PSN-L Email List Message

Subject: RE: Damping CDR for HS10-1
From: "Blair lade" blairl@...............
Date: Sat, 2 Jul 2011 13:55:45 +0930

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.. 


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?


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
> here!
> However, the increase in noise from using a 3 opamp IA config compared to
> 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
> 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
> 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
> 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
> 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
> nyquist frequency (approx 1/2 your sampling rate)?
> Also note that induced magnetic fields at the coil end are not common
> Blair
> -----Original Message-----
> From: 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
> 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"
> 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
> 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
> 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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