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

d0ct0r time at patoka.org
Sat Apr 26 01:03:13 EDT 2014



Just to confirm: MC100LVEL16 solved the issue with signal levels.


On 2014-04-25 14:54, d0ct0r wrote:
>> 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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