Increasing speed of Opamp used as Comparator

You won't find a better quad for the price.

Best regards, Spehro Pefhany

The output may be considered as followers.

As I said before, negative feedback clamping would be generally less than worthless.

...Jim Thompson

Another way of saying that is that it's hard to find a worse opamp at any price.

John

Wouldn't it keep the internal nodes from winding up? I've seen opamps that took *seconds* to come off a rail.

John

That's why I mentioned input *bounding*. If I knew the input range there might be a solution... of course feedback limits output swing.

...Jim Thompson

Look at the Linear Technology data sheets for the LT1006. and for the LT1013 dual and LM1014 quad versions of the same amplifier at

The LT1006 is much better behaved than the LM324 (and much more expensive) so the data sheets include a small gloat - see sheet 9 in both data sheets.

They also give some information about the performance of the parts as comparators.

Pity John Fields won't read this - the creep is claiming that I post "Basically nothing but opinions formed long ago and 'You're not doing it the way I would, so you must be wrong '"

Bill is so full of himself that he had to sign it twice! :)

Yeah, too bad a technical post couldn't stay a technical post. I guess we know what really matters here.

John

snipped-for-privacy@ieee.org skrev:

I have tried the idea by Jon:

LM324 with no tricks, in saturation has about 15us of delay before output reaches VCC/2

LM324 with the clamping trick has about 6us before VCC/2

LM324 with a BJT connected like this:

has 7us of delay until output shifts to VCC/2

The nice thing about the last one is that it is cheap and has logic level output for use with microcontrollers and so on

Comments?

Regards

Klaus

Klaus Kragelund skrev:

Another circuit:

This circuit has a response of 5us

Regards

Klaus

Well if you're going to use all that then go discrete all the way and get 50ns response.

OK for an intellectual exercize, but in real life, why add 11 or 12 additional discrete parts to turn a bad opamp into a mediocre comparator? I doubt this saves space or money.

John

My apologies. I should know better than to post a reply at four in the morning, when I really ought to be in bed. but the LT1006/LT1013/LT1014 datsheets were just too tempting to let me hit the sack. Bad judgement.

I count 7 or so *additional* parts (over using an additional comparator chip with pullup resistor), with a parts cost and power consumption less than adding the cheapest IC comparator, but I agree it looks a bit messy. It's not completely insane for huge volume.

Best regards, Spehro Pefhany

I like using LVDS receivers as comparators. They're cheap, have infinite Zin, brutal cmos outputs, and are blindingly fast.

Or how about using current-steering diodes into an opamp that stays linear? That's more like 4 additional parts.

John

According to the data sheet, even if you keep it linear, and hit the inputs with sledgehammer (a 2.5V step), the output slews at a leisurely ~0.5V/usec.

The simple RTL inverter on the output of an unadulterated comparator-connected amplifier looks like the best bang for the buck to me (7usec, the OP says), and only 1 or 2 additional very cheap & small jellybean parts. It works because he only needs fast response in only one direction, and the single supply output clamps itself at ground with no additional parts. If he feels lucky he could try biasing the base more positive by a couple hundred mV with another resistor. ;-)

Best regards, Spehro Pefhany

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Another possibility is to go the other way: depending on what you're doing, LM339 comparator sections can used as nasty, slow, horrible op amps[1], and the left-over section makes a dandy comparator.

Or use the LM392, which combines an LM324 op-amp and LM339 comparator in one package (any quads out there?)

Cheers, James Arthur

Any particular standouts?

Regards, James Arthur

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May you step on an LM324N, pins up, barefoot. [3]

John