One TI Quad Op-Amp (TL074CN) – Ebay
Four 10 kΩ Resistors – Ebay
One 10 MΩ Resistor – Ebay
One Stepper Motor Controller – Sparkfun part number: ROB-11699
One 12V/1A power supply – Ebay
Four 9V battery plugs – Ebay
Four 9V batteries – Ebay
Breadboard – Ebay
Part List - Optional
One 14 pin DIP IC socket – Ebay
One custom printed circuit board (Amp Rev B) – OSH Park or similar.
24 Female Headers – Sparkfun part number: PRT-00115
24 Break Away Headers – Sparkfun part number: PRT-00116
Assorted Grommets – Home Depot part number: 032076074746
By using a good data acquisition system, the electrical system is simplified. There are only three amplifiers needed and they all are on a single chip. Two of them invert the X and Y scan voltages and the last one converts the tunnel current into a voltage. A stepper motor controller board is also needed.
The circuit diagram below shows all the connections. On the left, connections will be made to either the PCIe-6323 card or the USB-6001 device. The pin number for the PCIe card is before the slash and the pin name for the USB device is after the slash. For the best noise results, the amplifier is powered by four 9-volt batteries – two wired in series to provide -18 V and two wired in series to provide +18 V.
Never unplug or plug in the motor while the stepper motor controller’s power supply is on. It will burn out the stepper controller. Make sure you don’t reverse the connections and connect 12 volts to GND.
Always unplug the batteries when you are not using them so they do not discharge.
Unplug the stepper motor power supply when you are not using the motor so it doesn’t heat up too much.
To avoid crashing the tip into the sample, it is recommended that the half-step mode be used rather than the full-step mode on the stepper motor. This change is made on the stepper motor controller board by connecting the MS1 pin to +5 volts and leaving the MS2 & MS3 pins connected to ground. In this configuration, four half-steps move the sample through the vertical piezo range. The quarter-step mode could also be used, but the reduced risk of crash does not seem to justify the slower approach speed.
Build Option 1 – Fast and Noisy.
The fastest build will run direct wires to each component and build the circuit on a bread board. The best results are obtained by making the wire from the sample to the amplifier as short as possible. A typical system is shown below. The STM is placed on foam to minimize vibrations. The connections to a PCIe card are made through the green and blue breakout boards purchased from DaqStuff. The problem with the above configuration is that it picks up too much noise on the wire from the sample for good resolution.
Build Option 2 – Custom PCB.
A better build option is to use a printed circuit board and removable headers. The PCB can be placed closer to the sample which reduces noise and the headers make trouble-shooting the system much easier. The PCB board file can be downloaded from this link and sent to a vendor such as OSH Park for fabrication. In the pictures below, the board and schematic are shown.
The components should be on the side with the labels and all the soldering should be done on the opposite side. All the headers are identified by a “J” label. Pin 1 of each header is the one closest to the “J” label. The connections to each header are listed below:
J5 Port 0 - Analog Connection
21/AO1: Z from Computer
J2 Port 1 – Analog Connection
22/AO0: X from Computer
68/AI0+: Tunnel Current
21/AO1: Y from Computer
34/AI0-: Tunnel Current Ground
J6 Piezo Connection
J4 Oscilloscope Connection
Z from Computer
This circuit board can be attached to the housing of the STM right by the sample. The board can be glued to the housing or attached to grommets. In the picture below, the grommets are glued to the housing and hold the board tight enough during operation, but allow it to be removed if needed.