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DS90LT012AQ / DS90LV011AQ fail safe biasing

Jesse Muszynski
Prodigy 80 points
Other Parts Discussed in Thread: DS90LV011A, DS90LV012A, DS90LT012A

Hi, 

We have signals being transmitted between circuit cards with DS90LV011AQ LVDS differential drivers and DS90LT012AQ LVDS differential receivers.  On the receiver side we have fail safe biasing resistors installed consisting of a 2 kΩ from the positive input to the +3.3 V supply and a 2 kΩ from the negative input to common ground.  For the purpose of failure mode effects analysis, in the event one of these biasing resistors were to fail open would the receiver still be able to decode transmissions from the driver?  The receiver and driver have separate 3.3 V power supplies connected to the same common ground, so there should be very little ground potential difference.

Page 4 of the receiver data sheet indicates signals greater than 100 mV can be detected over a ±1V common mode range centered around the +1.2 V driver offset voltage.  From the driver datasheet, it is not clear how strong the driver offset voltage is? In the event of an open fail safe bias resistor, would the remaining 2 kΩ resistor be strong enough to pull the offset voltage outside the 1.2±1 V region?

Regards,

Jesse

  • 0
    TI__Mastermind 43795 points

    Hi Jesse,

    The DS90LV011A driver supplies its own common mode bias at ~ 1.2V.  Losing one or both of the 2K resistors would not keep the DS90LV012A receiver from correctly interpreting the DS90LV011A transmissions.

    If the DS90LV011A is not transmitting, the 2K ohm resistors are designed to pull the P/N signal voltages apart slightly to aide the DS90LV012A receiver to hold a constant output state.  The DS90LV012A is designed to hold a steady output state even with the 2K resistors removed or shorted across the P/N input signals.

    From the datasheet (page 1)  The DS90LV012A and DS90LT012A support open, shorted, and terminated input fail-safe.  The DS90LV012A receiver output will be "High" for all failsafe conditions.  See also the truth table on page 2 of the DS90LV012A datasheet.

    Regards,

    Lee