Prototyping techniques help verify analog-circuit performance

The result is a board similar to the final manufactured double-sided pc board, except that the final board lacks plated-through-hole capability, and you must wire and solder on both sides any vias between the two layers of the board. Minimum trace widths of 25 mils (1 mil=0.001 in.) and 12-mil spacing between traces are standard, although you can achieve smaller trace widths. The size of the milling bit, which is typically 10 to 12 mils, dictates the minimum spacing between lines.

An example of such a prototype board is a daughterboard that interfaces an AD9562

dual PWM board in a 44-pin PLCC package to a test-set motherboard (Figure 4). The

leads are on 50-mil centers, and the traces are approximately 25 mils wide. This board

illustrates the resolution of the milling machine, but you can use the technique to

produce more complex boards.

IC sockets can degrade the performance of high-speed or high-precision analog ICs.

Although low-profile sockets ease prototyping, even these sockets often introduce

enough parasitic capacitance and inductance to degrade circuit performance. If you must use sockets in high-speed circuits, an IC socket comprising individual pin sockets, sometimes called "cage jacks," mounted in the ground plane board may be acceptable (Figure 5). Clear the copper on both sides of the board by about 0.5 mm around each ungrounded pin socket and then solder the grounded pin sockets to ground on both sides of the board. Both capped and uncapped versions of these pin sockets are available from electronic assembly Amp (Harrisburg, PA) (part no. 5-330808-3 and 5-330808-6, respectively). The pin sockets protrude through the board far enough to allow point-to-point wiring interconnections between them.

The spring-loaded, gold-plated contacts within the pin socket make good electrical and mechanical connection to the IC pins. Multiple insertions, however, may degrade the performance of the pin socket. The uncapped versions allow the IC pins to extend from the bottom of the socket. After the prototype is functional and requires no further changes, you can solder the IC pins directly to the bottom of the socket, thereby making a permanent and rugged connection.

Special prototyping considerations

These prototyping techniques apply only to single- or double-sided pc boards. Multilayer pc boards do not easily lend themselves to standard prototyping techniques. If a design requires multilayer-board prototyping, you can use one side of a double-sided board for ground and the other side for power and signals. You can use point-to-point wiring for additional runs, which would normally exist on the additional layers a multilayer board provides. Unfortunately, it's difficult to control the impedance of the

point-to-point wiring runs. The high-frequency performance of a circuit prototyped in this manner may differ significantly from that of the final multilayer board. Other difficulties in prototyping may occur with op amps and other linear devices that have bandwidths greater than a few hundred megahertz. Variations of greater than 1 pF in parasitic capacitance between the prototype and the final board can cause subtle differences in bandwidth and settling time.

If you use DIP packages for the prototype but SOICs for the production packages, you can see differences between the performance of the prototype and the final pc board. For instance, the AD8001 current-feedback op amp (approximately 800-MHz bandwidth for G=1) is available as both an eight-pin DIP and an eight-pin SOIC.Table 1, which contains data collected with the use of an evaluation board, reflects the difference in performance between the two packages after you optimize the feedback (RG) and feed-forward (RF) resistors (Figure 6a). The resistor values in Table 1 produce the highest 0.1-dB flatness bandwidths. The SOIC package's bandwidth is higher because of lower package parasitics. All resistors and capacitors on the board are surface-mount types for low parasitics.

Evaluation boards can be extremely useful in evaluating new analog ICs; these boards let you verify the IC's performance with minimum effort and without constructing your own prototype. Evaluation boards can range from relatively simple ones, with just op amps, for example, to rather complex ones for mixed-signal ICs, such as ADCs. ADC evaluation boards often have onboard memory and DSP μPs for 6 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 analyzing the ADC's performance. IC manufacturers often provide software so that these more complex boards can interface with a PC to perform complex signal analysis, such as histogram and FFT testing.

Most manufacturers of analog ICs provide evaluation boards, usually at a nominal cost. Regardless of the product, the manufacturer takes proper precautions regarding grounding, layout, and decoupling to ensure optimum device performance. The layout of the components on the evaluation board can guide both the prototype electronic assembly and the final pc-board layout. The artwork or CAD file is usually free, should you wish to copy the layout directly or change it to suit the application.