Does the noise density function increase if an OpAmp goes into slew rate limit?

Good question... I don't know... you would expect, since the output stage is, in essence, disconnected from the input during slew-limited operation, that the "gain" no longer exists, and noise should go down ??

I'll see if I can find an OpAmp model that is all device-level. If I can, I'll break it apart and see what I get. ...Jim Thompson

Hmm or up because now one see's the noise of the output stage? And not the low noise input.

My question (to Robert) is why would you care about the noise when it's over driven? (and finding those few nV/rtHz's in a few volt signal will hard.)

George H.

I've found that you can't trust a simulator when it comes to doing things to items, like an OP-amp in your case, to behave the way it would be in real life.

I don't think the models were ever written in such a way to simulate out of spec conditions.

After looking at the TINA simulator it appears they believe an opamp of various models should just flat line when over driven, which in fact most of the various op-amps will be have differently.

But getting to your question, I would think noise would increase since you have effectively created a low hz zone. Just a thought..

Jamie

If you believe *any* model will behave outside of its specifications, you're in for an interesting time. It would be interesting to see models that admit that the part does a phase inversion but don't expect the manufacturer to advertise the part that way.

You're lucky if they model reality even when used properly.

That was my thinking, too. The output stage becomes isolated and all the contributing noise sources get to supply energy. Like the feedback resistor, output Z, even Rload.

Sadly, I don't have access to the 'inside' of the OpAmp model, so the noise modeling consists of external noise sources thus I can only 'trust' to be working below gain crossover, at around 500kHz.

However, the MC1496 multiplier, I got to put all the noise sources 'inside' and using that model was a bit of an eye opener.

But it doesn't. During slew limiting, the "input" to the output stage is "grounded" thru the compensation capacitor. Now I'm not sure if that's helpful or a hindrance

Can you provide me with your "netlist" that will run on any simulator? I'm not quite following what you are doing. ...Jim Thompson

Does that feed the noise on through to the output, or short the noise at the output back the input ??

Can you do a simulation using a FULL component OpAmp model to find out?

that model is over 68MB in size !!!

The graph in the datasheet seems to think it peaks around 400kHz.

Some other low noise amplifiers have a single stage design that apparently lacks the bonus noise bump.

--sp

The input has no effect during the slew-limited condition.

Just for a 1496 ?:-} ...Jim Thompson

Obviously, slew rate limiting is a kind of saturation, and reduces the amplifier gain (therefore will block input-stage noise from the output). The test for 'noise', however, includes any deviation of the output from amplified-input. So, there's a LOT of noise in the sense of a bridge-balance noise determination, but that noise is an obvious waveform distortion (so an observer would call it distortion, while a mechanical measurement would only catch the deviation's existence).

Broadband noise (high frequency) would go down, and harmonic noise (distortion) would go up, in other words.

Do you have anyway to find the noise density function during slew rate limiting? or, does anybody?

That size is for an 'almost' full blown MC1496 model using Alex Bordodynov's CA3046 NPN transistors [I used them because they were slower than all the other models, so I trusted them a bit more] Actually the size depends on EXACTLY what you're going to do, too. could be twice that. If we can talk Mike into putting .tranoise into LTspice, the file would be around 2k.

Do you own Matlab, or install octave? Or what do you use for math analyses?

PSpice solves non-linear differential equations just fine >:-} ...Jim Thompson

It hardly matters. The output of the op amp deviates from A_VCL times V_in, so why do you care if it's noise or something else?

The LT1028 is a squirrelly mess with a seriously tendentious datasheet, which conveniently edits out its noise peaks at ~300kHz and ~2MHz. Check out the ADA4898 or ADA4899 as greatly improved replacements.

Cheers

Phil Hobbs

Not just for that. Last night I very changed to the LT1115 OpAmp instead of the LT1028 driven with 1MHz sine wave first by 7mVpk then by 7Vpk. The .noise analysis resulted in 6.5nV/rtHz and the 7V drive using standard .tran BUT with 1nS max steps showed approx 2Vpk is all that could get through. ok

Next, I used my .tranoise and used 0.9nVrms/rtHz and 1pArms/rtHz input noise, not the more likely 2 or even 3pA. when the circuit was kept in the 'linear' mode driven with 7mV the output noise was a respectable 4.5nVrms/rtHz BUT when driven with 7Vpk this number went up to 12nVrms/rtHz !!

Thus, the models both react similarly, overdrive and you increase your noise floor.

Also, big footnote, the LTspice FFT display had numerous artifacts in the spectral plot. I'm talking even a spike near 10kHz, but exporting V(out), processing with octave, the FFT's were VERY clean, I assume the artifacts generated by LTspice come from the elaborate processing methodology that Mike puts the signal through. However, for my work, most of the information is down in all those artifacts! Plus, having the noise floor come up unexpectedly explains a lot! I can send the .jpg plots to you if you want to see.

Matters a great deal to me. THAT's where all the information is for what I'm doing! Johnson [and shot] noise limit the system's performance.The lower the noise, the better everything works.

what is A_VCL? and V_in you speak of?

Again, the energy in the output does not matter, but the noise floor 'around' that signal does.

Historically, I never liked to use the LT1028, but back then when the 'equivalent' Analog part had horrible CURRENT input noise spec! But now the LT1115 is far superior to the LT1028. True there is a 'bump' of noise around 400kHz that you cannot get rid of where the bump is around twice the base line. Didn't see a noise peak at 2MHz ?? Only a bump *if* you don't put a cap around Rfdbk. LTspice models clearly show that to you. Haven't looked at Analog parts, sadly their models are less than stellar, but will take a looks, thanks for the numbers. Do you have a URL straight to the data sheet? Analog's website hangs all my systems.

Did not mean to mislead, I'm not selecting OpAmps here, Instead I'm looking for interactions between non-linear operation and noise. Need to verify what I found is real and not a simulation artifact. Again. PSpice and LTspice have .noise and .tran analysis, but those analyses are woefully lacking if you're trying to understand anything non-linear, like mixers, multipliers, OpAmps overdriven, etc. That's where my .tranoise analysis gives much better understanding. The time waveform plots look just like a scope trace complete with fuzz and meandering [1/f noise] and the spectrum noise floor actually tells you something. Plus, you can find all the bumps and dragging around noise that did not show up using 'separate' analysis techniques. Plus, the 'new' FFT displays do not contain artifacts from the spice fft processing. They're much cleaner so more useful clear down to the noise floor.

But in slew limiting, it isn't a linear system, so your linear systems approximation goes out the window. Worrying about the noise floor in that situation, when you've got maybe a volt of input error, is just cracked.

Use Octopart.

If you believe SPICE down to that accuracy level. It's asking a lot of a crappy macromodel, for sure. Unless I'm very much mistaken, it would be a lot faster and cheaper to make a couple of boards and have a look.

Cheers

Phil Hobbs

what? The system compensates for the input error. Even when signals go through the non-linearity there is still information outside harmonics of the non-linearity. Are you saying that the 1MHz signal distorts the

will look for it

It's not so much 'believe' the answers as much as 'gain insight' and know what to look for AFTER building the PCB up. And especially seeing what might need to be changed. as in 'where to perturb the beast'

I just checked... the model call for an LT1115 _symbol_ is also LT1028... open the symbols (.asy) with a text editor, and see for yourself.

The LT1028 model is 100% behavioral, so I suspect your results are totally meaningless... sorry.

I'll try to find you an all-device-level OpAmp model to play with. ...Jim Thompson

oops, forgot that!

do I only need small input section and small output section? But if you find the whole thing is ok.