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Official Definition of Noise Gain

Except that noise gain has more to do with loop stability than noise ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson
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Somewhere in my over-loaded office. Thanks for the pointer, Steve! That is indeed the "bible".

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

But nary a mention of "noise gain" :-(

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

As probable cause and effect, how does one separate them?

Reply to
Don Bowey

All of the transfer functions for all the nodes that are possible inputs have the same denominator, for a linear network, whether its active or passive or whatever, right. This may be where the noise gain is again useful for characterizing the circuit frequency response in a general way.

Reply to
Fred Bloggs

It is called out in Dostal on pages 120 and 237, where he matter of factly states it is the inverse of the feedback factor, 1/beta, and often approaches the ideal closed loop gain.

Reply to
Fred Bloggs

From the NON-INVERTING Input.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

On pp 23-24 of my 3rd edition.

It boils down to closed-loop gain from the non-inverting input.

I currently have modified my LoopGain symbol/macro to get loop gain, loop phase and closed-loop gain with data from a 3-pass AC analysis macro 'd into PSpice Probe.

I thought I had enough data to also get noise gain, but it looks like I need another pass... blink of an eye but it gets me everything displayed at once... the customers like that ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

But the problem with one figure for noise gain would seem to be that noise sources at different points within the system 'see' different gains. Lumping them all to one point (an op-amp input, for example) and then applying a single gain figure to them, you would lose visibility of the stability margin due to noise at each point.

Lumping stuff at one point might be useful as a rule of thumb to see how close you are pushing things, but eventually you'll have to pull the pieces apart again to see what contributes the most. If you are going to model the system anyway, you've already got that.

--
Paul Hovnanian	paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.
Reply to
Paul Hovnanian P.E.

With PSpice it's trivial to get a device-by-device listing of noise contributions.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

I'm guessing that the noise gain figure is something your customers requested.

This sort of begs the questions of why you need it and whether there are better ways of analyzing the system. I'd just give them a figure that fits what the majority of the various sources seem to think it is and then include a footnote explaining how you derived the figures you gave them.

--
Paul Hovnanian	paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.
Reply to
Paul Hovnanian P.E.

[snip]

The Presentation is EVERYTHING ;-)

And everyone continues to miss the usefulness of Noise Gain as a stability analysis tool, for example...

FEEDBACK PLOTS DEFINE OP AMP AC PERFORMANCE composite amplifiers Graeme sboa015.pdf

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

So, got your road show projector by now?

The CFO of a client calls them dog and pony shows.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Back a LOOOOOng time ago when I was in the corporate engineering world, we'd always ask if we had a barking part in a proposed dog and pony show.

Jim

Reply to
RST Engineering (jw)

Yep, Bought myself a ViewSonic PJ256D Portable DLP Projector for Christmas ;-)

(The lightest projector available.)

Yep. But that's what works at design reviews... PROVIDED you can answer every question asked without choking up ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
         America: Land of the Free, Because of the Brave
Reply to
Jim Thompson

Might want to make sure there is a spare bulb :-)

Yes, one "oh dang" answer can ruin the credibility rather fast.

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Well, I know what gain and phase margin are. But how does one use noise gain in evaluating system stability?

A lot of what I see in that paper just looks like classic Bode plots of open and closed loop gain.

--
Paul Hovnanian     mailto:Paul@Hovnanian.com
------------------------------------------------------------------
On a clear disk, you can seek forever.
Reply to
Paul Hovnanian P.E.

Maybe this helps, 2nd page:

formatting link 

--
Regards, Joerg

http://www.analogconsultants.com/
Reply to
Joerg

Yet one more rule of thumb relating noise gain (and open loop gain) to stability. What I'm after is an explanation of how noise gain relates to the underlying causes of instability.

--
Paul Hovnanian	paul@hovnanian.com
-----------------------------------------------------------------------
Procrastinators: The leaders for tomorrow.
Reply to
Paul Hovnanian P.E.

The noise gain is also the absolute ratio |AVol/(1+AVloop)|. Increasing the noise gain is the same as reducing the loop gain, which directly impacts stability as we know. Which to focus on is a matter of taste and of what's in view. For stability, the loop gain is where it's at, as you say, especially since in speaking of noise gain we usually ignore phase. For noise, crosstalk, EMI and so forth, the noise gain is conceptually more appealing, at least to me.

BTW it isn't necessary to specify which of the op amp inputs the noise gain is defined at--they're fully differential, so that makes zero difference--but it has to be calculated right at the op amp input, not at the input to the stage.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

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