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

d0ct0r time at patoka.org
Fri Apr 25 14:54:41 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.


That was my first impression. However its not working for some reason. 
AD9832 "do not oscillate". Normally, as REFCLOCK is presented on its 
input (pin69), it should start to generate "IO UPDATE" signal by 
default. Its not happens in my case.
But if I apply another source (the direct output from OCXO) to the same 
pin AD9852, its accept that. The chip to heat up and generate "IO 
UPDATE".
On the AD9852 evaluation board, AD is using MC100LVEL16 as a level 
converter. I would like to try that and see how it will be.


>> ________________________________
>> 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.
>> 
>> =============================================================================
>> 
>> 
>> ---
>> WBW,
>> 
>> V.P.
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>> 
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-- 
WBW,

V.P.



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