PSN-L Email List Message

Subject: Re: Coil + magnet sensors = long period noise?
From: "GMV" gmvoeth@...........
Date: Thu, 19 Feb 2009 00:15:09 -0700

That publication seems to be referencing db to
one meter per second squared ( acceleration)  and I was
just wondering if it would be calculated

10(log(n/(1m/s^2))) or 20(log(n/(1m/s^2))) 

n being what you want to measure against the chart ?

anybody know  what they are using here ?


----- Original Message ----- 
Sent: Wednesday, February 18, 2009 5:29 PM
Subject: Re: Coil + magnet sensors = long period noise?

> In a message dated 18/02/2009, lconklin@............ writes:
> In a  recent post, Chris Chapman commented "Coil + magnet systems detect  
> velocity and are likely to suffer noise problems at very long  periods".
> Could you elaborate a little on this?  What is the source of  the noise, 
> and what sort of periods are we talking about?
> Larry  Conklin
> lconklin@............
> Hi Larry,
>    Apart from the background seismic noise, you have  to consider the input 
> voltage and current noise of the amplifier, the intrinsic  noise of the input 
> resistances on both inputs and the 1/f (flicker) noise of the  system. Note 
> that a coil has noise determined by it's resistance. The 1/f noise  is likely to 
> be a limiting factor below about a few Hz. It can be avoided by  using a 
> chopper or a CAZ amplifier. Note that CAZ amplifiers like the  MAX420, MAX430, 
> LTC1150 etc., still have much more noise than true chopper  amplifiers, although 
> their drift and VLF noise is reduced when compared to  ordinary amplifiers. 
> See AN-45 from _www.linear.com_ (  
>    The RMS voltage noise of a resistor =  SQRT(4.k.T.R.B) where k = 
> 1.38x10^-23 in J/K, T is the absolute temperature in  Deg Kelvin (Deg C + 273.14), R 
> is the resistance on Ohms and B is the bandwidth  in Hz.
>    Professional seismometers usually use variable  capacitor sensors and 
> chopper amplifiers measuring position. See AN-87, p87 at  _www.linear.com_ 
> (  There is usually no  significant intrinsic noise associated 
> with a capacitor, as there is with a  resistor or an inductor. These give the 
> same output per mm of movement whether  this occurs over 1 or 1000 seconds.  
> However, a coil + magnet velocity  detection system would only give 1/1000 the 
> voltage output for the 1000 second  signal as compared to a 1 second signal, 
> so you quickly reach the amplifier  noise limits as the period increases. 
>    The seismic background noise is likely to be  greater than the 'ordinary' 
> amplifier noise for periods up to ~ 30 seconds,  maybe quite a bit more. The 
> complicating factor is the variation in the minimum  background seismic noise 
> seen at different periods. 
>    See 
> _ 
> ( 
>    I hope that this helps.
>    Regards,
>    Chris Chapman

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