Dear Sirs,
in order to unterface correctely the TLK2541 SerDes with a LVPECL Optical Transceiver, I would like to know the Digital Standard of TLK2541 Differential Inputs and Outputs ( DOUTTX and DINRX). Are they still LVPECL ?, have you any reference design to interface it with standard LVPECL inputs/outputs ?
Hi Andrea,
The high-speed serial inputs and outputs (DINRX and DOUTTX) are VML. This IO type generally has the same differential swing as an LVPECL signal, but uses a lower common mode voltage. Here is a graphical comparison of the two:
To interface an LVPECL output to a VML input, there are two options: DC-coupling and AC-coupling. DC coupling does not translate between common mode voltages, so you have to make sure that your VML receiver can accept the higher LVPECL voltage. The TLK2541’s input common mode range is from 1000 mV to 2250 mV (see p. 10 of the datasheet), so in this case a 2-V LVPECL common mode is OK. You should look at the output swing and common mode generated by your optical transceiver, though, and compare them against the TLK2541's input specs to be sure. If the levels are compatible, the LVPECL driver can be directly connected:
Note that no external termination is necessary since the TLK2541 has integrated 100-Ohm differential termination. This matches the characteristic impedance of the transmission line and prevents signal reflections.
Since the high-speed data is 8b/10b encoded and therefore DC-balanced, it is possible to AC-couple as well. This has the benefit of allowing the driver and receiver to operate at different common mode voltage. The TLK2541 will set its own optimum common mode internally, so no external components are necessary on the receive side. However, pulldown resistors (140 Ohms to 220 Ohms) will be necessary at the LVPECL outputs to provide a DC path to drive the falling edges. Here is an example:
Again, it is necessary to check the datasheet of your optical transceiver to see if the signals are compatible. For instance, many optical transceivers have internally AC-coupled I/O. If this is the case, there is no need to externally AC-couple.
There are similar concerns in connecting a VML output to an LVPECL input. If your LVPECL device has a wide input common mode range (capable of accepting 1.25 V), then the VML output can be DC-coupled. Just make sure that the line is properly terminated at the far end by either external resistors or (if present) internal termination inside the LVPECL receiver. If the receiver's input common mode range is not compatible, then you will need to AC-couple the TLK2541 output and re-bias it to the proper level. Here is one way this can be accomplished:
Often there is not a convenient termination voltage (Vterm) available. In this case, the single termination resistors can be replaced with voltage dividers:
The resistors need to be chosen such that their equivalent resistance seen by the transmission line (R1 || R2) is equal to the line's characteristic impedance (50 Ohms) and their divide ratio sets the proper common mode voltage for the LVPECL receiver. If the supply voltage (Vcc) is 3.3 V and the desired common mode voltage is 2 V, then R1 should be 83 Ohms and R2 should be 130 Ohms.
The diagrams above assume that the LVPECL receiver does not have any internal termination. Many optical transceivers do integrate this termination, though, so it is essential to check the datasheet of the device in question to determine whether or not external termination is necessary.
Let me know if you have any questions.
Best regards,
Max Robertson
Analog Applications Engineer
Texas Instruments
m-robertson@ti.com
