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 <[email protected]>
To: [email protected]
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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--
WBW,
V.P.
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