What is overshoot? How to solve the problem of overshoot in high-speed circuit signals

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What is overshoot? How to solve the problem of overshoot in high-speed circuit signals [Copy link]

1. What is overshoot?

　　When a faster signal drives a longer trace and there is no effective matching in the trace topology, overshoot often occurs. The main problem caused by overshoot is that the "1" level is higher than the input voltage value (VIHmax) of the receiving device, or the "0" level is lower than the minimum input voltage value (VILmin) of the receiving device, which may cause potential cumulative damage to the device, shorten its working life, and thus affect the long-term stability of the product.

　　2. The general method to solve overshoot is matching, or termination. The central idea of matching is to eliminate the impedance mutation at the end of the signal path. In summary, it can be summarized as follows:

　　There are two forms: serial matching at the source end (as shown in the PCB below) to eliminate secondary reflections, and parallel matching at the terminal to eliminate reflections. Not every matching method is suitable for all occasions. For example, 50ohm parallel matching is generally not used for level logic such as LVTTL/LVCMOS, because the power consumption on the resistor is too large to accept. In addition to matching, there is another effective way to improve overshoot, which is to slow down the signal edge at the driver end, making the original high-speed signal less "high-speed". The most common method to slow down the signal edge is to reduce the driver's drive current. This method is particularly common in FPGA/CPLD design.

　　3. Ringing: Overshoot is often accompanied by ringing, or in other words, overshoot is part of ringing. The sub-peak voltage generated by ringing is overshoot. The reason why the two should be distinguished is that in addition to overshoot, the harm of ringing is that the voltage fluctuations it generates may cross the threshold voltage of the logic level many times, causing misjudgment at the receiving end. For CMOS devices, the ringing process may also extend the time that the upper and lower MOS tubes are turned on at the same time, sharply increase power consumption, and affect the life of the device. Since the generation mechanism of ringing and overshoot is the same, the way to deal with it is no different from dealing with overshoot. Here is only a brief theoretical explanation.