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Level Translator - 5V PECL Differential - LVDS_33

Joachim Wuerker
Mastermind 23275 points
Other Parts Discussed in Thread: SN65ELT21, SN65ELT20, SN65LVDS180, SN65LVDS1, SN65LVDS2, SN65LVDS100

My customer is looking for a level translator that can be used to translate from 5V PECL Differential Standard <-> LVDS_33 Differential Standard.
Other option would be e.g. to translate from differential PECL <-> Single Ended LVCMOS/LVTTL and then <-> LVDS_33.

Do we have any solution to do that?
The SLL guys pointed me to the ICP group as they do not have a solution.

  • 0
    Genius 4530 points

    Hi,

    AFAIK, every differential signal is mainly characterised by its DC Bias voltage. So the simplest solution would be to convert the PECL signal DC voltage to LVDS input DC bias voltage.

    This can be done by connecting 2 resistors -> 51 + 33 Ohms in series and the entire combination in parallel to 120 Ohms. The entire combination is connected to 3.3V and is used individually for each signal. The DC bias voltage thus derived for each LVDS signal is 0.825 mV.

    Regards,

    Sid

  • 0 in reply to Sid
    TI__Mastermind 23275 points

    As we don't seem to have an integrated solution I suggested the following discrete solution:

    Differential PECL <-> TTL
    - SN65ELT21 - 5V PECL to TTL Translator
    - SN65ELT20 - 5V TTL to Differential PECL Translator
     
    (LV)TTL <-> LVDS:
    - SN65LVDS180 - Single Full-Duplex LVDS Transceiver
    - SN65LVDS1 - Single LVDS Transmitter
    - SN65LVDS2 - Single LVDS Receiver
    These LVDS devices have 5V tolerant inputs so they should be able to work with SN65ELT21.

    Any comments?

  • 0 in reply to Joachim Wuerker
    TI__Genius 11825 points

    Hi Joachim,

    We'll get back with you on this one shortly.

    Actually, this one belongs in the High Speed Interface area.  We're still trying to adjust to this new website.

  • 0 in reply to Joachim Wuerker
    TI__Guru 62920 points

    Joachim,

    The two-part solutions you mentioned above should work for both LVDS-to-PECL and PECL-to-LVDS conversion.

    It is also possible to convert PECL to LVDS levels using a simple resistive divider network.  This is described in detail in the attached Application Note SLLA101: "Interfacing Different Logic With LVDS Receivers."  See Figure 6 on page 6 for a suggested implementation.  The divider is used to (1) provide the Vcc-2V termination voltage necessary for the PECL signal, (2) establish an acceptable common mode at the LVDS input, (3) attenuate the high differential swing of the PECL signal, and (4) match the transmission line's characteristic impedance.

    Additionally, the SN65LVDS100 may be used for PECL-to-LVDS translation.  It is an LVDS repeater with a wide input common-mode range that allows for single-ended input voltages up to 4 V.  Figure 37 on page 14 of the part's datasheet (attached) illustrates how the device can be configured to accept PECL inputs.  Please note that certain PECL-output devices may produce output "high" levels greater than 4 V, so it is important to check the specs of whatever device the customer intends to use.

    To convert from LVDS to PECL, the SN65ELT20 paired with the SN65LVDS2 (as you mentioned) looks like a good option.  I don't think at this time we have an integrated solution for +3.3V LVDS to +5V PECL.

    Best regards,

    Max Robertson