The computer is connected to a custom built I/O board. The I/O board input provides 16-bit ADC sampling. On the output side, it has five 16-bit DACs giving the PC software control of the piezo movement in two dimensions, the tunnel bias voltage Vb, and the target tunnel current Iref as well. There are also some logic level output lines being used to drive the stepper motor for coarse approach. The PC connects to this board via a standard parallel (Centronics) port.
Four high voltage amplifiers drive the piezo-ceramic actuator. Each amplifier feeds one of the four electrode quadrants coating the outer tube surface.
The tunneling current is a function of the small D.C. voltage applied between sample and tip, and of gap width. This tiny current is translated to a measurable voltage by the electrometer amplifier.
The feedback circuit maintains a constant gap between the tip and the sample. It does this by continuously monitoring the magnitude of the tunnel current. During scanning, the software drives the tip across the sample surface in a raster pattern similar to the one used on a CRT display. The height of the tip as it moves along, follows the undulations of the sample surface and is recorded on the PC. These co-ordinates are then processed to provide a three-dimensional relief.
The PC used is a standard IBM compatible, with an intel 80486 processor. (DX4 - 100MHz - floating point co-processor). This may seem somewhat primitive by today's standards, but it is important to note that the software uses plain MS-DOS. Note that most of the latest windows based operating systems provide a DOS emulation mode, but no real DOS operation. This is important because the software works by continuous polling, and the windows system interrupts would be prohibitively disruptive.
I/O card Tunneling unit Pre-amplifier
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