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Overshoot fix
8

Overshoot fix

Overshoot fix

(OP)
I have a circuit here that essentially minimally amplifies a voltage coughed up by a DAC. It's a non-inverting amp that has a wee bit of gain because the DAC is only good for 4.096V and the output needs to reach 10V. Meanwhile I need to also PWM the output. Hence the FET that yanks the input to 0V without bothering the DAC.

It all works as I desire with the exception of the overshoot seen in my sketch. Does anyone have a simple solution to it?

Keith Cress
kcress - http://www.flaminsystems.com

RE: Overshoot fix

I haven't messed with electronics in quite awhile and I'd have to stop and think to figure out the equation for gain of op amp.

To keep my head from hurting thinking about the Op Amp, my very crude thinking is to put a cap on the non-inverting input to create an input low-pass filter with time constant R*C where R = 2.5 kohms.

Just a thought... I'm sure you'll treat it with due grains of salt.

=====================================
(2B)+(2B)' ?

RE: Overshoot fix

The cap of course would be connected from non-inverting input to ground

=====================================
(2B)+(2B)' ?

RE: Overshoot fix

Better yet, from the "+" sign to ground. (I'm not too sure of my terminology)

=====================================
(2B)+(2B)' ?

RE: Overshoot fix

There seems to be some charge injection from gate to Drain and from there to the N.I. input of the opamp.

Try reducing the gate drive. Or, better, use a simple 4066 analogue gate. They have very low charge transfer. Four times overkill, but you are in a generous X-mas mode. Aren't you?

Of course, you can slow down the opamp by adding a capacitor parallell to the feed-back resistor. But you want a sharp, clean edge, I think.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

How about a 10 Volt zener across the output??

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Overshoot fix

A variable zener would then be needed since voltage range is 0 - 10 V and I guess that Smoked wants that "overshoot" (which it isn't, technically, but still) to go away at all levels.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

"...charge injection from gate to Drain..."

Could this be fixed by a resistor from gate to ground?

RE: Overshoot fix

Thanks Gunnar. I misunderstood the problem.
mmm Variable zenner. May be a good project for early in April.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Overshoot fix

Some guesses, educated or not. You do not mention the frequency or rise time of your signal. But the output looks like as if the feedback was "too slow". The feedback path should be low-inductance and low-capacitance, maybe even low-resistance. Initially, you might try with resistances in the 100 ohm range. Better still, use some high-frequency amplifier, no op-amp at all.

Another question (where I am guessing even more) is what happens, when the output signal approaches the maximum value available, about Vcc-1.5V. Strange things may happen. A higher supply voltage, say 15 V might help, maybe.

RE: Overshoot fix

ijl
The resistors are fine. No risk that a 358 would show erratic behaviour caused by parasitic inductance (or capacitance). Also, it cannot supply enough current for 100-ohmish resistors.

Bill: Start a write-up on those zeners! Editors need long time to go from e-mail to publishing.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

I think Skogsgurra suggestion of charge injection is it, and if it wasn't for the fact that your op-amp negative rail is ground, you would probably see a similar spike at the negative transision. The correction is to use a analog switch IC, and lots of choices are available. If there is still a small amount of overshoot after you address the switch issue, put a small capacitor across your 7.3K resistor. Maybe between 100pF and 1nF which will kill-off some of your HF gain.

RE: Overshoot fix

Gunnar,
I did not think the behaviour of the amplifier itself, but that the feedback path with the stray capacitances etc. together with the input capacitance may act as a low-pass filter. This together with the delay in the amplifier makes the feedback "slow". If the signal is fast enough, the output may overshoot before the feedback has time to counteract. Ok, 100 ohm is too small, but 1 to 2 kOhm might show some difference in the output.

RE: Overshoot fix

Sorry. I don't think that computes either. Or, to put it less bluntly: How would that work?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

cap around 7.3k gain resistor to slow op amp response to remove the overshoot? too big and rather than overshoot you are underdamped?

RE: Overshoot fix

ur fet pulls input lo and just lets go of it FAST - seems like it is a simple case of your freq response of letting it go is too fast for the skew rate of your op amp. perhaps get a faster (higher bandwidth) op amp that does not saturate causing the overshoot from this high freq input change, or at lo pass filter around the op amp to slow it down to eliminate the overshoot (add cap around 7.3k R)

RE: Overshoot fix

As you know, Smoked usually tells the truth. And is also true to details.

If this were a bandwith or marginal stability problem, he would have shown that in his waveform sketch with a damped ringing - or at least a classical ramp-up_overshoot_settle waveform. He doesn't.

Instead, he shows a typical response when injected charge causes an overvoltage, which then dies out more or less linearly as the 1+R2/R1 gain of the opamp comes into play. Which is a very clear indicator that this has nothing to do with resistors or parasitic phenomena other than the Cgd capacitance in the transistor.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

(OP)
Wow great stuff folks. Many thanks on all your considered contributions.

I built this thing on a perf-board with all short trimmed thru-hole parts flying on the top. Not much parasitic anything.

Sorry I didn't mention the PWM period. tsk tsk bad form -5pts. It's 200Hz. I'll be running it from an unsuspecting microcontroller via interrupts in it's spare time so I don't want the PWM service to be too fast.

The output slews very nicely as is at about 1V/us. This helps with EMI.

A well behaved four channel analog switch is a great idea! Especially since there will be four channels of the above circuit!!

As for the 100ohm output, that isn't the load. It's only to provide rudimentary protection to the opamp's output pin. The load will be about 2mA max(500uA typical). The 0-10V need not be precise. I'll be using 20mA PTC fuses with 100ohms cold value in lue of resistors. The LM358 is good for about 50mA of drive but I have no control of who hooks what to this thing.

I'll let y'all know if a cap across Rf works or less base-drive or an analog switch kills dear Spike. (My money is on the analog switch though the reduced base drive is the easiest to test.

Stay tuned.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Overshoot fix

itsmoked, sorry for somehow calling you a liar; it was by no means my intention. Guess I best stay in the back room hidden from other people with my apparent stereotypical engineer mentality (and incompetent one of those too with my poor suggestion) that causes what I write to come out as calling folks liars. Sorry.

RE: Overshoot fix

I don't think that anyone gets the idea that Smoked was lying. He always sez the truth. But that truth may have been misinterpreted on a few hands.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

(OP)
Mike: You need a bit thicker skin around here, don't sweat it. If you want to insult me you'd need to be much, much more direct. Your inputs have all been welcome, don't turn into an introvert on us.


Keith Cress
kcress - http://www.flaminsystems.com

RE: Overshoot fix

Reduce gate resistor to .5k and add a resistor about 1k to gate and ground. Also as was posted, a cap of 1-2nF in parallel with 7.3k resistor may be a solution; what tell about rising time of PWM signal?

RE: Overshoot fix

I built the circuit with some available components: Different transistor, constant voltage instead of the DAC. The result was an "overshoot" in the wrong place, at the trailing edge instead of the leading edge, see picture. The "overshoot" disappeared when I removed the resistor at the gate.

This makes me wonder about the reason of the original overshoot.

RE: Overshoot fix

2
I doubt it's the problem but you checked your scope probe compensation, right?

RE: Overshoot fix

I have been bothered from start because of the "wrong" place for the voltage increase. ijl shows how it should be if the problem is charge injection from gate, namely at the trailing part. It is when positive charge is injected to turn the transistor on that some of that charge spills to drain and causes a temporarily increased voltage, just as ijl shows.
Question to Smoked: Did you observe the trace through a mirror?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

Since overshoot is not at point that can be caused by injection voltage issue, I probably will be called a liar again for repeating myself, but it still appears you are exceeding the slew rating of your LM358.

You say it is changing at rate of about 1v/us, yet the LM358 is only rated at .3v/us, about 1/2 of even a 741. when I suggested a faster op amp, I assumed something with capability of around 100v/usec.....

the other scope pix shown has what looks like 8v/30usec so if he used same lm358 he prob would be ok and not see the leading rise overshoot in my opinion.

so i stand by my comment to add a cap around 7.3k resistor to slow down to match the BW availabe on your LM358 or change from 10 cent to 50 cent op amp.

RE: Overshoot fix

No-one called you a liar before. But we are coming close to that situation now. A "reverse" action at the start of a transition can not be caused by any of the mechanisms you propose. Nor can it be cured by any of the changes you mention. Period.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

(OP)
iop995; I didn't try any R in the fet's D or S yet.

ijl; Wow! Thanks for the effort. I too am using a constant voltage instead of a DAC in this study.
When I added the large gate resistor it didn't fix the leading edges and added the small overshoot you show!

LionelHutz; I haven't checked the comp and should have, but, I also, on my first sketch, failed to notice that I DID actually have the negative spike too. It was a bit buried by the cursor bar. However you'll see in the pics below that I can get rid of one of them so I don't think it's scope comp.

Skogs: Nope no looking glass.

Hi Mike; I see looking at the data sheet 1v/us. Why are you saying a faster one would cure this? Is it that the amp would respond faster and hence would allow it to correct the overshoot? I could use a 50 cent amp without a problem. As for the cap 'solution' see below.


All;
Here's a picture of the unchanged situation as not completely drawn correctly. Note the negative glitch too.
ASIS

Here's the large gate resistor as suggested. Note, no real improvement but now the trailing spike shows up.
LGR

I tried many caps across Rf. None did much until I got up into 10,000pF which is shown here where it wiped out the negative one but also threw a wiggle into the trailing edge.
CAP

Turned out I didn't have an 4066 laying around even though I turned my lab inside out looking for one. I did lay this all out in the product with provision for an Rf cap and using the 4066 so we'll see if that does the trick after I get the boards back. Seems charge injection has been argued out of contending anyway. :)

Keith Cress
kcress - http://www.flaminsystems.com

RE: Overshoot fix

OK. I should have known it. Smoked IS the liar.
The traces show that bad probe compensation is the problem.
Easy as that. Shame on someone. Do not sketch traces. Show them!

If you have an overcompensated probe and reduce the tf while keeping the tr more or less intact, you get exactly the traces shown in Smoked's last picture.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

2
This thread has generated lots of discussions. One could base a seminar on what has been said. I restrained myself to a short paper on probe compensation. I hope that it is OK with you, Smoked?
http://www.gke.org/presentationer/files/Smoked%20p...

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

(OP)
Hi Skogs. No problemo. I'm still not convinced it's probe comp since I haven't been back to my lab yet to confirm it. I will say when I see things like this probe comp is my first thought. But since I didn't see the negative one buried in my on-screen cursor I discounted it. Maybe tomorrow I'll be able to make by my lab.

Nice paper. What you don't seem to describe really is why an overshoot. You show the RC curves expected with caps and resistors but not why that would cause a higher spike because of bad compensation.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Overshoot fix

That's a good point. I think there will be a sequel. I'll add Bode diagrams for over- and undercompensated probes. And I see that I didn't include the smoking probes. Stand by - watching the New Year's consert from Vienna now.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Overshoot fix

(OP)
Juries in.. LionelHutz nailed it. It was the compensation. I on the other hand. Blew it.

New Years Resolution:
Check the compensation, especially when the horns are there.

I'm still glad I switched to a 4066.
Thanks for the help everyone and have a good new year.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Overshoot fix

Skogsgurra, great posts, and great pickup. Takes me back about 25 years when I, as a junior, and a senior engineer spent about a week looking for a phantom, very similar to this case.

That wasn't as embarrassing as having the cro set to AC input, and wondering why my square wave wasn't acting correctly :)

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