Electronics

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A/D: Digital to Analog Converter
The Analog to Digital (A/D) converter is an on-chip module that periodically samples external analog signals and produces corresponding digital values. The A/D converter is typically used to measure analog inputs like motor speed, temperature, and pressure or fluid levels.

Aperture Lists
Increasingly, vector Photo Plotters are being replaced by the laser Photo Plotter, which emulates the older style machine in a raster (bit-map) fashion. While use of the term "aperture" to describe a pad or trace shape persists, the term "aperture wheel" is now being replaced by "aperture list", which implies the greater flexibility now available to the designer.

There are four principle advantages with aperture lists on raster plotters:

Aperture shapes can be easily generated in software, thus eliminating the need to design a physical wheel.

The aperture shapes can be described in the Gerber file, if certain extended-Gerber formats are supported ("RS274X", "MDA FIRE AutoPlot", etc.). This reduces chances of getting the wrong list.

More apertures can be defined on a list.

Allowable apertures sizes are typically (but not always) greater than those imposed by the physical dimensions of an aperture wheel.

Aperture Tables (RS-274D)
Aperture tables must contain the following information:

Custom apertures can be accommodated but must be accompanied by a detailed dimensioned drawing.

Note: Aperture lists must be entered into our CAM software in order to correctly view your Gerber data. Aperture lists, when entered by hand, are susceptible to human error. A more desirable method is to use software to convert the supplied aperture table. Circuit Graphics has software to convert aperture lists output from the most commonly used CAD packages to a format recognizable by our CAM software. For this conversion process to work, you must tell us what your CAD package is and ensure that you do not edit the aperture file in any way. We have conversion programs for the following CAD systems:

Aperture Wheel Setup for Vector Plotters
The setup of an aperture wheel is an exacting and time consuming process since each aperture in the wheel must be hand-mounted and aligned. In order to avoid repeated setup costs, designers have the Photo Plotting vendor keep a wheel on file and are forced to always use that same set of apertures. This has obvious drawbacks, both in terms of design flexibility and the ease of migration to other vendors.

RS-274D files were not designed to communicate any information about the apertures in use, only to specify where they are used. This has led to a great deal of confusion between designers and fabricators since designers aren't aware that a Gerber file alone is not sufficient to define the board - an external document describing the apertures is also needed.

Aperture Wheels
Traditionally, the Photo Plotter counterpart to a pen plotter's pen rack has been the aperture wheel. The aperture wheel is a disk with 24 or 70 apertures arrayed radically along its circumference.

When the Photo Plotter selects an aperture, the aperture wheel is rotated to place the desired aperture between the light source and the film. Apertures are themselves pieces of film and can be made to any shape required, although in practice this is a time-consuming process and there is a physical limitation on size.

Arcs and Circles
It is vital that any arcs and circles created be represented in the CAD database and the resultant data file (see below) as true circular entities. Many CAD systems create and/or output arcs and circles as hundreds of short line segments, which are problematic to CAM systems. If the designer's artwork CAD system is incapable of creating true arc data, a suitable 2D mechanical CAD program should be used to create board profiles.

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E-Mail: info@pcbdesignandfab.com

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