PSN-L Email List Message

Subject: Re: nice online Opamp Input Noise Voltage Tutorial
From: chrisatupw@.......
Date: Mon, 30 Sep 2013 10:13:25 -0400 (EDT)

From: Geoff 
To: psnlist 
CC: David Saum 
Sent: Mon, 30 Sep 2013 6:18
Subject: Re: nice online Opamp Input Noise Voltage Tutorial

On 9/29/2013 3:28 PM, David Saum wrote:
> this tutorial helped me better understand some of the considerations
> for selecting opamps for seismometers
> enjoy,
> Dave

VunderBar, I have never seen a better lecture on NOISE.

What is the noise at DC ?? it seems a voltage regulator might do better.

****High ! A voltage regulator CONTAINS an amplifier !!

I have heard below 1Hz you want to use bipolar devices only ??

****Mostly. FET chips have their own, usually larger, problems, with
impulse type noise..

There has to be one best opamp for 100Hz to 100seconds,
What Op Amp is it ??

****If you want to improve the noise level of an opamp, you can always
put a very low noise long tailed pair of discrete transistors on the=20

One best opamp in the whole universe.

****One best opamp - but for what purpose and frequency range ?

****If you need to get extremely low noise and ZERO 1/f low frequency
noise, use a CHOPPER AMPLIFIER. This technique works brilliantly for
providing IMMUNITY to 1/f noise. DON'T BOTHER with the chopper
stabilised-input opamp chips !

It would be so much easier if you could just
ask an egghead some where to tell you the truth
as to which is the one and only low noise op amp to use
below 1Hz.

****OK, I am listening !

I bet it would be cooled with liquid hydrogen.
Who can afford that ?

****Who would WANT that ? You tend to end up with Dewar vessels with
a paste of solid air and liquid hydrogen in the bottom. This is NOT=20

****FET ONLY opamps will work at very low temperatures. Bipolar opamps
start to loose gain below about -40C and may be useless below -80C --->
solid CO2 cooled. But you would need to refrigerate the whole board.=20
resistors and capacitors have temperature coefficients. The X7R and Z4U
ceramic chip capacitors have HUGE temperature coefficients.=20
capacitors may be destroyed if the electrolyte is frozen below -40C.
Tantalum capacitors are characterised down to -50C, but should work at
lower temperatures. From practical experience, the epoxy coatings are=20
to crack at much lower temperatures. They definitely don't like being=20
in liquid Nitrogen and neither do 1% resistors !


     Chris Chapman

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