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Are the 78 ohm resistors required for the LXT901 on the AUI port when implementing an on board (without the AUI cable) interface with another device?
Yes, the 78 ohm resistor sets up the voltage bias points for the interface, and should always be implemented. It is also recommended that 2kV caps are used when implementing capacitive coupling in order to stay compliant with the IEEE 802.3 voltage spike tolerance.
Can I leave the LEDT/PDN pin on the 901A/907A, 905, and 908 float if I am not using its LED functionality?
No, if you are not using the LEDT functionality you must pull this pin up with a 10k 1% resistor so that noise can not put the device in powerdown mode.
On the 905 datasheet, what should the transformer turn ratio be? What value should I use for the termination resistors?
The transmit twisted pair interface ratio should be 1:2 with 25ohm series termination resistors or a transformer ratio of 1:2 with 11.8 termination resistors. This makes the effective lineside impedance 100W. Revision 2.7 of the LXT905 datasheet has been updated to maintain consistency with a turn ratio of 1:2 with 11.8 resistors.
What are the differences between the LXT907 and the LXT907A?
The major difference between the LXT901/907 and the LXT901A/907A products is the supply voltage. The LXT901/907 are 5V devices and the LXT901A/907A devices are 3.3V devices. Some additional differences exist in the AC and DC specs, therefore it is recommended that you download both datasheets and compare side by side. Also, the LXT901A/907A devices are available in extended temp.
Is the LXT970A a drop-in replacement for the LXT970 device?
The LXT970A is the B9 to B11 stepping revision of the LXT970, and is pin for pin compatible. Although, the LXT970A has fixed many of the existing known errata for previous stepping revisions of the LXT970, and it is therefore recommended that you acquire the LXT970A Fast Ethernet Transceiver Revision B11 Stepping document for known design considerations.
Can I capacitively couple the twisted-pair (TP) interface on the LXT971A Fast Ethernet PHY Transceiver, since the centertap on the transformer (magnetics) requires a voltage source?
Yes, it is possible to connect two twisted-pair ports without using transformers. For recommendations, please see our application note titled "Magnetic-Less Ethernet Point-to-Point Over a Backplane." This application note and all product documentation can be found on our web site.
What is the optimum slew rate for the 971A?
The optimum slew rate is design and layout specific therefore we cannot give you an optimal value. We recommend setting it to its fastest setting (2.5ns) and testing to confirm that the 971A is able to transmit and receive. If it is not able to transmit and receive, slow the slew rate down. If it does work, leave it as 2.5 ns and begin testing for EMI. If the device is passing EMI tests, this is your optimal slew rate. If it does not pass, slow it down until it does pass EMI and this is your optimal slew rate.
Is a reference design available for a twisted-pair to fiber converter application?
Intel now offers the LXT973, a dual-port 10/100Mbps transceiver. This chip, along with an external FIFO, can be used for twisted-pair to fiber conversion. The reference design for this application is not currently available.
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