## 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 ?

thanx
geoff

----- Original Message -----
From:
To:
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_ (http://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_
> (http://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
> _http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node28.html_
> (http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node28.html)
>
>    I hope that this helps.
>
>    Regards,
>
>    Chris Chapman
>
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