.00. In the last 10 years, the speed of the CPU in most computers have increased to the point that the CPU can run the feedback loop in addition to running the operating system, displaying data, and getting input from the user. The requirements for the computer CPU for this microscope are not very demanding by today’s standards. A computer with a dual or quad core CPU should work for this project.

The heavy lifting from the data standpoint is taken care of by the data acquisition card. Although the cost is reduced by not needed a DSP-based system, the data acquisition card is still the most expensive part of the project. Ideally the card should have the following features:

4 voltage output channels (DACs) with a voltage range of -10 to +10 volts at a minimum of 14 bit resolution (16 bit resolution is preferred).
1 voltage input channel (ADC) with a voltage range of -10 to +10 volts at a minimum of 14 bit resolution (16 bit resolution is preferred).
2 digital out channels.
Ability to read and write DC voltages (sound cards don’t do this).
Ability to read a single value and write a single value and repeat this combination at a minimum of 1000 times per second (2000 times per second is preferred).

This feature list is out of the range of most low-cost solutions but the National Instruments Corporation makes boards that fit both the minimum and preferred requirement list.

Part List - Option 1: Slow but Simple.

The minimum option costs $378 and is based around the NI USB-6001 device. Each device only has two voltage outputs, so two have to be purchased for this project. Each device includes screw terminals so connection to the microscope is easy. Because the USB interface is not optimized for the single read /single write operations that a feedback loop needs, the bandwidth of the feedback look is slow so image acquisition will take longer.

National Instruments USB data acquisition boxes.

Part List - Option 2: Fast and Expensive.

The preferred option costs $1800 and is based around the NI PCIe-6323 card. This card is available from National Instruments for about $900.00. To connect the card to the electronics, two cables and two breakout boards are needed which add another $900.00 to the total cost. In the picture below, the connections from breakout boards to the microscope are shown.

There are a couple of ways to lower this bill. The company Winford.com sells cables and breakout boards https://www.winford.com/products/cbvh68.php that are much cheaper. Two of these cables and breakout boards have a total cost of $300. Another method to lower the cost is to use the older National Instruments PCI-6229 card. This card uses the older PCI bus and can be found used on Ebay for about $400.00. There does not seem to be a difference in STM operation between the PCI and PCIe cards.

National Instruments break-out boxes for the PCI and PCI-e data acquisitionboards.

Part List - Option 2b: Fast, Used, and Some Wiring.

As the PCI connectors disappear from computers, the price on the National Instruments PCI-6229 card is beginning to fall. In January 2018, Ebay had several listings for sale in the range of $375. If a computer with a PCI connection is available, this now the preferred solution. In addition to the card, a way of connecting to the microscope is needed. A single $50 “12 foot Shielded VHDCI Male Cable” http://www.daqstuff.com/100768_vhdci_mm_cable.htm can be purchased and cut in half. Headers can be added to the following conductors and plugged straight into the STM. The wire color is correct for the 100768 cable only.

J2	Plug1	Wire Color
1	22 – X	Center bundle: White with pink stripe.
2	21 – Y	Tan with yellow stripe.

J5	Plug0	Wire Color
1	21 – Z	Tan with yellow stripe.
2	55 – Ground	Yellow with brown stripe (paired with 21).
3	68 - Tunnel Current Read	Purple with pink stripe.
4	34 – Tunnel Current Ground	Pink with purple stripe (paired with 68).
5	22 – Bias	Center bundle: White with red stripe.

BigEasy	Plug0 – Motor	Wire Color
GND	18: (D GND) Ground	White with blue stripe (paired with 52).
STEP	17: (P0.1) Motor step	Tan with brown stripe.
DIR	52: (P0.0) Motor direction	Blue with white stripe (paired with 18).

The connections are explained more fully in the electrical section of the build instructions. The only change between this list and the connections listed in the electrical section is that the digital ground for the stepper motor moves from pin 50 to pin 18. The reason for this change is that pin 50 is close to pin 52 on the connection box but pin 18 is twisted with 52 in the cable. This solution moves the total cost of the data acquisition system down to just over $400.