Geoff, 
The issue with the bias path is it is part of the input circuitry, so it
should be treated as such.
Most IA's only have good CMR at low freqs, typically around the mains freq
which is why they are used...
The shield is used to shield the input cables from high freq signals that
will not get rejected (as well) by the IA's CMR circuitry..

Ferrites will help with RFI but they need some help as well.
The inputs to a typical IA are many 100's of Kohms, may be even up into the
10's of Mohms and ferrites work by increasing the impedance of the cable at
rising freq, but if the input Z of the IA is high at high freq, the effects
of adding ferrites is diminished, to make them work well, you have to reduce
the input Z of the IA's at high freq.
Small capacitors 100s of pf across the inputs ,in the feedback paths and
from inputs to ground are commonly used to 

1/ limit the bandwidth of the IA's to the range of signals expected

2/ reduce the input Z at high freq so that ferrites have something to work
against..

Ie , the ferrites increase the effective resistance of the cables, and the
capacitors short the inputs at high freqs..


The shield is used to reduce RFI introduced into the signal cables.


Driven shields are used to reduce the capacitive loading on the signal
cables to improve the high freq response of the signals, and this is not
always successful, with instability and oscillations sometimes resulting.

I'm not sure what you mean by a sense wire in an IA,

Power supplies for strain gauge based transducers use sense wires as part of
the control method for removing voltage drops in cables in the excitation
applied to the sensor (strain gauge)... as the output from a strain gauge
(full, 1/2 or 1/4 bridge circuits) is ratiometric with respect to the
excitation voltage applied... if the excitation changes , then the output
signal will as well, so a (or 2) sense wires are used to compensate the I2R
(voltage drop) losses down the cable. Typical power supplies for strain
gauges are held to < 10mV accuracy.

Blair



-----Original Message-----
From: psnlist-request@.............. [mailto:psnlist-request@...............
On Behalf Of gmvoeth
Sent: Monday, 4 July 2011 7:10 PM
To: psnlist@..............
Subject: Re: Damping CDR for HS10-1

Why not use the Shield ?
It has two surfaces,

If you put ferrite beads around the shield
should not that keep the RFI away ?

Some instrumentation amplifiers have
a sense wire, could this be that
power supply wire of which you speak ?


geoff
----- Original Message ----- 
From: "Blair lade" 
To: 
Sent: Friday, July 01, 2011 9:25 PM
Subject: RE: Damping CDR for HS10-1


> Mark, Geoff,
> 
> 2 options,
> 
> 1/ if the electronics are at the instrument then the bias resistor from
the
> centre tap to analogue ground would work, note that shouldn't go to the
> screen, a separate wire within the screened cable back to the analogue
> ground of the instrumentation amp (psu 0Volts) is required.. 
> 
> 
> or 
> 
> 2/ not worry about the centre tap..
> 
> 
> -----Original Message-----
> From: psnlist-request@..............
[mailto:psnlist-request@...............
> On Behalf Of Mark Robinson
> Sent: Friday, 1 July 2011 11:46 PM
> To: psnlist@..............
> Subject: Re: Damping CDR for HS10-1
> 
> Good thread.
> 
> I seem to remember Geoff's rig has a centre tap on the coil.
> 
> How should he connect that?
> 
> Mark
> NZ
> 
> On 02/07/11 00:51, Blair lade wrote:
>> Geoff,
>>
>>   Blair here in Aust,
>>
>> I do lots of (work) stuff with instrumentation amps, matching is
certainly
>> easier using some adjustable components...0.01% resistors are about $30
> each
>> here!
>>
>> However, the increase in noise from using a 3 opamp IA config compared to
> a
>> traditional single input amp has to be weighed against the better common
>> mode rejection with a true instrumentation amp.
>> If you don't have to deal with Common Mode noise, then don't go down the
> IA
>> path..
>> There is much literature on the pluses and minuses of single opamp input
>> verse 3 opamp IA input designs from a seismic point of view.
>>
>> With instrumentation amps (infact with all opamps), there has to be some
>> bias current path from the inputs back to ground, usually provided by the
>> excitation circuitry (strain gauge stuff) but if you are just 'going in'
>> with a floating signal from a coil, you will need to provide some bias
>> resistance / reference to ground, probably a pair of 1meg resistors would
>> suffice , one from each input to ground (0 volts) depending on the IA
> input
>> resistance, ofcourse, these could be your dampening resistors, the total
>> value being split and the centre going to ground...
>>
>> I prefer the 1 megs as any mismatch in low resistance values here will
>> affect your CMR a lot.
>>
>> So the dampening resistor is across the inputs with a couple of 1megs to
>> ground.
>>
>> Need also to be careful to balance the capacitance of your input leads to
>> ground as well, but if you are using good quality 2 core screened
> microphone
>> cable (with the screen tied to grounded only at the amplifier end) that
>> should be fine for the frequencies of the signals you are looking at.
>> The other end of the screen (seismo end) would normally be tied to the
>> sensor's ground and should not be 'hard grounded'(eg. earth staked)
unless
>> you are expecting lots of trouble from lightning.
>>
>> I'd not bother with having a guard amp driving the screen for seismic
>> signals.
>>
>> With careful design and correct choice of components, CMRRs of 150db at
> low
>> frequencies with respect to output are achievable without trimming but if
>> you have that much common mode noise you should do something about it!
>>
>> The 1.25 hz periodic noise... what is your sampling rate and do you have
>> some sort of low pass filter that lobs everything off before you get to
> the
>> nyquist frequency (approx 1/2 your sampling rate)?
>>
>> Also note that induced magnetic fields at the coil end are not common
> mode..
>>
>> Blair
>>
>> -----Original Message-----
>> From: psnlist-request@..............
> [mailto:psnlist-request@...............
>> On Behalf Of Geoff
>> Sent: Friday, 1 July 2011 3:59 AM
>> To: psnlist@..............
>> Subject: Re: Damping CDR for HS10-1
>>
>> I think I just wasted a lot of time
>> trying to get rid of a signal which
>> is real differential and not
>> common mode,
>> there seems to be a machine
>> owned by a neighbor which is not
>> always used.
>>
>> I am totally unable to rid the artifact of
>> about 1.25 Hz which is periodical.
>>
>> Creating the multiple resistor pairs
>> was a waste of time.
>> A pot, most likely, is the beat way
>> to balance the two against the ground.
>> Like 100 Ohm or 10 Ohm 15 turn
>> between two resistors matched already.
>> 1 ohm is the best my DMM can do
>> without help.
>>
>>
>> I have trouble keeping my website
>> to be real time. So its not
>> 100% reliable.
>>
>> I think my website shows the
>> instrumentation amp with the geophone
>> on the negative leads of the instrumentation
>> amp, will have to change that
>> with a new schematic.
>>
>>
>> Regards,
>> geoff
>>
>>
>>
>> -----Original Message-----
>> From: Geoffrey
>> Sent: Saturday, June 25, 2011 8:39 PM
>> To: psnlist@..............
>> Subject: Re: Damping CDR for HS10-1
>>
>> Interesting Bob,
>>
>> But I'm using an instrumentation amplifier.
>> In such an arrangement of three op amps
>> you are using two positive inputs which means
>> the input impedance is mega ohms to giga ohms.
>> The only input is the the resistors which are
>> split against ground. So in my case the you
>> have verified my numbers to be basically correct.
>>
>> I have learned something new to myself in the past
>> few days about this input.
>>
>> There seems to be common mode signals
>> of an electrical nature coming in on the
>> geophone input. The only way to balance out
>> this unwanted signal has been to
>> make several pairs of identical split resistors
>> and see which pair will after installed eliminate the problem.
>> It seems my test equipment can not resolve the measurements
>> fine enough to properly match these two resistors.
>> Therefore it is a matter of chance that the right
>> combination can be achieved.
>>
>> I have never been able to do this balancing
>> act with any configuration other than an instrumentation
>> amplifier.
>>
>> It is my ignorance in combination with
>> people who simply refuse to talk about this
>> which has caused me years of headaches.
>>
>> In my case the Ge seems to reduce to
>> (2.99 * 1302)/1742 or 2.234 v/(in/sec)
>> But this is not how I handle this figure.
>> I treat it as an overall loss of 20log(2.234/2.99) or -2.53dbv
>> when calculating the final amplifier gain.
>>
>> Thanks for your feedback.
>>
>> Regards,
>> geoff
>>
>>
>>
>> -----Original Message-----
>> From: Bob McClure
>> Sent: Saturday, June 25, 2011 6:11 PM
>> To: psnlist@..............
>> Subject: Re: Damping CDR for HS10-1
>>
>> For whatever it is worth, here is my computation of the shunt resistance
> to
>> be applied to the HS-10 geophone to obtain a
>> damping coefficient of 0.707. It confirms Geoff's latest results, but
also
>> allows for the loading provide by the amplifier itself.
>>
>> HS-10 properties
>>
>> Sensitivity, E = 2.99 V/ips = 117.7 volts per meter per second
>> Natural Frequency = 1 Hz = 2*PI radians per second
>> Natural damping = 0.031
>> Inertial Mass = 33 oz = 0.936 kilogram
>>
>> Erhard Wielandt, in his chapter "Seismic Sensors and their Calibration"
> in
>> the Manual of Observatory Practice
>> presents a formula for electromagnetic damping.
>>
>> The formula is h = (E^2 / 2* M * wo * Rd) , where
>>     E is the output in volt-seconds/meter,
>>     h is the damping coefficient (0.5/Q),
>>     M is the effective pendulum mass in kilograms,
>>     wo is the natural frequency of the pendulum in radians/sec, and
>>     Rd is the total shunt resistance.
>>
>> The recommended total damping is 0.707. Since the HS-10 has an open
> circuit
>> damping of 0.031, we want the electromagnetic
>> contribution to be 0.707 - 0.031 = 0.676.
>>
>> so,
>>
>> Rd = E^2 / (2*h*M*wo) = (117.7)^2 / (2 * 0.676 * 0.936 * 2 * PI) = 1742
> ohms
>>
>> Let us say the coil resistance is 440 ohms. The input resistance of the
>> amplifier and its applied shunt resistor must then
>> equal 1742 - 440 = 1302 ohms. The 1302 value is that of the external
shunt
>> resistor in parallel with the amplifier input
>> resistance.
>> Say the amplifier input resistance is 10K ohms.
>> 1/Rext = 1/Rt - 1/Ramp
>> 1/Rext = 1/1302 - 1/10000 = 0.000768 - 0.000100 =  0.000668
>>
>> Rext = 1497 ohms
>>
>> The applied load will reduce the sensitivity of the geophone. The output
>> will be Rshunt/(Rcoil + Rshunt) times the open
>> circuit value.
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