PSN-L Email List Message
Subject: Re: Coil + magnet sensors = long period noise?
From: "tchannel" tchannel@............
Date: Wed, 18 Feb 2009 17:47:57 -0700
Hi Chris, Earlier I posted this question about this "Jar Project" =
http://tinkeringcaveman.googlepages.com/home On his posting he used an =
LED and phototransistor, instead of a coil and magnet. Could you or =
anyone explain the benefits of each? The coil and magnet are more =
common, is it better?
Thanks, Ted
----- Original Message -----=20
From: ChrisAtUpw@..........
To: psn-l@.................
Sent: Wednesday, February 18, 2009 5:29 PM
Subject: Re: Coil + magnet sensors =3D long period noise?
In a message dated 18/02/2009, lconklin@............ writes:
In a recent post, Chris Chapman commented "Coil + magnet systems =
detect=20
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,=20
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=20
The RMS voltage noise of a resistor =3D SQRT(4.k.T.R.B) where k =
=3D 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.=20
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.=20
See =
http://www.geophys.uni-stuttgart.de/oldwww/seismometry/man_html/node28.ht=
ml
I hope that this helps.
Regards,
Hi Chris, Earlier I posted this question about this "Jar=20
Project"
http://tinkeringcav=
eman.googlepages.com/home =20
On his posting he used an LED and phototransistor, instead of a coil and =
magnet. Could you or anyone explain the benefits of=20
each? The coil and magnet are more common, is it =
better?
Thanks, Ted
----- Original Message -----
Sent: Wednesday, February 18, =
2009 5:29=20
PM
Subject: Re: Coil + magnet =
sensors =3D long=20
period noise?
In a=20
recent post, Chris Chapman commented "Coil + magnet systems detect=20
velocity and are likely to suffer noise problems at very long=20
periods".
Could you elaborate a little on this? What is the =
source=20
of the noise,
and what sort of periods are we talking =
about?
Larry=20
Conklin
lconklin@............
Hi Larry,
Apart from the background seismic noise, =
you have=20
to consider the input voltage and current noise of the amplifier, the=20
intrinsic noise of the input resistances on both inputs and the 1/f =
(flicker)=20
noise of the system. Note that a coil has noise determined by it's =
resistance.=20
The 1/f noise is likely to be a limiting factor below about a few Hz. =
It can=20
be avoided by using a chopper or a CAZ amplifier. Note that CAZ=20
amplifiers like the MAX420, MAX430, LTC1150 etc., still have much more =
noise=20
than true chopper amplifiers, although their drift and VLF noise is =
reduced=20
when compared to ordinary amplifiers. See AN-45 from
www.linear.com
The RMS voltage noise of a resistor =3D=20
SQRT(4.k.T.R.B) where k =3D 1.38x10^-23 in J/K, T is the absolute =
temperature in=20
Deg Kelvin (Deg C + 273.14), R is the resistance on Ohms and B is the=20
bandwidth in Hz.
Professional seismometers usually use =
variable=20
capacitor sensors and chopper amplifiers measuring position. See =
AN-87, p87 at=20
www.linear.com There is usually =
no=20
significant intrinsic noise associated with a capacitor, as there is =
with a=20
resistor or an inductor. These give the same output per mm of movement =
whether=20
this occurs over 1 or 1000 seconds. However, a coil + magnet =
velocity=20
detection system would only give 1/1000 the voltage output for the =
1000 second=20
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=20
greater than the 'ordinary' amplifier noise for periods up to ~ =
30=20
seconds, maybe quite a bit more. The complicating factor is the =
variation in=20
the minimum background seismic noise seen at different periods.
I hope that this helps.
Regards,
Chris=20
Chapman
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