![]() What I mean is, if the line is slightly longer than the limit for an electrically short line and there is some residual mismatch, the (line + filter) input impedance may be quite different from the transmission line’s characteristic impedance. ![]() I mentioned above that the source may need to be matched to the input. Matching to Transmission Line Input Impedance Finally, In Step 3, you may need to apply an additional matching network to match the source impedance to the (line + filter) input impedance. In Step 2, the target (equivalent) impedance you calculated in Step 1 becomes the load used in the input impedance calculation in Step 2. You can find a great tutorial at this link (starting on ). Step 1 is foundational in circuit analysis so I won’t show an entire explanation here, I’ll probably cover this in another article. ![]() The “Matching” block is often an L-filter. Standard topology for terminating a capacitive load. The designer can pick high pass or low pass functionality in the matching network and apply circuit analysis to get Zeq. Typically, the matching network will be an L-match network or a pair of capacitors/inductors in a pi arrangement with the load. The different impedances used for a capacitive load are shown below. Set the source impedance to match the input impedance calculated in Step 2.Calculate the input impedance for your particular transmission line as seen at the input end using the standard formula.This is easily done with a series/parallel transformation and by solving for the L and C values. Determine the LC filter circuit required to set the (load + filter) equivalent impedance to the target impedance. ![]() To fully impedance match an entire interconnect, you should use the following process: This reflects the fact that, for a real load, it will not be purely capacitive. Specifically, a group of two capacitors or an L-filter will normally be used to determine the The key here is this: the real part of the load impedance must be greater than zero. If you’re only working at a specific frequency, or at a very narrow range of frequencies, then you want to use a standard LC filter circuit. With this in mind, let’s explore two main cases where capacitive impedance and termination are considered: narrowband analog signals and broadband digital signals. In other words, the element is approximated as a capacitor, even if it does not exactly act as such. Stubs on transmission lines, depending on their length and termination.A capacitive input impedance generally decreases with increasing frequency and causes current to lead voltage in phase. Instead, in electronics, when we say a load is capacitive we are most likely referring to a component’s impedance, or specifically its input impedance. In electronics, and specifically in PCBs, a load that appears to be capacitive will only do so in a certain frequency range, and the capacitance may not arise due to intentionally placed capacitors. Even banks of capacitors are not truly “capacitive” loads, at least from a reactive power perspective. Very simply, purely capacitive loads do not exist. Let’s take a look at this important aspect of interconnect design and see what it really means to terminate a capacitive load. Yes, capacitors exist, but all capacitors are non-ideal, and it is this deviation from a theoretical capacitance that determines how to impedance match a load that exhibits capacitive behavior. I see there are a lot of great videos for impedance matching and designing the width of traces to match any incoming cable impedance, but what about matching capacitive loads?Ĭurrent generation MOSFETs and GaN HEMTs are capable of sub 100 ns switching at this point and so I feel like matching to the gate of a power device is going to become a much bigger issue in future…Īs I’ll explain below, the idea of a purely capacitive load is something of a fallacy. I recently received an interesting question from a viewer that asked about terminating capacitive loads: For those of you who pay attention to Altium’s Youtube channels, you know that signal integrity is a hot topic and it generates a lot of questions.
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