Op amp vs. comparator

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:

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Reply to
Michael
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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.

Reply to
Ancient_Hacker

In article , Michael wrote: [....]

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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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

Reply to
Michael

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

Reply to
John Larkin

"switching point"="grey"/"undefined" value. Your input signal is too stable and stays in this place too long so the amplifier tries to make sense of your order by looking at output voltage which will feedback values compatible with this input. And it does it at "feel spud" as there is no stable point. It is NOT DC amplifier. Disregard it and feed it from fast input signal.

Have fun

Stanislaw Slack user from Ulladulla.

Reply to
Stanislaw Flatto

In article , John Larkin wrote: [....]

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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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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

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John

Reply to
John Larkin

Hi Michael

Comparators don't have built in hysteresis. The main differences:

Comparators are designed to have exact two possible output values, HI and LOW. Opamps are designed to have analog output voltage, normally not HI or LOW. Only R2R Output reaches the rails. The design of such output stages differs very much. The input design is very similar.

Some OPAmps have a ugly behaviour if the inputs are too different. Some types inverts then the Out-Signal. May be there is such situation in your design. In general: Use the devices in that mode that they are designed for. Use a comparator if you need one, not a Opamp ;-)

Regards

Marte

Reply to
Marte Schwarz

In article , John Larkin wrote: [....]

Yuck.

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

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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

Jorgen dj0ud

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Reply to
Jorgen Lund-Nielsen

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

Reply to
John Larkin

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

Reply to
John Larkin

Many do. JT's classic ecl design, with a shared latch/hysteresis function, is still popular.

Actually, no. They are designed to have high gain and no compensation. Most do actually have linear transfer functions, if you ignore oscillation.

Opamps are designed to have analog output voltage, normally not HI or

Opamps make excellent comparators if you don't need speed. They are far less liable to oscillate or to have their external hysteresis loops teased. One gotcha is that some opamps don't behave well if railed.

John

Reply to
John Larkin

OK, there are OP available with fixed gain etc. I mean general purpose comparators, there i would not expect included hysteresis. At least I wouldn't want to have it, because I want define the hysteresis tha I need.

What else than HI or LOW should anyone want on an output of a comparator. That's exactly the point they are designed to.

I aggree, but that is not the goal, this is one way to go there.

Great linear transfer fuction with oscillatoins :-)

No they don't do it generally. Many of them are not capable to drive good digital levels. Mostly they take too much time to slew up or down. Almost every comparator have to detect a time that one voltage reaches a reference voltage or other condition. Then you want to detect it imediately. Sure you can tahe a R2R videoOPAMP for $2 to get the performance of good old lm311 :-)

Well I do this with standard comparators, too. I don't see the advantage of any misused OPAMP for comparing voltages.

Marte

Reply to
Marte Schwarz

In article , John Larkin wrote: [....]

The LM339 can be compensated by adding a capacitor on the output to make it work like an op-amp. I have done this when using the LM339 in a DC-DC converter. The circuit is sort of like this:

Vcc ! Vref----/\\/\\---- / ! \\ FB --/\\/\\--+--/\\/\\--!!--+-+-----!-\\ ! ! ! ! >--+----!+\\ --!-\\ ! Is-!+/ ! ! >-- Gate driver ! >------+ --- !-/ Vref-----!+/ ! --- --- ! --- GND ! GND

Some parts are left to the reader, but you can see how being able to make one section look linear, reduces the parts count. Also unlike a typical op-amp circuit, the up swing is limited to Vref even though the LM339 is running on a higher non-reference voltage.

[...]

You can also take advantage of the limited slew rate of an op-amp. If you use an op-amp as a comparitor to drive the gate of a MOSFET, the switching of the MOSFET won't make quite so much RF noise. As a result the circuit won't quit working each time it tries to run on the "I'm working" LED.

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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

On Fri, 8 Sep 2006 13:17:23 +0000 (UTC), Ken Smith wrote in Msg.

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

robert

Reply to
Robert Latest

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

Why?

robert

Reply to
Robert Latest

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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kensmith@rahul.net   forging knowledge
Reply to
Ken Smith

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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Reply to
Ken Smith

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