Yes, lots of 60Hz common mode stuff
floating around here. I can measure
60Hz at like 23Vrms but there is little
current relating to it.
I think my DMM is like 20Meg or more.
If i try to measure the current I get
nothing.
And static Electricity is bad also.

I have everything residing on aluminum
sheet metal which is tied to both
Circuit(chassis) and Power entrance (Earth)
ground.

I like the balanced nature of the Instrumentation
Amplifier especially since I use a split power supply.

Regards,
geoff

----- Original Message ----- 
From: "Blair lade" 
To: 
Sent: Sunday, July 03, 2011 7:18 AM
Subject: RE: Instrumentaion Amps and transformers


> Hi Geoff, 
> The transformer idea could be made to work , however, you have to take it
> apart a bit (you need to change the magnetic assembly from a closed field
> (which is how a transformer works ) to an open field (which is how a
> solenoid works)... 
> 
> If you pull the "Es" and "Is" apart and reassemble the transformer with just
> the "Es" all pointing the same way it will sort of work.
> "C" core type transformers don't work well!
> hope this makes sense to everyone reading this.....
> 
> Transformers are typically used with ac signals to change the level or give
> isolation to the signal (ac voltage) which incidentally is why they are
> called transformers, and while seismic signals are an ac signal, the
> frequency of seismic signals is so low (typically 0.01 hz to 20hz) that any
> transformer off the shelf just wouldn't have enough core in the right
> configuration to respond at these frequencies as a transformer.
> 
> Just on the side....
> 
> Typical mains (power) transformers in Aust are designed for 50Hz and have
> more core and inductance and are physically larger and heavier than the ones
> you have in the US which are designed for 60Hz, and likewise motors designed
> for Aust conditions have more core (the soft iron metal) than ones designed
> for the US.
> US transformers and motors don't perform well in Aust, they get hot and
> suffer from poor efficiency. Likewise , when Aust motors and transformers
> are used in the US at 60Hz they don't like it either..and it's got nothing
> to do with northern verse southern hemispheres and that water goes down the
> sink in a different direction.....
> 
> 
> (Good) Audio transformers typically have enough core in them to work from 20
> or 30Hz upwards. They also use different core material.
> 
> If you look at a low current relay coil with a pole piece, it has very large
> amounts of wire and reasonable amounts of metal core (which is again
> different metal to that used in transformers).
> 
> These could be used as a sensor for seismic but there are issues with
> linearity etc if you have large amounts of movement between the magnet and
> the core of the winding, again they are only able to respond to ac signals.
> 
> To test this, hook a coil up to a multimeter on dc volts and slowly move a
> magnet to and fro, you will see a varying dc signal, which is of course an
> ac signal.
> 
> Now place the magnet on the end, once the meter settles down, you will see
> no voltage produced as the magnetic flux isn't changing. 
> 
> 
> It is a problem for all magnetic type sensors, poor dc (or low ac frequency)
> signal response.
> 
> To a certain extent, the more core the better the low freq response, but
> there are practical limits.
> 
> 
> Getting back to Instrumentation Amps, why the decision to go down the IA
> path?
> Do you or do you think you have a common mode noise issue that has to be
> addressed?
> 
> Blair
> 
> -----Original Message-----
> From: psnlist-request@.............. [mailto:psnlist-request@...............
> On Behalf Of gmvoeth
> Sent: Sunday, 3 July 2011 10:52 PM
> To: psnlist@..............
> Subject: Re: Damping CDR for HS10-1
> 
> 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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