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

Started by RobertMacy May 28, 2014
On Wednesday, May 28, 2014 7:40:58 AM UTC-7, Robert Macy wrote:

> Does the noise come up [because of the inability of the OpAmp to reduce] > when the OpAmp is driven too fast at too large a signal level?
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.
On Wed, 28 May 2014 11:14:17 -0700, Jim Thompson  
<To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:

> On Wed, 28 May 2014 11:00:00 -0700, RobertMacy > <robert.a.macy@gmail.com> wrote: > >> ...major snip.... >> that model is over 68MB in size !!! > > Just for a 1496 ?:-} > > ...Jim Thompson
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?
On Wed, 28 May 2014 17:29:05 -0700, RobertMacy
<robert.a.macy@gmail.com> wrote:

>On Wed, 28 May 2014 11:14:17 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote: > >> On Wed, 28 May 2014 11:00:00 -0700, RobertMacy >> <robert.a.macy@gmail.com> wrote: >> >>> ...major snip.... >>> that model is over 68MB in size !!! >> >> Just for a 1496 ?:-} >> >> ...Jim Thompson > >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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On 5/28/2014 10:40 AM, RobertMacy wrote:
> Trying to sort out circuit simulation artifact from something that may > be real. > > Does the noise come up [because of the inability of the OpAmp to reduce] > when the OpAmp is driven too fast at too large a signal level?
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?
> > Circuit is a simple LT1028 in an 'inverting' configuration with power of > +/-12Vdc. GNDed non-inverting input. Rinput = 1k. Rfdbk = 1k. Cfdbk = > 20pF. Load is a simple 10k in parallel with 10pF. VERY simple inverter.
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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Wed, 28 May 2014 18:20:28 -0700, Jim Thompson  
<To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:

> On Wed, 28 May 2014 17:29:05 -0700, RobertMacy > <robert.a.macy@gmail.com> wrote: >> ...snip.... >> 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
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.
On Wed, 28 May 2014 18:39:11 -0700, Phil Hobbs <hobbs@electrooptical.net>  
wrote:

> On 5/28/2014 10:40 AM, RobertMacy wrote: >> Trying to sort out circuit simulation artifact from something that may >> be real. >> >> Does the noise come up [because of the inability of the OpAmp to reduce] >> when the OpAmp is driven too fast at too large a signal level? > > 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? >
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.
>> >> Circuit is a simple LT1028 in an 'inverting' configuration with power of >> +/-12Vdc. GNDed non-inverting input. Rinput = 1k. Rfdbk = 1k. Cfdbk = >> 20pF. Load is a simple 10k in parallel with 10pF. VERY simple inverter. > > 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
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.
On 05/29/2014 09:37 AM, RobertMacy wrote:
> On Wed, 28 May 2014 18:39:11 -0700, Phil Hobbs > <hobbs@electrooptical.net> wrote: > >> On 5/28/2014 10:40 AM, RobertMacy wrote: >>> Trying to sort out circuit simulation artifact from something that may >>> be real. >>> >>> Does the noise come up [because of the inability of the OpAmp to reduce] >>> when the OpAmp is driven too fast at too large a signal level? >> >> 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? >> > > 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.
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.
>>> >>> Circuit is a simple LT1028 in an 'inverting' configuration with power of >>> +/-12Vdc. GNDed non-inverting input. Rinput = 1k. Rfdbk = 1k. Cfdbk = >>> 20pF. Load is a simple 10k in parallel with 10pF. VERY simple inverter. >> >> 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 > > 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.
Use Octopart.
> > 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. >
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 -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
On Thu, 29 May 2014 06:46:19 -0700, Phil Hobbs  
<pcdhSpamMeSenseless@electrooptical.net> wrote:

>> ...snip.... > 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.
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 10.1kHz signal? Hmmmm. I should ADD the other signal, and do this again. Perhaps, what I'm seeing is a form of IM between noise floor and 1MHz.
>> ...snip... > Use Octopart. >
will look for it
>> ...snip... > 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
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'
On Thu, 29 May 2014 06:16:55 -0700, RobertMacy
<robert.a.macy@gmail.com> wrote:

>On Wed, 28 May 2014 18:20:28 -0700, Jim Thompson ><To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote: > >> On Wed, 28 May 2014 17:29:05 -0700, RobertMacy >> <robert.a.macy@gmail.com> wrote: >>> ...snip.... >>> 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 > > >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.
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 >.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. >
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 -- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: skypeanalog | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
On Thu, 29 May 2014 07:48:08 -0700, Jim Thompson  
<To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:

>> ...snip.... > 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.
oops, forgot that!
>> ...snip.... > 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
do I only need small input section and small output section? But if you find the whole thing is ok.