Issue link: https://iconnect007.uberflip.com/i/1522071
34 DESIGN007 MAGAZINE I JUNE 2024 en, the source impedance must be extracted from the input/output buffer information speci- fication (IBIS) model of the driver IC. Subtract- ing the source impedance from the trace char- acteristic impedance gives the required series terminator value. Unfortunately, drivers do not have the same impedance as the transmission line (typically 10–35 Ω) so series terminations are used to balance the impedance, match the line, and minimize reflections, particularly on long traces. Impedance matching slows down the rise and fall times, reduces the ringing (over/ undershoot) of signal drivers, and enhances the quality of a high-speed signal. Now that the impedance of the transmis- sion line is solved, the next step is to extract the driver (source) impedance from the IBIS model of the device. But how do you extract the source impedance? IBIS models can be used to characterize I/V output curves, ris- ing/falling waveforms and pin parasitics of the device packaging. e models' source imped- ance should be based on these I/V curves. I recall a renowned signal integrity expert once saying that it is impossible to extract the source impedance from the IBIS driver model. However, it is possible (otherwise simulators would not work), but it is a very time-intensive process, and tackling the I/V curves to deter- mine the impedance should be le to precision soware to solve (Figure 3). One also needs to consider the on-die ter- minations (ODT) for high-speed memory devices. Although the termination resistors on the motherboard reduce some reflections on the signal lines, they are unable to prevent reflections resulting from the stub lines that connect to the components on the module card. As you can see, the Micron DDR4 device has various termination options, and each of these will result in a different waveform at the load. On-die terminations can be used with some memory devices to match the transmission line and dampen the reflections. Incorporating a resistive termination within the DRAM device improves the signaling environment by reduc- ing the electrical discontinuities introduced with off-die termination. DDR4 and DDR5 Figure 3: Micron 4 Gb DDR4-3200 SDRAM with ODT. (Source: iCD Termination Planner)