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

Started by May 28, 2014
```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?

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 drive is a sinusoid 1MHz of 7Vpk, which the LT1028 can't keep up with,
making the output a 'triangular' signal around 2 Vpk.

Now I wish to consider noise: Using LTspice I get an 'expected' noise
density function over the range of ?? to 1MHz that seems reasonable for
the LT1028. With almost no input voltage and doing what I call .tranoise
analyses, where .tran and .noise analyses are combined, I get
approximately the same noise density function. HOWEVER, when the above
sinewave is applied which obviously overdrives the LT1028, the noise
density function increases around 10dB.

Thus, my question. In real-life should I expect the noise density function
from an overdriven OpAmp to increase?

```
```On Wed, 28 May 2014 07:40:58 -0700, RobertMacy
<robert.a.macy@gmail.com> 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?
>
>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 drive is a sinusoid 1MHz of 7Vpk, which the LT1028 can't keep up with,
>making the output a 'triangular' signal around 2 Vpk.
>
>Now I wish to consider noise: Using LTspice I get an 'expected' noise
>density function over the range of ?? to 1MHz that seems reasonable for
>the LT1028. With almost no input voltage and doing what I call .tranoise
>analyses, where .tran and .noise analyses are combined, I get
>approximately the same noise density function. HOWEVER, when the above
>sinewave is applied which obviously overdrives the LT1028, the noise
>density function increases around 10dB.
>
>Thus, my question. In real-life should I expect the noise density function
> from an overdriven OpAmp to increase?
>

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
--
| James E.Thompson                                 |    mens     |
| 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 Wednesday, May 28, 2014 10:55:40 AM UTC-4, Jim Thompson wrote:
> On Wed, 28 May 2014 07:40:58 -0700, RobertMacy
>
> <robert.a.macy@gmail.com> 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?
>
> >
>
> >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 drive is a sinusoid 1MHz of 7Vpk, which the LT1028 can't keep up with,
>
> >making the output a 'triangular' signal around 2 Vpk.
>
> >
>
> >Now I wish to consider noise: Using LTspice I get an 'expected' noise
>
> >density function over the range of ?? to 1MHz that seems reasonable for
>
> >the LT1028. With almost no input voltage and doing what I call .tranoise
>
> >analyses, where .tran and .noise analyses are combined, I get
>
> >approximately the same noise density function. HOWEVER, when the above
>
> >sinewave is applied which obviously overdrives the LT1028, the noise
>
> >density function increases around 10dB.
>
> >
>
> >Thus, my question. In real-life should I expect the noise density function
>
> > from an overdriven OpAmp to increase?
>
> >
>
> Good question... I don't know... you would expect, since the outpu
> stage is, in essence, disconnected from the input during slew-limited
> operation, that the "gain" no longer exists, and noise should go down
> ??
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'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
>
> --
>
> | 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.

```
```In article <op.xgkq2kzo2cx0wh@ajm>, robert.a.macy@gmail.com says...
>
> 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?
>
> 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 drive is a sinusoid 1MHz of 7Vpk, which the LT1028 can't keep up with,
> making the output a 'triangular' signal around 2 Vpk.
>
> Now I wish to consider noise: Using LTspice I get an 'expected' noise
> density function over the range of ?? to 1MHz that seems reasonable for
> the LT1028. With almost no input voltage and doing what I call .tranoise
> analyses, where .tran and .noise analyses are combined, I get
> approximately the same noise density function. HOWEVER, when the above
> sinewave is applied which obviously overdrives the LT1028, the noise
> density function increases around 10dB.
>
> Thus, my question. In real-life should I expect the noise density function
>  from an overdriven OpAmp to increase?

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

```
```On Wed, 28 May 2014 12:32:57 -0400, "Maynard A. Philbrook Jr."
<jamie_ka1lpa@charter.net> wrote:

>In article <op.xgkq2kzo2cx0wh@ajm>, robert.a.macy@gmail.com says...
>>
>> 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?
>>
>> 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 drive is a sinusoid 1MHz of 7Vpk, which the LT1028 can't keep up with,
>> making the output a 'triangular' signal around 2 Vpk.
>>
>> Now I wish to consider noise: Using LTspice I get an 'expected' noise
>> density function over the range of ?? to 1MHz that seems reasonable for
>> the LT1028. With almost no input voltage and doing what I call .tranoise
>> analyses, where .tran and .noise analyses are combined, I get
>> approximately the same noise density function. HOWEVER, when the above
>> sinewave is applied which obviously overdrives the LT1028, the noise
>> density function increases around 10dB.
>>
>> Thus, my question. In real-life should I expect the noise density function
>>  from an overdriven OpAmp to increase?
>
>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.

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.

> But getting to your question, I would think noise would increase since
>you have effectively created a low hz zone. Just a thought..
>
>Jamie
```
```On Wed, 28 May 2014 09:32:57 -0700, Maynard A. Philbrook Jr.
<jamie_ka1lpa@charter.net> wrote:

>> ...snip...
> 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
>

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.
```
```On Wed, 28 May 2014 10:33:02 -0700, RobertMacy
<robert.a.macy@gmail.com> wrote:

>On Wed, 28 May 2014 09:32:57 -0700, Maynard A. Philbrook Jr.
><jamie_ka1lpa@charter.net> wrote:
>
>>> ...snip...
>> 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
>>
>
>That was my thinking, too. The output stage becomes isolated

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

>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.

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

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| 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 Wed, 28 May 2014 07:40:58 -0700, RobertMacy
<robert.a.macy@gmail.com> 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?
>
>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 drive is a sinusoid 1MHz of 7Vpk, which the LT1028 can't keep up with,
>making the output a 'triangular' signal around 2 Vpk.
>
>Now I wish to consider noise: Using LTspice I get an 'expected' noise
>density function over the range of ?? to 1MHz that seems reasonable for
>the LT1028. With almost no input voltage and doing what I call .tranoise
>analyses, where .tran and .noise analyses are combined, I get
>approximately the same noise density function. HOWEVER, when the above
>sinewave is applied which obviously overdrives the LT1028, the noise
>density function increases around 10dB.
>
>Thus, my question. In real-life should I expect the noise density function
> from an overdriven OpAmp to increase?

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

http://www.speff.com/2014-05-28 14_01_34-1028fb-4.pdf - Adobe Acrobat
Professional.png

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

--sp

```
```On Wed, 28 May 2014 10:39:54 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:

> On Wed, 28 May 2014 10:33:02 -0700, RobertMacy
> <robert.a.macy@gmail.com> wrote:
>
>>> ...snip....
>> That was my thinking, too. The output stage becomes isolated
>
> 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
>

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?

>> ...snip...
>> However, the MC1496 multiplier, I got to put all the noise sources
>> 'inside' and using that model was a bit of an eye opener.
>
> Can you provide me with your "netlist" that will run on any simulator?
> I'm not quite following what you are doing.
>
>                                         ...Jim Thompson

that model is over 68MB in size !!!
```
```On Wed, 28 May 2014 11:00:00 -0700, RobertMacy
<robert.a.macy@gmail.com> wrote:

>On Wed, 28 May 2014 10:39:54 -0700, Jim Thompson
><To-Email-Use-The-Envelope-Icon@on-my-web-site.com> wrote:
>
>> On Wed, 28 May 2014 10:33:02 -0700, RobertMacy
>> <robert.a.macy@gmail.com> wrote:
>>
>>>> ...snip....
>>> That was my thinking, too. The output stage becomes isolated
>>
>> 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
>>
>
>Does that feed the noise on through to the output, or short the noise at
>the output back the input ??

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

>
>Can you do a simulation using a FULL component OpAmp model to find out?
>
>>> ...snip...
>>> However, the MC1496 multiplier, I got to put all the noise sources
>>> 'inside' and using that model was a bit of an eye opener.
>>
>> Can you provide me with your "netlist" that will run on any simulator?
>> I'm not quite following what you are doing.
>>
>>                                         ...Jim Thompson
>
>that model is over 68MB in size !!!

Just for a 1496 ?:-}

...Jim Thompson
--
| James E.Thompson                                 |    mens     |