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| PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS PM5345 S/ U N ITM 155 PECL OUTPUT TERMINATIONS APPLICATION NOTE ISSUE 3: SEPTEMBER 1997 PMC-Sierra, Inc. 105 - 8555 Baxter Place Burnaby, BC Canada V5A 4V7 604 .415.6000 PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS PMC-Sierra, Inc. 105 - 8555 Baxter Place Burnaby, BC Canada V5A 4V7 604 .415.6000 PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS CONTENTS 1 WHAT TERMINATION IS RECOMMENDED FOR THE PECL OUTPUTS ?1 i PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS LIST OF FIGURES FIGURE 1 -........................................................................................................ 1 FIGURE 2 -........................................................................................................ 2 ii PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS LIST OF TABLES TABLE 1 -........................................................................................................ 2 iii PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS iv PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS 1 WHAT TERMINATION IS RECOMMENDED FOR THE PECL OUTPUTS ? In a typical application, only the PECL data outputs (TXD+, and TXD-) are used. The TXCO+ and TXCO- outputs are left open. For this case, two termination schemes are possible. The first scheme is described in the PM5345 datasheet (see the application examples section), and involves calculating RVT1 and RVT2 values based on the required PECL output levels and the termination impedance. We have observed that the signal waveform shapes associated with this termination can be improved by the following modification: Figure 1 - connect VT1 to VSS. IOH current source is disabled IOH Zo R1 R2 IOL VDD RVT2 VDD The IOH current source is disabled by tying VT1 to VSS. R1 and R2 provide two functions: 1. the voltage divider set up by R1 and R2 sets the PECL output high voltage (nominally 4.2V for VDD = 5V). 2. the Thevenin equivalent impedance of R1 and R2 terminates the transmission line. In addition, the equation used to calculate RVT2 is changed from the equation in the datasheet. The new equation becomes: RVT2 = 12.564*Zo where Zo is the characteristic impedance in 1 PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS For a 100 characteristic impedance transmission line, recommended resistor values are: Table 1 R1 R2 RVT2 626 120 1260 This termination is recommended for all new designs, and must be used if both PECL output pairs (clock and data) are used in an application. How do the PM5345 PECL outputs operate? The PECL outputs, TXD+, TXD-, TXCO+, and TXCO-, are implemented using CMOS technology, and can be modeled as ideal current sources. This is the dual of a traditional ECL output which is typically modeled as a voltage source. The output high and low currents (IOH and IOL) are set using resistors RVT1, and RVT2. IOH and IOL generate the output high and low voltages across the termination resistor, RO. Figure 2 VDD RVT1 IOL VDD RVT2 VDD Ro IOH Zo The advantage of this method is that the PECL output levels (VOH and VOL) are independent of variations in the VDD supply rail, and true PECL output levels are achieved across the complete VDD voltage range (4.75 to 5.25 V). The output 2 PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS levels will of course be affected by variations in RVT1 and RVT2, as well as the terminating resistor, Ro. 3 PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS NOTES 4 PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS NOTES 5 PM5345 APPLICATION NOTE PMC-930920 ISSUE 3 PECL OUTPUT TERMINATIONS CONTACTING PMC-SIERRA, INC. PMC-Sierra, Inc. 105-8555 Baxter Place, Burnaby, BC Canada V5A 4V7 Tel: Fax: (604) 415-6000 (604) 415-6200 document@pmc-sierra.com info@pmc-sierra.com apps@pmc-sierra.com http://www.pmc-sierra.com Document Information: Corporate Information: Application Information: Web Site: None of the information contained in this document constitutes an express or implied warranty by PMC-Sierra, Inc. as to the sufficiency, fitness or suitability for a particular purpose of any such information or the fitness, or suitability for a particular purpose, merchantability, performance, compatibility with other parts or systems, of any of the products of PMC-Sierra, Inc., or any portion thereof, referred to in this document. PMC-Sierra, Inc. expressly disclaims all representations and warranties of any kind regarding the contents or use of the information, including, but not limited to, express and implied warranties of accuracy, completeness, merchantability, fitness for a particular use, or non-infringement. In no event will PMC-Sierra, Inc. be liable for any direct, indirect, special, incidental or consequential damages, including, but not limited to, lost profits, lost business or lost data resulting from any use of or reliance upon the information, whether or not PMC-Sierra, Inc. has been advised of the possibility of such damage. (c) 1997 PMC-Sierra, Inc. PMC-930920 (R3) Issue date: September 1997 PMC-Sierra, Inc. 105 - 8555 Baxter Place Burnaby, BC Canada V5A 4V7 604 .415.6000 |
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