Input Noise Cancelation - LME 49990 - LT1028 - diyAudio
Go Back   Home > Forums > Source & Line > Analog Line Level

Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 29th October 2014, 06:52 PM   #1
diyAudio Member
Join Date: Oct 2011
Default Input Noise Cancelation - LME 49990 - LT1028


I worked the whole day on calculating the noise of a instrumental amplifier. My goal is to have a analytical model of the noise equations, so i can automaticly find the "best" possible resistor combination (and get a little bit more knowlage how to deal with noise).

I'll do my calculation with Matlab and wrote a little script to try a lot of combinations.

The interisting Question is: should i do amplification with the differential Amplifier and the first stage?
Should the resistors of the first stage be low ore in the differential Amplifier?

Additional I'd like to cross check a few combinations with LT-Spice.

And There my Problem begin:

All "manual" Noise calculations are spot on in the first stage were the opamp sees only a resistor at one input (if the source impedance is zero). But when i do the calculations of the differential Amplifier the "manual" calculations are always worse than the calculations with LT-Spice.

Do anyone know if LT Spice calculates input Noise cancelation?
In the Datasheet of the LT 1028 there is a diagramm of this feature. For the LME 49990 input Noise cancelation is'n named at all?
Is there a way of calculating it?
  Reply With Quote
Old 30th October 2014, 12:26 PM   #2
diyAudio Member
Join Date: Jul 2013
Sorry I have nothing more specific, but the LME49990 model is pretty basic and somewhat false. For example, the amp's 5 terminals do not obey Kirchoff's current law - any output current developed at the output is not somehow sourced from the other 4 terminals (most specifically the power supply terminals).

However, the LT1028 seems to be a pretty realistic model, far beyond a simple Boyle op amp model. I would trust it to a much higher degree, but also try to verify it with specific, pathological circuits that zero in on this behavior. In the end, verification and comparison to actual breadboarded circuits will guide your model a lot more accurately, albeit more slowly.
  Reply With Quote
Old 1st November 2014, 11:20 PM   #3
gerhard is offline gerhard  Germany
diyAudio Member
gerhard's Avatar
Join Date: Oct 2005
Location: near Stuttgart, south Germany
Am I right when I assume that you build the usual instrumentation amplifier
from 3 op amps?

If you are interested in noise, the first stages should have gain,
since then the noise of the second stage will play a lesser role.
It's analogous to the Frijs'schen Formel as its known from RF.
A voltage gain of 10 gets you most of the advantage. The thermal
noise of the feedback network counts also and is added geometrically
to the noise of the other inputs. Small resistors rule, but make sure
that the opamps can drive them. With the small voltages involved
this won't be much of a problem.

The Opamp input stage is run with large current, so the bases will
require a base current that cannot be ignored. LT made sure that
the bias currents of both inputs come from the same source and
thus are correlated. Thus the noise effects on the + and - input cancel.
That works only if the resistance on both inputs is the same, since
noise current * resistance = effective noise voltage at the input.

So if you minimize the resistance of your signal source to get less noise,
you may get punished from an unexpected side if you cannot do the
same for the feedback network.

Other solutions would be to need less bias current such as in FETs or
super-beta-transistors, but FETs usually have more noise and the
LT1028 may be quite super-beta-ish already.

I have seen in the data sheet that they now confess the noise peak
at 400 KHz, that might hurt you. The AD797 and the ADA4898-2
no not have that. The 4898-2 may be interesting for you since you
need several opamps anyway.
Nevertheless, you can get your 1 nV/sqrt HZ from a LT1028 any time.

feel free to email me.

Gre aus Stuttgart
Everything has been said already - but not yet by everyone. (Karl Valentin)
  Reply With Quote
Old 2nd November 2014, 07:20 PM   #4
diyAudio Member
Join Date: Sep 2004
Location: virginia
I suggest buying this book. You can find them used for less than $20. I bought a copy over twenty years ago.
  Reply With Quote
Old 2nd November 2014, 09:09 PM   #5
gerhard is offline gerhard  Germany
diyAudio Member
gerhard's Avatar
Join Date: Oct 2005
Location: near Stuttgart, south Germany
Good book. I think there is a newer edition in the meantime.
Everything has been said already - but not yet by everyone. (Karl Valentin)
  Reply With Quote
Old 3rd November 2014, 05:44 PM   #6
diyAudio Member
scott wurcer's Avatar
Join Date: Jan 2004
Location: cambridge ma
Originally Posted by gerhard View Post
I have seen in the data sheet that they now confess the noise peak
at 400 KHz, that might hurt you.
"The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important."
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
noise cancelation slimslim Headphone Systems 0 7th October 2012 06:52 PM
LME 49713 + LME 49600 pre amp klewis Chip Amps 16 4th February 2011 02:15 PM
Splitting line array into sections for dipole cancelation? Ang Multi-Way 8 18th October 2010 11:43 PM
Phono stage with LT1028 MC input andyjevans Tubes / Valves 10 22nd January 2010 01:09 AM
Crosstalk cancelation chip fcserei Swap Meet 1 23rd July 2007 04:01 AM

New To Site? Need Help?

All times are GMT. The time now is 06:55 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2