I tried to use a centertapped step up transformer
but was unable to get that to work.

It was like no signal at all.

I guess you cant just grab a transformer off the shelf
to make such a thing work.

Also tried a centered tapped coil with a magnet
which did work but too complex to make
it is better to make two coils and use then
together. or so it seems to me.

regards,
geoff

----- Original Message ----- 
From: "Mark Robinson" 
To: 
Sent: Friday, July 01, 2011 7:16 AM
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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