Increasing speed of Opamp used as Comparator

[John Larkin]

Any particular standouts?

Regards,
James Arthur

DS90C402/SO8. Powered from +6 or so, puts about 4.5 volts into 50 ohms
in 640 ps. Don't tell anybody.

John

 

[Michael A. Terrell]

Klaus Kragelund skrev:

[email protected] skrev:

Klaus Kragelund wrote:
John Larkin skrev:

On 11 Jul 2006 07:12:37 -0700, "Klaus Kragelund"

Hi

I have an application where I have a quad LM324 opamp and I'm using
only three of the opamps in the package. I would like to use the last
opamp as a comparator.

For simplicity the opamp is supplied from 5V and the value I compare to
is 2.5V. The input is fed directly to the other input. My problem is
though that the opamp saturates against either rail and is slow when it
needs to change the output state. (input stage i saturated due to V+/v-
is not operating closed loop

Are there a fancy way to actively clamp the output just below either
rail to speed up the "out of saturation" time?

Regards

Klaus


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.


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)

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

http://www.linear.com/

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 '"

--

I have tried the idea by Jon:

www.microdesign.dk/tmp/LM324Simulation_Sch.pdf
www.microdesign.dk/tmp/LM324Simulation_Plt.pdf

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:

www.microdesign.dk/tmp/LM324_plusBJT.pdf

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

Another circuit:

www.microdesign.dk/tmp/LM324_plusBJTClamp.pdf

This circuit has a response of 5us

Regards

Klaus

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

[1] Not that an LM324 sets much of a standard[2].
[2] Though I'm still a fan of the nasty little beggars.

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

John

[3] I have. Maybe that's why I dislike them so much.

I always seem to find 6502 chips that way.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Klaus Kragelund]

Spehro Pefhany skrev:

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.

Yes, correct.

I must admit I got carried away - had to follow through though.

If you look at the "BJT on the output" solution it is only 3 added
components and two of them are resistors (dirt cheap). And this
solution was the best I found with respect to the response time (5us)

Regards

Klaus

 

[Jim Thompson]

[snip]

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

John

[3] I have. Maybe that's why I dislike them so much.

I always seem to find 6502 chips that way.

I've only stepped on an LM324 once.

West Virginia sand hornets cured me of going barefoot outdoors.

The LM324 cured me of going barefoot indoors ;-)

...Jim Thompson

 

[Michael A. Terrell]

I've only stepped on an LM324 once.

West Virginia sand hornets cured me of going barefoot outdoors.

The LM324 cured me of going barefoot indoors ;-)

Jim Thompson

I don't have a lot of choice. I have to wear surgical support
stockings as much as possible, so I only wear shoes when i leave the
house. The stockings are bad enough, but adding shoes makes my feet hurt
and swell even more. Luckily I haven't stepped on any chip barefoot in
about 10 years, but I've pried enough 4164 DRAMs out of the soles of my
shoes over the years.

BTW, Email me when you have the time. I have an idea you might find
amusing.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Ken Smith]

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)

Maybe not, how nasty can the three amplifiers be?

The LM339 can be made to work as a very bad op-amp by placing a capacitor
on the output.

Also, you may be able to just have the LM339s oscillating away and doing a
ugly sort of PWM.

 

[[email protected]]

<need-to-know info snipped>

Thanks.

Powered from +6 or so, puts about 4.5 volts into 50 ohms
in 640 ps. Don't tell anybody.

John

Your secret's safe with me ... and Google -- I think they're
steadfast, but I'm not as sure.

Cheers,
James

 

[[email protected]]

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

[1] Not that an LM324 sets much of a standard[2].
[2] Though I'm still a fan of the nasty little beggars.

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

John

[3] I have. Maybe that's why I dislike them so much.

Gee, you pique-o-second guys... I stepped on a fast DIP-28 SRAM,
once, maybe twice--clenched its pins in my heel--and I still like
SRAMs. A lot.

Grins,
James Arthur

 

[Robert Latest]

On Wed, 12 Jul 2006 10:56:45 -0700,

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

John

[3] I have. Maybe that's why I dislike them so much.

Aah, childhood memories. It's amazing how much pain a DIL package can
inflict.

robert

 

[John Larkin]

On Wed, 12 Jul 2006 10:56:45 -0700,

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

John

[3] I have. Maybe that's why I dislike them so much.

Aah, childhood memories. It's amazing how much pain a DIL package can
inflict.

There's a term for people who deliberately step on IC's barefoot:
they're called pin sluts.

John

 

[Fred Bloggs]

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

Is that the best you can think of, woman? LOL

 

[John Larkin]

Is that the best you can think of, woman? LOL

You apparently consider "woman" to be an insult. That sort of attitude
must surely complicate your love life, if any.

John

 

[Jim Thompson]

You apparently consider "woman" to be an insult. That sort of attitude
must surely complicate your love life, if any.

John

Fred HAS a love life ? Bwahahahahahaha!

...Jim Thompson

 

[Fred Bloggs]

You apparently consider "woman" to be an insult. That sort of attitude
must surely complicate your love life, if any.

John

Is that the best you can do, Mary?

 

[Jim Thompson]

Is that the best you can do, Mary?

Frederica? Is that you ?

...Jim Thompson

 

[Fred Bloggs]

Fred HAS a love life ? Bwahahahahahaha!

...Jim Thompson

I'm not into the monogamy thing- I am a poly-something...

 

[Spehro Pefhany]

I'm not into the monogamy thing- I am a poly-something...

Polymastic? Polymath?


Best regards,
Spehro Pefhany

 

[[email protected]]

Polymastic? Polymath?

Polynominal?

Cheers,
James Arthur

 

[John Larkin]

Polymastic? Polymath?

Pollywog? [1]

John

[1] I think that means you have a lot of Italian relatives.

 

[Michael A. Terrell]

I'm not into the monogamy thing- I am a poly-something...

Polyunsaturated.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida