can the AD790 Comparator Logic Supply Pin be powerd with 3.3V, to Interface directly with LVTTL or LVCMOS33 ?
The AD790 datasheet shows that the logic supply range is 4V to 7V... So no, it can't be powered by 3.3V.
There are other comparable comparators that you can consider, such as the AD8468/ADCMP608.
Thanks a lot for the quick reply,
the comparators you recomanded are not working for me because I need a wide Input Voltage Range of min +/-7V
and I have to Interface with and 3.3V CMOS CPLD. Forgot to mention that. So I think I have to work with Logic-Level shifting then.
I have exactly the same problem and I'm going to experiment operation the Vlogic at 3.3V since that's my only option. The other parts recommended have too small Input Voltage Ranges as you have noted and I need +/-5V. It doesn't make sense why the Vlogic is limited to 4V! It needs to work down to 3.3V and my guess is they haven't tested it at the lower voltage. AD is notorious for not handling +3.3V TTL very well. But that is another issue.... to be continued. Stay tuned....we're running tests!
OK after LOTs of testing...sorta.... the AD790 works GREAT at Vlogic at +3.3V... even down to about +2V. I didn't and can't run all the temperature and load variations but FOR OUR APPLICATION, Vlogic = +3.3V which will drive the input to a CPLD/FPGA just fine! Just take a look at the pictures. Enjoy.
I may be a little late chiming in here, but I agree that a high quality comparator like the AD790 needs to be available with 3.3V logic support. I checked the AD8468/ADCMP608 data sheets, and they do not support the analog +/-15V range as the 790 does.
I note that ITCSteve did some testing at 3.3V and below, which is encouraging, but I am very hesitant to take the risk of designing this into a client's product......severe liability issues if the performance varies between lots, etc etc. AD SUPPORT FOLKS: PLEASE, PLEASE upgrade the 790 to handle +3.3V logic, document it as supporting 3.3V if it consistently works, or consider a new product.
This comparator beats all the others out there, but 3.3V logic for CPLDs and CPUs is simply too ubiquitous nowadays to restrict operation to 5V.
Thank you for your interest on the AD790. We're currently checking the possibility of the AD790 for supporting +3.3V logic and if we would be able to release a new grade or new product that could handle this condition. May we ask what specific application you or your client are working on that needs the wide input range comparator?
Hi Rainier -
Take a look at my website: www.rockfieldresearch.com.
About 60% of my business is control systems, often for HV pulse power.
I have a contract right now for a rather large radar system transmitter
that senses klystron arcs, waveguide arcs, etc, and shuts down HV or RF
These response times must be uber-fast, the environment is noisy and
signals are in the +/-10V range,
and I have intelligence buried in Atmel CPLDs (ATF150x series) for fast
programmable logic, as well
as in NXP CPUs (LPC23xx series) for state logic, serial interface
muxing, event logging, etc.
The CPUs are +3.3V, but have +5V tolerant I/O supposedly. Some of the
pins, as the fine print says,
are not +5V tolerant, though, and I've often found that the tolerance is
poor..... Interfacing with +5V logic
requires 2k or better series for current limiting to guarantee no
latchup on transients. That makes it a
pain when there are 50 or more signals shared between CPU and CPLD,
since banks of series R tend
to fill up the board.
I want to use the +3.3V CPLDs, but then I have to contend with the fault
signals that come in to the CPLD
(which latch and combine them) are coming from comparators that only do
+5V logic. Ugh. I cannot find
anybody who makes a +3.3V to gnd output comparator with +/-15V analog
front end and similar speed.
The AD790 is a gem. I can always divide the output of each AD790, but
that is another bank of resistors,
and with the parasitic capacitance and input cap of the CPLD, it slows
down the comparator output some.
I've used the AD790 in many similar projects, both in my own company and
in a previous job at Diversified
Technologies Inc. (look at the client page on my website and click on
"DTI projects"....there are some cool
500 kV solid-state pulse modulators I designed there). The heart of
the modulator protection in all those
is the AD790.
I hope you enjoyed the tour. I look forward to seeing a +3.3V
Please feel free to call for questions.
Rockfield Research Inc.
Las Vegas, NV
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