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?
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
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
=====================================
(2B)+(2B)' ?
RE: Overshoot fix
=====================================
(2B)+(2B)' ?
RE: Overshoot fix
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
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Overshoot fix
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Overshoot fix
Could this be fixed by a resistor from gate to ground?
RE: Overshoot fix
mmm Variable zenner. May be a good project for early in April.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Overshoot fix
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
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
RE: Overshoot fix
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
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Overshoot fix
RE: Overshoot fix
RE: Overshoot fix
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
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
RE: Overshoot fix
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Overshoot fix
Keith Cress
kcress - http://www.flaminsystems.com
RE: Overshoot fix
RE: Overshoot fix
This makes me wonder about the reason of the original overshoot.
RE: Overshoot fix
RE: Overshoot fix
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
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
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Overshoot fix
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
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
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
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
Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.
RE: Overshoot fix
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
The Sequel: 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
Much better. Thanks Gunner.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Overshoot fix
That wasn't as embarrassing as having the cro set to AC input, and wondering why my square wave wasn't acting correctly :)