Prototyping techniques help verify analog-circuit performance

Figure 2 shows another prototype in which a single-sided copper-clad Vectorboard has

predrilled holes on 0.1-in. centers. Power buses are at the top and bottom of the board.

The power pins of each IC have their own decoupling capacitors. Because of the loss of

copper area due to the predrilled holes, this technique does not provide as low a ground impedance as a completely covered, copper-clad board.

A variation of this technique is to mount the ICs and other components on the non-copper-clad side of the board. The holes serve as vias, and you do the point-to-point wiring on the copper-clad side of the board. To prevent shorts, you must drill out the copper surrounding each hole used for a via. This approach requires that all IC pins be on 0.1-in. centers. You can use low-profile sockets for low-frequency circuits, and the socket pins allow for easy point-to-point wiring.

One commercial breadboarding system has most of the advantages of robust ground, screening, ease of circuit alteration, low capacitance, low inductance, and several additional advantages. The system is rigid, has components that are PCB Assembly close to the ground plane, and lets you easily calculate node capacitances and line impedances. The product is available as "Mini-Mount" in Europe from Wainwright Instruments

GmbH (Andechs-Frieding, Germany) and as "Solder-Mount" in the United States (where the trademark "Mini-Mount" is the property of another company) from Wainwright Instruments Inc (San Diego, CA).

Solder-Mount comprises small pieces of pc board with etched patterns on one side and

contact adhesive on the other (Figure 3a). The pc-board pieces stick to the ground plane, and you can solder components to the pieces. The board pieces are available in a variety of patterns, including ready-made pads for eight-pin SOICs to 64-pin dual-in-line types; strips with solder pads at intervals ranging from 0.040 to 0.25 in., including strips with 0.1-in. pad spacing, which you can use to mount dual-in-line devices; strips with 50, 60, 75, or 100_ conductors to form microstrip transmission lines when you mount them on the ground plane; and a variety of pads for mounting various other components.

The conductor-strip feature of Solder-Mount at VHF is convenient. You can use these strips for 3 von 8 9/25/00 4:09 PM EDN -- 02.15.96 Prototyping techniques help verify analog-circuit performance http://www.ednmag.com/reg/1996/021596/04df3.htm

transmission lines, impedance matching, or power buses. Glass-fiber/epoxy pc board is somewhat lossy at VHF and UHF, but the losses are probably tolerable if microstrip runs are short. Self-adhesive, tinned copper strips and rectangles (LO-PADS) are also available as tie-points for connections. These strips have a relatively high capacitance to ground and, therefore, serve as low-inductance decoupling capacitors. The strips come in sheet form, and you can cut them with a knife or scissors.

The main advantage of Solder-Mount construction over bird's nest or dead-bug construction is that the circuit resulting from using Solder-Mount is stiffer and, if an application requires, smaller. The latest Solder-Mounts for surface-mount devices let you construct breadboards scarcely larger than the final pc board. It is generally more convenient if the prototype is somewhat larger than the final breadboard, however. Solder-Mount is durable enough for low-quantity production. A 2.5-GHz PLL prototype built with Solder-Mount (Figure 3b) is a high-speed circuit, but the technique is equally suitable for the construction of high-resolution, low-frequency analog circuitry.

Both the dead-bug and Solder-Mount techniques become tedious for complex analog or mixed-signal circuits. Formal layout techniques often produce better prototypes of large circuits. One approach to prototyping more complex analog circuits is to lay out a double-sided board using CAD. PC-based layout packages offer easy layout and schematic capture to verify connections. Such packages are available from PADS Software (Marlborough, MA) and Accel Technologies (San Diego, CA). Although

most layout software offers some autorouting capability, this feature is best left to digital designs. After you place the components in position, manually route the interconnections following layout guidelines (Reference 3). After you complete the layout, the software verifies the connections against the schematic diagram's netlist.

Many design engineers find that they can use CAD techniques to lay out simple boards or work closely with a layout person who has experience in analog-circuit boards. The result is a pattern-generation tape or Gerber file, which you normally would send to a pc-board-manufacturing facility, which makes the final board. Rather than use a pc-board manufacturer, however, you could use automatic drilling and milling machines that directly accept the pattern-generation tape. These prototype-board cutters are

available from LPKF CAD/CAM Systems Inc (Beaverton, OR) and T-Tech Inc (Atlanta). These systems directly produce single- and electronic assembly double-sided circuit boards by drilling all holes and then using a milling technique to remove copper, create insulation paths, and create the finished board.