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Need an op-amp expert !

All,

I think I've messed up an op-amp (actually a comparator) design at work.

I'm using an LMV321 (dual low voltage op-amp) operating at 4V.

Using one half as an amp, it seems to amplify a 50KHz square wave to about 1V peak to peak quite happily.

Then comes the trouble !

The 1V square wave is connected to a clamp (DC restorer) with the output square wave bottom side approx 1.6VDC (so top side should be

2.6V). That connects directly to the - input of the other half of the LMV321 used as a comparator.

The reference voltage (2.1V) connects to the + input of the comparator via 22K. There is a 330K feedback resistor from output to + input for Schmitt trigger action.

The output of the comparator seems to change the mark/space duty cycle (weird). The problem gets much worse at cold. Adjusting the voltages slightly on the comparator inputs seems to vary the output duty cycle (weird).

My best theory is that an LMV321 is not fast enough as a comparator (even though it amplifies the 50KHz to 1V peak to peak quite happily).

I have cunning plan. An LMH6658 is an RF op-amp that is pin compatible and shouuld drop in for the LMV321.

Has anybody any experience with using the LMH6658 as a comparator ? Will it make an OK fast(ish) comparator ? Are there any circuit tweaks to speed up the LMV321 ? Do op-amps get slower at cold (-30C) ?

Please help !

Thanks,

Jim

Reply to
Jim
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Reply to
snoop.dog99

Everything I've read through the years, strongly suggests that using op amps as comparators is a poor idea. The symptoms are usually as you have experienced - the op amp's speed as a comparator is often much poorer than its gain-bandwidth product would suggest, and the lag time can vary unpredictably.

I gather that the reason is that when you use an op amp as a comparator (i.e. open-loop) you're pushing its internal circuitry into overdrive/saturation. When the signal turns around, it takes the differential input stage quite a while to pull enough charge back through the various internal capacitances to cause the output to change state.

Op amps are designed to be used in a closed-loop environment, where the voltage difference between the inverting and noninverting input is very close to zero (maintained by negative feedback). When you use 'em open-loop (as a comparator) the differential input voltage is much higher, the internal circuitry saturates, and (in some cases) you can damage the differential input stage.

"Real" comparators are designed for fast open-loop operation, with very high differential gain, high tolerance to large differential input voltages, and fast (and predictable) output switching behavior. They don't have to be designed to be stable in a low-gain closed loop environment (they make lousy op amps!).

Overdrive recovery time for an op amp is often _not_ a characterized number - you won't find it in the data sheet, and can't depend on whatever value you happen to measure for a given part. The next batch of the "same" parts you get from the same manufacturer, might behave differently.

The only really reliable solution I can suggest, is that you use a real comparator.

--
Dave Platt                                    AE6EO
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Reply to
Dave Platt

-- like most other op-amps, most likely the capacitance in the package to prevent oscillation is causing a phase shift, much more so than what you may see in a real comparator.

You may want to look at "Miller effects" It might shed some light on it.

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I don't know if you want to believe everything you read from the wiki but it's a start.

Have you tried a fast comparator ?

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Reply to
Jamie

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