Comparator vs. OpAmp: pros & cons?

[Joseph Hynes]

I was searching for a high speed comparator, and soon realized that there seem to be more high performance op-amps out there than comparators.
If you don't need open-collector TTL outputs, are there any advantages to using a hi-speed comparator vs a high-speed opamp connected as a comparator?

Comps seem to have lower gain (3V/mV) than the open-loop gain of op-amps, and also have lower input resistence than opamps.

One question:
If you don't use neg feedback on an OpAmp (but maybe use a bit of pos FB for hysterisis), is the (-) input still a virtual ground? or does it become high-impedance, lke the (+) input?

Joseph

[lineup]

Op-amps and comparators are basically the same.
Op-amps were from beginning made for being comparators
in digital applications.
They could be '0' or '1'.

Op-amps compare both their inputs, as you know.

If your intended use is not very special or extreme you can use op-amp.
Your choice of op-amp or comparator is from what your need is.
Can be from very simple low cost IC to higher precision chips.

Low cost Op-amp LM324 can for example be used as 4 comparators
with only 0.7mA total supply at +-1.5 to +-16 Volt.

http://www.national.com/pf/LM/LM324.html

[poobah]

Joe,

To answer your first question; op-amps and comparators are similar but the not same. You can use an op amp for a comparator but not exactly vice-versa; comparators do not have the internal integrator that is key to an op amp. If you need speed, the comparator is the way to go. This is why you don't see gain and bandwidth figures in comparator specs.

Comparators in general have not the development in recent years as much as op amps have. ADC convertors and microprocessors with ADC's take over their former applications.

If using an op amp for a comparator, greater off the shelf accuracies are attainable but usually with some loss in speed. An op amps output will "slew" AFTER the internal integrator has charges or discharges... to maximize speed you must use zeners (or other methods) around the op amp to ensure that the outputs don't saturate.

For your second question... both inputs of an op amp have similar impedances... virtual ground is created by the circuit it sits in... and here again if the output is saturated... the virtual ground is not there.

Post a scematic of what you would like to do...

Cheers,

[anatech]

Hi lineup,
Comparators have a high impedance between the + and - inputs. You can stick one input at +8V and the other at ground, as long as you don't exceed supply voltages or input range. An op amp will not be happy with you if you do this. A bipolar input op amp may break down reverse b-e with these voltages, a comparator will not. Comparators do not have differential inputs as a rule.

If you want to use an op amp as a comparator, you can as long as you watch input voltages and instantaneous voltages at the input pins. Some sort of current limiting may be in order.

-Chris

Hi poobah: X post with you, but I'm right

[lineup]

thanks for info, anatech and poobah
this will give the topic poster info he wants

I imagine dedicated comparators use 'switch transistors'
with very short rise and fall time

Another thing may be full rail voltage at output
( even if there are some op-amps with rail-to-rail output, too )

A parameter often seen in comparators is 'settling time'
I have only a slight thinking of what this may be,
but do not know exactly

[poobah]

Chris??

What does "X post" mean???

[anatech]

Hi poobah,
It's my secret language. X= cross, therefore, cross posted or posted while you were doing the same.

-Chris

[poobah]

Oh... I thought "x" meant we were disagreeing or something. I REFUSE to disagree with you: 1) you're too nice 2) you're probably right 3) I forgot...

[Eva]

Comparators are not required to operate in closed loop, thus they don't require any internal frequency compensation (at the expense of reduced slew rate). Also, op-amps may have higher open-loop gain at low frequencies, but comparators have higher gain at high frequencies, due to the absence of compensation.

As it has been mentioned, op-amps have evolved a lot in the past decades and hundreds of new models have appeared as opposed to what happened with comparators, but these facts are quite easy to understand. Lets take a LM393 dual comparator and a uA741 dual op-amp as a reference, both are very old devices from 1970s, but while the op-amp is very slow and too noisy for most today's applications, the comparator is fast and is still fine for new designs (my LM393 are swinging from rail to rail in 200ns or so provided adequate input overdrive and output bias).

Don't underestimate devices just for being old. There is still a lot of old stuff regarded as industry standard because it still fits today's needs quite well. Personally, I use a lot of LM393, CD4000 CMOS gates, TL431, BC546/556, BD139/140, LM358, 1N4007, 1N4148 etc... and I like these devices.

PD. Using op-amps as comparators works but produces poor performance.

[anatech]

Poobah,
I would really prefer that you disagree with me in the future.
1) I'm a cranky old coot.
2) I just look right (works in the board room)
3) What were we talking about ??

-Chris