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LTC6957 failsafe

adowd
on Jan 20, 2021

I have a SE clock signal (Sine-wave, 50 ohm) that I want to feed into the LTC6957-x, but the circuit requires a failsafe.  In other words, when the input clock is disconnected, it can't oscillate, even internally.  This is a low noise application any free-running oscillations is a concern.  One option that I considered is to disable it with the SDx pins.  But its not clear the SDx disable the first amplification stage.   Is there a bias circuit I can use on the front end to avoid oscillations and not degrade the phase noise performance? 

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    •  Analog Employees 
    on Jan 21, 2021 7:45 PM 1 month ago

    There is a small voltage offset designed in at the LTC6957 inputs that should prevent the oscillation in most cases.  This assumes the external noise on the inputs is around 1mV or less. 

    When both SD1/SD2 are high the LTC6957 is in full power down mode, which includes powering down the input.  This should resolve your concern even in a high noise environment.