Increasing speed of Opamp used as Comparator

Klaus Kragelund skrev:

Perhaps unclear. Input signal is fed to Opamp + input, 2.5V (half supply) is fed to opamp - input. When the signal passes 2.5V I want a pretty fast response to detect that level

Regards

Klaus

What is the character of the input signal? Perhaps a slow DC loop to hold the output centered?

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
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You should realise that the 324 is a pretty sluggish op-amp and that's likely to be most of the trouble !

You could always use a small 'speed-up cap' from O/P to + I/P.

Graham

I think it was analog devices that had an article in their "Analog Dialogue" newsletter on this subject. It may be useful.

Sorry about the previous "blank" post!

Connect a voltage divider from output to ground. Calculate values such that when the output is above the max allowable output (4V, for example), the divider output is +2.5V. Conect a diode from divider output to the Inv input (cathode to the Inv input). This will prevent the output from rising above the max allowable. . Do the same thing for the low output value, except the low end of the divider must go to a negative supply. Calculate values such that when the output is below the min allowable output (1V, for example), the divider output is +2.5V. Connect the diode cathode to the output of this divider. This will limit the lower output level to 1V. . For this scheme to work, the input voltage to the NI input must be limited to the maximum alowable output excursions (+4, +1 in the example given). Regards, Jon

Klaus, I left out one step: You must place a resistor between the +2.5V reference and the Inv input, so that you can get negative feedback. Regards, Jon

Good link - thanks. But are they really serious in Fig. 1?

"Figure 1. Responses of single-supply amplifier (63 pV) and comparator (280 mV) models to a ?1-mV inputvoltage difference."

It's not a typo. The figure shows 62.82pV, so he rounded up. Bob Pease has an article that may be useful: "What's All This Comparator Stuff, Anyhow?"

Regards,

Mike Monett

How much output voltage swing is required as a response? The smaller the swing, the faster the response.

Maybe add a little positive feedback? Depending on the impedances at the inputs, a 100k to 1 meg resistor from output to + input, then a

10K resistor in series with the input. That will add a 1 to 10 percent hysterisis and a snap-action. A small capacitopr across the feedback resistor will also help improve the snap.

I've restled with the same problem once. Easier maybe to add LM393 (20 cents American) for a real comparator.

Luhan

So what's "pretty fast"? Is your problem with the time it takes to reach 50% output or the slew rate?

Best regards, Spehro Pefhany

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Adding positive feedback will increase the transition speed and improve the noise immunity, but it will not help pull the output stage out of saturation faster. Positive feedback can be used in conjunction with the circuit I suggested in my earlier post. Regards, Jon

Be aware that, in many LM324's, driving one amp to its rails will do very strange things to the other three amps in the package. So a feedback-type clamp is necessary even if speed isn't an issue. So you may as well use some other amp, or better yet a separate comparator.

John

Eeyore skrev:

to

The idea behind using the LM324 is that it is the cheapest I can dig up (0.1USD) and we are running high volume.

I have the option to use the LM139 comparator - but will add an entire new IC to the design (penalty = 0.1USD)

Regards

Klaus

And how would you clamp a follower anyway? And feedback won't work without the input being carefully bounded.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Ancient_Hacker skrev:

I tried adding the pos feedback before posting and it did nothing (i guess when the amp finally slewing the overdrive is significant and no more pos feedback matters)

I may however add a NPN from output to GND (output to base via resistor, emitter to GND and collector pulled up). This will make the output respond fast (reducing the slew from GND to VCC) in one direction (I only need fast response in one direction.

Regards

Klaus

John Larkin skrev:

The datasheet for 5 different manufactors I looked up didn't mention this and I have never heard of this. Is this some soft of undocumented feature that is constrained to us westerneers so the terrorist bombs explode prematurely due to the quirks of the LM324s? (bad joke, sorry)

Regards

Klaus

Not maky datasheets have a list of non-features. Instead of bullets, they could use little frowney-faces.

I did a box that used two '324 sections as regular amps and two as comparators. Every time a comparator switched, the other three sections made a huge output glitch. Jim says that some 324's may do it and some may not, depending on how they shared some internal current mirrors I guess. I dropped in an LF347 and it worked fine.

LM324 is a rotten opmap anyhow. Don't even think about pulling any of the inputs even a little below ground.

John