Differential input comparator

Hello to all.

First post on this forum, I 'd like to say I am pretty happy I found a place it seems I can got some help (and exchange ideas!).

I have this project where I am testing the behavior of comparators when they are fed with two (continously changing) inputs into is terminals,
instead of having a signal in one input and a "voltage reference" to the other.

See schematic below.

The main reason for this post is that it seems I have encountered a problem which I found suprising:

Since the signals fed into the comparators' inputs are starting at the same point, during low voltage/fast signals the comparator is not being able to cope with that and does not trigger (start) a pulse from the point it should; but from a point a little later.
Can anyone please advise me on the nature/causes of the problem?

I have been asking some firends and they said that this could be solved by a delay line into the "red" input" (?)
Yet, I am a little concerned about the fact that a delay line could cause a considerable amount of addition of noise.

So, is there someone of you who who can think of a "noiseless" delay circuit?

or perhaps another solution to my problem?

Regards,
Futurama.

You need to provide a schematic with component values, and waveforms with time and voltage scales. All comparators are not created equal. Each type has common mode voltage limits, response time limits, differential offset voltage, etc.
We may be able to suggest a solution if you can provide this info. I seriously doubt that you need a delay line.

For any circuit there is something called delay (response time)
Which is defined as the time required by output to change its state when the input changes.
So in your case when the (+) input becomes higher than the (-) input after the delay time (specified in the datasheet of comparator) the output will change

and also there will be certain difference required in the two input voltages (may be equal to differential output voltage I am not sure about this value but it might be there in datasheet) for the output to be changed

Hello,
Ur descriptions kept me guessing the type of comparator u meant. Op amp comparators or what.

Just provide a circuit diagram.

Hello!

Sorry I didn't post earlier, was on vacations.

Nothing fancy as a schematic, just a function generator in the lab feeding a LM393.

The signals going into LM393 are of a same voltage, but the red is twice as "slow" as the blue one.

I did this experiment to see how a comparator works when it has to analyse a differential input signal.

The signals vary from 60mVs to 6 Volts, I have experienced the "problem" below 200mVs or so.

What do you think is the problem?
I am missing that "low signal" area, and I am vey curious as to why...?

Can u help?

futurama.

futurama said:

Hello to all.

First post on this forum, I 'd like to say I am pretty happy I found a place it seems I can got some help (and exchange ideas!).

I have this project where I am testing the behavior of comparators when they are fed with two (continously changing) inputs into is terminals,
instead of having a signal in one input and a "voltage reference" to the other.

See schematic below.

The main reason for this post is that it seems I have encountered a problem which I found suprising:

Since the signals fed into the comparators' inputs are starting at the same point, during low voltage/fast signals the comparator is not being able to cope with that and does not trigger (start) a pulse from the point it should; but from a point a little later.
Can anyone please advise me on the nature/causes of the problem?

I have been asking some firends and they said that this could be solved by a delay line into the "red" input" (?)
Yet, I am a little concerned about the fact that a delay line could cause a considerable amount of addition of noise.

So, is there someone of you who who can think of a "noiseless" delay circuit?

or perhaps another solution to my problem?

Regards,
Futurama.

Click to expand...

The first plot you posted doesnt look like a problem at all. Your "comparator theory" plot shows the comparator switching in zero time when its two inputs are equal (+) & (-).

In reality, the comparator needs to first see a certain amount of difference (the amount is determined by its gain) and then it will drive internal circuits to trigger the output. This obviously happens at a later time. The ouput happens at a later time not from the time the inputs were equal, but from the time the inputs differed by a sufficient amount such that the comparator could "see" the difference. A comparator output state is undefined when the two inputs are identical. Noise will push it on eway or the other but probably not for long. This is why hysteresis is added to the circuit.

So your second plot of "reality" is doing just what I described.. so whats the problem? The comparator works as advertised. You read the datasheet right?


I think you dont have a problem at all _OR_ I am really really way off in understanding what you are talking about. But I agree with RON, the notion of needing a delay line seems absurd. You may be trying to fix a perceived problem that really isnt a problem. Maybe what you need is a faster comparator. If microsecond delays offends you, you can get comparators with sub 10 nanosecond delays.

>>In reality, the comparator needs to first see a certain amount of difference (the amount is determined by its gain) and then it will drive internal circuits to trigger the output. This obviously happens at a later time. The ouput happens at a later time not from the time the inputs were equal, but from the time the inputs differed by a sufficient amount such that the comparator could "see" the difference.

Ok, how much is "delay time" for LM393?I mean, what is the parameter I have to check in datasheet?
Is it slew rate?


>>A comparator output state is undefined when the two inputs are identical. Noise will push it on eway or the other but probably not for long. This is why hysteresis is added to the circuit.

NOISE:
Ok, considering the lowest signal is peak-to-peak around 60mV, if I add 200mV of noise am I causing the problem I described?And in what sense?Is voltage gain reduced?

>>So your second plot of "reality" is doing just what I described.. so whats the problem? The comparator works as advertised. You read the datasheet right?

The problem is that the comparator seems not to match its specifications.
For instance, if a voltage is 40,000 , the comparator should output a pulse when the differential input is more than 7.5 mV or so.
But I don't get that behavior.Practical it is quite slower (needs more difference)
Again, is noise the most-contributing factor to such phenomenon?

>>I think you dont have a problem at all _OR_ I am really really way off in understanding what you are talking about. But I agree with RON, the notion of needing a delay line seems absurd. You may be trying to fix a perceived problem that really isnt a problem. Maybe what you need is a faster comparator. If microsecond delays offends you, you can get comparators with sub 10 nanosecond delays.

Can u suggest a faster comparator? Or better yet, a comparator specially designed for entering into its + - inputs two (different) continous signals?

One last thing: Does a dc offset fed into the comparator inputs (into both and of the same level) ALTERS the degree of how "easy" the comparator changes output state?
For instance, does the comparator have the ability to respond to changes of its output state faster (more efficient operation) when its inputs are both zero or both 1volt? (This refers to dc offset appearing when no input signal exists)

Thanks.
Futurama

The LM393 (and all true differential comparators) responds to the differential input signal. If you want to see what the comparator is driven by, subtract the inverting input from the noninverting input. The switch point will be where the diff. input crosses zero, with some (possibly a lot of) propagation delay. This wiil be true until you get to a relatively high frequency, where common mode rejection starts to degrade. The problem is, the comparator response is not specified for slowly slewing signals, so it is difficult to predict what the response will be. The comparator has a lot of gain, but very low bandwidth. With very little overdrive, the delay will be long because of the low bandwidth.
The response time is only specified for a fast risetime, 100mv step with 5mv, 20mv, and 100mv overdrive. See p. 6 in the **broken link removed**.

SOme much faster comparators..
Just from one company (linear technologies)
www.linear.com


LT1715, LT1719, LT1712, LT1721, LT1720, LT1711, LT685
LT1395, LT1714, LT1713, LT1016, LT1116, LT1671, LT1011, LT319A

The LT1715 has a prop delay of 4 nano-seconds.

Other folks who sell fast comparators, national semi, texas instruments, analog devices.. you can find lots of em out there but you will pay for the higher speeds. And some of them require smaller supply rails and run at higher power levels.. so there are tradeoffs.. but they exist if you really need them.

It is not clear to me that what you are seeing is not simply the expected prop delay in your circuit for that comparator and I think you are having a hard time explaining it (at least to me anyhow). Some things to keep in mind.

Your function generator has noise on it that you are likely not setup to properly measure.

Your function generator has offsets on its output and one channels will be different than another. If you think you have nulled these out show us how. If you are trying to measure single mV with a cruddy hand held multimeter, you are probably not getting the right answer. All of these things can slightly affect the comparators trip point. And as RON mentioned, the specs only show prop times for specific overdrive voltages so if you are not providing these amounts (including none), you can expect to find a prop time that will show up nowhere in the spec.

Only _generally_ speaking can we tell you that the more overdrive you give, the faster it will switch. If you do less, it is anyones guess as to what you might see.

The other fun thing about all this is if you put a different part in there but same type & part #, the times you observe will be different still. Put a heat gun on it and watch the times change again.. maybe not by alot but sounds like you are trying to pin point a single cause with some amount of precision.

Why are you doing all this anyways, if I may ask?