Op amp vs. comparator

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I have a system with op. amp. -based comparator (AD8051; 5V single
supply; 2.5V reference 1.2V wide hysteresis loop made of 1k and 4k
resistors). The thing oscillates every once in a while (100+MHz) when
input signal approaches "switching point". I am not suspecting the
layout.
:o(
Here is the question.
Aside from the built-in hysteresis, what do comparators (e.g. LMV7239)
have  that op-amps (e.g.: AD8051) do not?

Yes, I understand that there are tons of OA topologies. There are
probably quite a few comparator topologies too....................
:o?
Thanks
Michael

datasheets:
http://www.analog.com/productSelection/pdf/AD8051_2_4_b.pdf#search=%22ad8051%22
http://www.national.com/ds/LM/LMV7235.pdf


Re: Op amp vs. comparator



Michael wrote:
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Do you have a bypass capacitor right by the IC?  What size and type?

It might help to add some extra hi-freq hysteresis with say a 5pF
capacitor across the feedback resistor.

I dunno what the differences are between op-amps and comparators.  As a
wild guess, comparators are optimized for speed at the detriment of
phase shift, op amps otherwise.


Re: Op amp vs. comparator



Ancient_Hacker wrote:
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Re: Op amp vs. comparator


[....]
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In general:

An op-amp is slower to react to the inputs passing each other. The output
starts moving later and ramps more slowly to its final value.

Comparitors are designed to have a largish difference between the inputs.  
Some op-amps have clamps on the inputs at a few diode drops.


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Re: Op amp vs. comparator


On Thu, 7 Sep 2006 13:54:52 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken
Smith) wrote:

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And at least one comparator has back-to-back input diodes, the
Max9691. If anybody knows of a stupider linear design idea, let me
know.

John


Re: Op amp vs. comparator


[....]
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Do you mean that Maxim really made such a thing or that they published a
datasheet to see if they got any bites?


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Re: Op amp vs. comparator


On Fri, 8 Sep 2006 01:17:24 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken
Smith) wrote:

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Oh, they made them, and still do. They even sampled me 2500 pieces.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2342


John


Re: Op amp vs. comparator


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Yuck.

I think I just came up with an idea dumber than making a comparitor with
clamp diodes.  it is <drum roll> designing in a comparitor with clamp
diodes.


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Re: Op amp vs. comparator


On Fri, 8 Sep 2006 13:17:23 +0000 (UTC), snipped-for-privacy@green.rahul.net (Ken
Smith) wrote:

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I designed in the Max9690, which didn't have clamp diodes. Maxim
subcontracted the fab to a small foundry in Minnesota, and they had a
process problem. Seems that some oxide layer was dirty or something,
and, over time, a pinch resistor would creep up in value. The symptom
was that units would fail at high temperature and, over time, that
temperature would creep down, eventually hitting room temp. It took
over a year of production until we started seeing field failures.
Interestingly, heating the boards would anneal out the defect and make
the boards work again for a few months, which confused the hell out of
us at first. After months of stonewalling and less than candid
behavior, I finally found someone honest at Maxim who told me the
truth and got me free replacements, but they added the clamp diodes in
the redesign. We had to make an adapter board that soldered into an
SO-8 footprint, that carried a tiny Max9691 and a few other parts, and
recall/rework about a hundred products. At least we're not R*******,
who used millions of the 9690's in defense systems.

The dumb thing was probably designing in a Maxim sole-source part in
the first place. But there wasn't anything close to it in performance
at the time. That situation remains the Maxim Dilemma.

John


 

Re: Op amp vs. comparator


On Fri, 8 Sep 2006 13:17:23 +0000 (UTC),

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What's the problem with clamp diodes in a  comparator?

robert

Re: Op amp vs. comparator


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They load the signal source with a non-linear loading.

If you are thinking in the frequency domain, this reflects all sorts of
harmonics and intermixing products back into the circuit driving it.  You
may have to insert a buffer to prevent this.

They can lead to an extra amplitude dependant phase shift that appears
right in the normal amplitude range of the signal.

Also, they are ugly.

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Re: Op amp vs. comparator



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They can clamp the input and/or the reference! My inputs are often
capacitor+current source ramps, and my references are often dacs. They
don't like being coupled to one another. And sometimes one signal
feeds more than one comparator, so think about the consequences of
that one.

John


Re: Op amp vs. comparator


John Larkin a écrit :
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Sure.
OTOH I wonder what would be the consequences of zenering the input
transistors EB junction and how much reverse voltage they can accept.

And you don't have an anti-parallel diodes pair, but 2 in series.


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Thanks,
Fred.

Re: Op amp vs. comparator


On Mon, 11 Sep 2006 18:20:41 +0200, Fred Bartoli

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Both Analog and Maxim have sub-ns comparators whose inputs can be
railed any way without zenering, so it's strange that the "drop-in"
replacement for the 9690 has clamp diodes. I could live with a maximum
differential voltage spec, but they didn't give me that choice.

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They leak seriously at about 1.3 volts at high temp, and that's often
a nasty.

John



Re: Op amp vs. comparator



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Ohmygod. I thought you were talking about protection diodes
against the supply rails, with which I can't see anything wrong.
But a closer look at the DS shows the "Differential Input Clamp
Voltage 1.7V"

Yes, that looks very dumb. So much that I could have prototyped
something with such a part and wondered why it wouldn't work, and
then would have found the comparator's inputs shorted. Well,
perhaps I would have stumbled on that line in the DS, but if not
I probably would have chucked out the "faulty" part and put in a
new one only to find the same fault.

Thanks for pointing out somthing that I didn't know existed.

robert

Re: Op amp vs. comparator



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That's odd, the data sheet says in the Absolute Maximum Ratings:
"Differential Input Voltage +-5V..."

Jorgen
dj0ud

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Re: Op amp vs. comparator


On Fri, 08 Sep 2006 15:56:44 +0200, Jorgen Lund-Nielsen

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Look at the datasheet some more. There are current-limiting resistors
ahead of the diodes, roughly 500 ohms total. It still makes big
trouble in the majority of comparator apps.

John


Re: Op amp vs. comparator


On Fri, 08 Sep 2006 07:45:17 -0700,

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Why?

robert

Re: Op amp vs. comparator


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See other post plus:

Consider this model of what is going on:

  6Vp-p       500             !     \
  Signal ----/\/\/----+---+---!Ideal >--- Squarewave out
                      !   !   !     /
                      V  ---
                     ---  ^
                      !   !
                     GND GND


When the signal swings 3V above ground, the 500 ohm resistor has:

           (3V - 0.7V)/500 = 4.6mA

flowing in it.  If the comparitor is being driven by a normalish op-amp,
this has used up a good fraction of the drive ability of the op-amp.





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Re: Op amp vs. comparator



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Becxause it demands that both of the comparator inputs be fed from
zero-impedance sources that can drive several milliamperes.

John




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