PSN-L Email List Message

Subject: Re: Op amp front end noise
From: John Popelish jpopelish@........
Date: Tue, 15 Mar 2005 12:05:01 -0500

> ChrisAtUpw@....... wrote:
> > John Popelish wrote:
> > Something interesting to me that does not show up in the
> > list I posted, but in the graphs is the fact that the
> > overall signal to noise ratio climbs as the coil wire size
> > is reduced, even though it results in more resistive
> > noise.  But some opamps have such low current noise and
> > the extra turns provide more signal voltage, so that there
> > is a steady climb in signal to noise ratio to coil
> > resistances around 100k, and then there is a second, even
> > bigger peak for coil resistances of 100 meg ohms, but I
> > wouldn't want to handle the wire.
>     The opamps have a design impedance when the current and voltage
> noise levels are about equal. If the coil resistance is less than
> this, it pays to add turns.

Agreed.  Processing the opamp data to take the turns count (in a given
winding volume) into account and displaying this graphically makes
this very clear.  I need to get set up to post these pictures to the
>     Are you taking the 1/f noise into account? This is usually
> fairly critical for seismic sensors, particularly when you are
> considering long period types.

Yes.  I selected or extrapolated all device specs to 0.1 hertz for
this comparison.  Amplifiers that have a low 1/f corner frequency or
chopper amps that actively correct 1/f noise do better in this range
than if you compare amplifiers at higher frequencies.  I selected this
frequency as representative of what long period instruments measure.
> > But the graph does show that there is signal to noise
> > value in going with the smallest size wire you can deal with.
> It will usually pay to choose a fairly low amplifier impedance
> for inductive systems. The coils are much easier to make and
> physically smaller, which allows you to take full advantage of the
> very high fields that can be produced by 'modern' NdFeB magnet
> systems.

My list is valid for any signal source, but the rising signal to noise
effect with impedance mentioned in that last paragraph refers to coils
that fit in a given volume.  This implies that doubling the turns
count raises the DC resistance by a factor of 4.
> The larger the coil, the more difficult it is to screen it from
> environmental noise. 

My comments referred to coils of similar dimensions.

> In general, most of us do not have the luxury
> of quiet seismic sites. The larger the inductance, the more
> susceptible is the wiring to picking up stray signals.

This is a separate problem.  
I think it is a good idea to electrostatically shielded any pickup
And the inductive pickup problem can be addresses with a hum bucking
coil design. 

I am presently in the middle of building a dual voice coil pickup that
is an attempt to buck out the effect of any external magnetic field
changes, like those from power distribution, lightning strikes and
aurora.  I finally got my 6 iron pole pieces from the machine shop and
assembled them with 20 ferocious NeFeB magnets.  I am creating the
coil bobbin now.  The Lehman pendulum is almost done and I may have
everything but the amplifier together by April fools day. 

> It can pay to put a ceramic capacitor across the input to the opamp.

I concur about the capacitor, but you have to be careful with
ceramics.  Some of them are quite microphonic, especially the larger
values in the high K class, like Y5V and Z5U.  I think I prefer film

I also like a pair of non light sensitive diodes across the input to
protect the front end from static when wires are not connected or
excessive generated voltage when the coil is bumped.

> The use of
> screened cable with a large dielectric loss can be an advantage.

I think there is value in getting at least the first stage of
amplification close to the coil, to lower the chance of stray signals
getting into the input.  I may mount the whole amplifier-filter right
beside the pendulum.  

John Popelish

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