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Why is a series resistor used in high speed signal data lines? Is there any reason to include a resistor or simply protection from the receiver end?

For example, I'd like to understand the reason behind the low value series resistor placed in processor-to-SDRAM data and address lines.

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    \$\begingroup\$ Can you give an example schematic of what you're talking about? A "series resistor" normally means one that is placed in series with the data line, not one that is connected between lines. \$\endgroup\$
    – The Photon
    Aug 18, 2017 at 5:17

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A series resistor between a high-speed driver and a trace is to match total impedance of driving source (impedance of driver + series resistor) with characteristic impedance of transmission line (PCB trace). If the source impedance is not matched to transmission line, multiple reflections would occur, such that the reflected waves would go back and forth, and disrupt signal levels at receiver end, vastly reducing signal quality of the channel and killing signal margins.

In modern (good) silicon designs, however, there is no need in impedance-matching resistors, because normal high-speed interfaces nowadays have controlled impedance, and board designers learn to design traces with perfect match. The resistors are used only in poorly-designed silicon chips where the driver impedance control and properly controlled termination is not implemented.

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  • \$\begingroup\$ So,In IC datasheet Normally source impedance of the driver won't be mentioned.then,how can we choose the series resistor value for matching with the impedance of driver. \$\endgroup\$
    – Murugesh
    Aug 21, 2017 at 6:34
  • \$\begingroup\$ @Murugesh, if you are using drivers with non-controlled impedance, you need to estimate the effective driver impedance based on its drive strength and VOH/VOL levels from datasheet. If you have a LVCMOS33 driver with 8 mA drive strength and datasheet says the VOL is 0.4V, then a ballpark value of the driver impedance is 400/8 = 50 Ohms, so no resistor for 50-Ohm trace, and 15 Ohm for a 65-Ohm trace. But if your driver is listed as 16 mA, then 400/16=25, and you will need 25-33 Ohm series resistor to match your traces. The match won't be perfect, but it is better than nothing. \$\endgroup\$ Aug 21, 2017 at 7:50
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Some literature:

TI Series / Parallel Termination, p.4 ff.

Lee Ritchey's Right the first time, ch. 21

The key concept is to have a transmission line with a continuous impedance. The additional resistor's (series termination resistor) task is to absorb energy reflected by the load at the source, thereby dampening overshoot/undershoot. As a main drawback you're limiting your data rate somewhat as you're putting more load on the transmitter which will reduce rise/fall times. Overall goal: Perfect signal integrity with no EMI issues.

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  • \$\begingroup\$ By adding the series resistor it will slow down the rise time. \$\endgroup\$
    – Murugesh
    Aug 18, 2017 at 7:45
  • \$\begingroup\$ The near end series resistor in a signal line does not "put more load on the transmitter". A parallel termination at the far end of a transmission line to GND or to some supply rail does increase load on the transmitter. \$\endgroup\$ Aug 18, 2017 at 9:28

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