IMPEDANCE MATCHING FOR HIGH-SPEED SIGNALS IN PCB DESIGN (2)

DIFFERENTIAL SIGNALING AND IMPEDANCE MATCHING

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• CML (Current Mode Logic): Specified input and output impedance of 50 Ohms, which is referenced to the single-ended impedance of each trace in a differential pair. CML chips may not have input termination resistors and require pull-up and pull-down resistors to match the input level to the Vdd level on the chip (see the application notes linked below).

PECL (Pseudo-Emitter Coupled Logic): Traces have 100 ohm differential impedance and 50 Ohms single-ended impedance. Outputs have low impedance (~5 Ohms), which requires pull-up/pull-down resistors for impedance matching.

HSTL (High Speed Transceiver Logic): There are four classes of HSTL for signaling between CMOS axx Sts, each requiring different termination methods.

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