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The First 12 Reasons for the Designation of DC Trace Impedance
Posted:10:00 AM November 20, 2018 writer: G

1. Reduced power supply voltage and associated logic threshold

- 1 volt is not a common voltage, but it can reduce noise. Especially processors and high current density devices will attract a few amps of supply current.

2. Both LVDS and Gigabit Ethernet contain DC terminated transmission lines

Excessive trace impedance can cause common mode problems in the receiver.

3. The combination 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 can significantly attenuate the signal.

4. The main OEMS has such a demand

- 3 mil (75 micron) and shorter traces

5. Some trace resistance will reduce the inflow current

In hot-swappable PC card applications, this method is just one of many ways to reduce the inflow current with inherent resistance. The goal is to achieve a low-cost method without adding physical components.

The functions used, other methods are the sense resistors in switching power supply applications, or the heating traces in low temperature applications.

6. Increased power track

Explain that the designer selects 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.

7. Some microwave path PCB boards have faults in which the path cannot be connected.

This phenomenon is more common at the beginning of use because the trace impedance through the microwave path is higher than expected.

8. PCB manufacturer controls impedance by changing line width

This method is commonly used by PCB manufacturers. The thinner the line, the smaller the line width, the more the attenuation on the transmission line will increase, and the rise time will be shorter.

9. Designers want to reduce EMC by reducing high frequency harmonic content

People use the inherent overall impedance of thin traces to "kill" high-frequency harmonics of clocks or high-speed data lines to ensure EMC compliance.

10. Not only are the traces getting thinner, but also:

The tendency to use 1/4 ounce of copper (0.35 thousandth of a inch), the reduction in geometry inevitably leads to a reduction in series resistance.

11. The ability of the designer to simulate performance has improved

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

12. In the field of mobile communications

The dc impedance is especially important in designs with size and space constraints, especially antennas.

Sum up

Power supply designers must always pay attention to DC impedance. Increasing PCB operating speed, shrinking geometry and power supply voltage, making DC impedance a lower speed PCB board

The main method of impedance in design. It is useful to reduce the impedance in some cases (for example: EMC), but in most cases it will limit the maximum operating speed and line length. Understanding the actual impedance and yield changes will help the designer's design.

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