PCB News - 12 reasons for DC trace impedance

More and more manufacturers require that the DC trace impedance on the PCB board be specified. The following explains the reasons for specifying and controlling the impedance of the DC trace from the designer's point of view...

1. Increase in power rails

It shows that the designer chooses the power rail again when there are not enough layers. Coupled with the increase in low-voltage logic circuits, it is necessary to determine the series resistance of these power rails, because in a low-voltage environment, although the power supply tolerance limit is the same in %, the actual millivolt value has been reduced before the fault occurs. NS.

2. Some microwave path PCB boards have a fault that the path cannot be connected...

This phenomenon is more common in the initial stage of use, because the impedance of the trace through the microwave path is higher than expected.

3 PCB manufacturers control impedance by changing the line width

This method is a common method used by PCB manufacturers. The thinner the line, the smaller the line width, the attenuation of the transmission line will increase, and the rise time will also be shortened.

4. The designer wants to reduce EMC by reducing the content of high-frequency harmonics

People use the inherent overall impedance of the thin traces to "eliminate" the high frequency harmonics of clocks or high-speed data lines to ensure that products comply with EMC regulations.

5. Not only the trace is getting thinner, but also:

The trend is to use 1/4 ounce copper (0.35 per thousandth of an inch), and the reduction in geometric size inevitably leads to a reduction in series resistance.

6. Due to the reduction of the power supply voltage and related logic circuit thresholds

-1 volt is not a common voltage, but it can reduce noise. Especially when the processor and high current density equipment will draw a few amperes of power supply current.

7. Both LVDS and Gigabit Ethernet contain DC-terminated transmission lines

Too high trace impedance can cause common pattern problems in the receiver.

8. The series connection of precision traces will greatly attenuate the signal

If the trace impedance is too high, long precision traces such as LVDS and Gigabit Ethernet and high-speed serial buses will greatly attenuate the signal.

9. Major OEMS have such a demand!

-3mm (75 microns) and shorter traces

10. Certain trace resistances will reduce the incoming current

In hot-swappable PC card applications, this method is just one of many ways to use inherent resistance to reduce the inflow of current. The purpose is to achieve practical functions through a low-cost method without adding physical components. Other methods include the sense of switching power supply applications. Resistance measurement, or heating traces in low temperature applications.

11. The designer’s ability to simulate performance has improved

If you want to obtain a reasonable Rdc in the finished product, you must specify the Rdc in the modeling and simulation tools.

12. In the field of mobile communications

In designs with size and space constraints, especially antenna designs, dc impedance is particularly important.

Summarize

Power supply designers must always pay attention to DC impedance. The ever-increasing PCB board operating speed, shrinking geometric dimensions and power supply voltage make DC impedance the main method to reduce the impedance in high-speed PCB board design. In some cases it is useful to reduce the impedance (for example: EMC), but in most cases it will limit the maximum operating speed and line length. Understanding the actual impedance and output changes will help the designer's design.