[time-nuts] CMOS level difference for LTC6957-3 and AD9852

Robert LaJeunesse rlajeunesse at sbcglobal.net
Fri Apr 25 14:00:26 EDT 2014

AD was comparing apples and oranges. The LTC6957-3 has CMOS outputs, which go from 0.2V to Vdd-0.3V (with a 3mA load). Perhaps they were looking at the LTC6957-1 which is LVPECL and would have a rather small swing. The LTC6957-3 should drive the AD9852 if its requirements are as noted below.

Bob LaJeunesse

> From: d0ct0r <time at patoka.org>
>To: time-nuts at febo.com 
>Sent: Friday, April 25, 2014 1:33 PM
>Subject: [time-nuts]  CMOS level difference for LTC6957-3 and AD9852
>To whom it may concern, here is the note regarding of CMOS level 
>difference for two product AD9852 from AD and LT6957 from Linear.
>My attempts to feed AD9852 directly from LTC6857-3 has failed. AD9852 
>just ignored the REFCLOCK signal coming from LTC6957-3. So, I'll need to 
>think about some solution how to match the levels and do not impact the 
>source signal too much.
>(Received from tech. department of AD):
>... the logic levels for the AD9852 and LTC6957
>don't line up. From page 5 of the AD9852 datasheet, VIH minimum is
>2.3V whereas the LTC6957 says it's VOH could be as low as 2.08V. This
>means the LTC6957 could output 2.08V for a logic high, but our part
>won't recognize it as it needs at least 2.3V to be a logic high.
>It's the same problem for low logic voltages. The AD9852 datasheet
>says VIL max is 1V whereas the LTC6957 says it's VOL max is 1.63V or
>1.67V depending on the temperature range of your part. So the LTC6957
>could output 1.63V for a logic low, but our part won't recognize it as
>it needs a voltage below 1V to be a logic low.
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