Geoff,

 I see you have updated the schematic on your web site,=20
From the previous schematic , I assumed that you had the hi impedance =
inputs grounded and the seismo coil connected where Rg the common gain =
setting resistor between the negative inputs is.

This was an unusual but interesting method of connection and one I'll =
explore at work some time. It has potential for extending the low freq =
response with negative resistance, although, I'm a bit unsure of =
stability with long leads.=20
Having the front end amplifier at the seismo might be the way to go with =
this method..
And that would certainly reduce any common mode signal pick up in the =
low level signal cables.

On your updated schematics,

the connections to the seismo are shown and much more conventional.
Again, all the gain setting resistors need to be very closely matched =
for good IA performance.

The 1M and 50k resistors set the DC (and AC) gain of either side of the =
differential amp, so any mismatch here will result in poor CMR =
performance.
The 4ufd in parallel with the 1M resistor slugs the hell out of the AC =
gain and values this large with 1M resistors will set the AC gain to 1 =
at fairly low frequencies..

While these capacitors set the AC gain to 1 and the differential amps =
will remove any common mode signal from its inputs, the gain is set to 1 =
at pretty well any freq above 0.05hz which is probably a bit low for =
seismic. (you probably want a bandwidth of dc - 10Hz here using a =
geophone, maybe Lower if using long period device and you don=E2=80=99t =
want to see local events or local seismic noise)

Most users of IA's would use  4 resistors the same value here (and only =
very small valued capacitors 100-1000pf) as any gain makes it much =
harder to get the desired CMR performance without trimming the gains to =
be the same.

The gain of the IA is usually just done in the front end (the 2 input =
amplifiers) and again the matching of the 2* 82k resistor is critical.
I'd be putting the 4ufd (or similar) capacitors around the front end and =
upping the gain here, no point amplifying broadband signals (+noise) in =
the front end then knocking it down further on up the chain.
These capacitors should also be matched in capacitance and inductance =
for best performance, (something very hard to do!!)

Increasing the gain in the front end and reducing the gain of the =
differential amp should also make it more temperature stable, not that =
you care as temperature changes should be much longer in the time domain =
than your signals and you would remove these very low freq signals by ac =
coupling to your filter section or to your digitizer (or both).
See how and where Larry does it in his amplifier / filter circuit for =
some ideas.

Gains of >1000 are typical of IA's with good noise performance.


I'd also be putting a couple of back to back diodes across each of the =
inputs of the IA to ground.

Reason here is 2 fold, it gives some protection against overloads... =
(induced large signals due to lightning) and also provides a short to =
the seismo when moving it around (very important with Wilmore =
seismometers for example)..
The diodes shouldn't affect the seismic signal as your signal is way =
lower than the 0.3-0.6 volts at which the diodes will conduct (which is =
why you have a gain of x 1000's anyway.)

Grounds,=20
All grounds should go to the 1 point (star fashion) usually to the 0Volt =
connection at the power supply and should not be daisy chained. Power =
rails aren't so critical in this respect but it's good practise to do =
the same. This way you avoid internal and external ground / hum loops.


A note about power supplies, the 470ufd caps after the regulators are =
probably a bit big, National Semiconductor suggest around 10ufd with =
0.1ufds in parallel and you really should have reverse diodes across the =
regulators with capacitors this size (see National Semiconductor's =
application notes for the reasons).

Bypass every chip and every board with 0.1ufds on each rail ( to the =
local board ground) and typically 1 * 10ufd on each rail to ground on =
each board if the supply leads are greater than about 6 inches.

The ARRL handbook has some guidelines about power supplies...

For now

blair


-----Original Message-----
From: psnlist-request@.............. =
[mailto:psnlist-request@............... On Behalf Of Geoff
Sent: Tuesday, 12 July 2011 12:37 AM
To: psnlist@..............
Subject: Re: Noise



-----Original Message-----=20
From: Blair lade
Sent: Friday, July 08, 2011 10:24 AM
To: psnlist@..............
Subject: RE: Noise



Geoff,



what is your sample rate and are you averaging the data?
1. 50 sps
2. don=E2=80=99t know, I use winsdr to fetch the data, however that =
works

How far away from the IA is the seismo?
1. 40 feet, two feet underground,grounded only at amplifier


I=E2=80=99ve had a look at your circuit,



for good common mode noise reduction , the 4 resistors around the 3rd op =
amp (summing amplifier) have to be very well matched,=20
typically , they need to be 0.01% tolerance resistors.  This is quite =
critical to the performance of the cancellation circuit. There=20
is an ic made by Analogue Devices, the  Amp03 which is often used for =
this, it has 4 x  25k laser trimmed resistors on the chip=20
which are matched typically to 0.01% and saves a lot of effort. (I use =
lots of them.)





The same applies to the  resistors around the 2 buffer amps driving the =
3rd op amp, oviously, if the gains of the input / buffer=20
amps are not exactly the same then there will be different levels of =
signals reaching the cmr circuit.



Your use of the IA is a little unconventional, IA=E2=80=99s are normally =
use to provide a very high impedance (many meg ohms) to the signals=20
so that there is no loading on the signal  and I2R losses down the cable =
are also reduced, that is why the +ve inputs of the op amps=20
are used.

1. you can consider the in+ tie points to be open circuits, whatever you =
do to balance things seems ok to me.
    the lower the input impedance the lower the circuit noise.

That=E2=80=99s not to say that the way you are using it wont work, =
it=E2=80=99s just not the normal method of using IA=E2=80=99s.
1. normally, I guess, you use a single IC IA instead of the combination =
I now use.
    It is cheaper for me to do it this way.

As you are not using the IA the =E2=80=98normal=E2=80=99 way and have =
the +ve inputs grounded, ignore the need to have bias resistors to =
ground in=20
this case..
1. It seems to work OK the way I'm using it now.


Having long wires from the seismo going to the virtual earth connections =
 of the buffer amps could result is all sorts of=20
instabilities within the IA .
1. I agree with you if I do not balance the Rd against ground.
    there are basically two different circuits here
    which join at the IA input.
    the geophone loop which transfers voltage to the IA, from there on =
it=E2=80=99s the amplifier stuff only.
    I look at at the two separately.

While it would seem that the connection is a virtual earth, in fact it =
is the connection between the feedback resistor and the gain=20
setting resistor that is the virtual earth, any signal (noise) that gets =
introduced between the virtual earth and the =E2=80=93ve input will=20
get amplified a very large amount. This includes voltages produced from =
temperature gradients and dissimilar metals (seebeck=20
effects)..
1. I know nothing about this it would take building several different =
circuits for me to test here.

(There were some audio mixing consoles in the 1980=E2=80=99s that used  =
virtual earth mixing busses and they all suffered from poor noise=20
and cross talk performance)
1. interesting
    with ground you are simply trying to keep things both iso-voltage =
and common mode equality without feeding output to input as=20
ground loops.

There is quite a lot written about extending the LF response of =
seismometers by applying overdamping using negative resistance, and=20
I guess that=E2=80=99s what you are attempting, using an IA for this is =
probably not the way to go unless you are using precision resistors,=20
it=E2=80=99s too hard to try and balance it all up.

Have a look at http://www.vaxman.de/publications/teach_gp.pdf for =
example

1. I find this circuit interesting it works different than I have ever =
seen anything before.
The amplifier shown seems to act as a damping resistor but no output
is derived from that amplifier.
I will one day have to play with this idea again.
I understand the principal of over damping but it seems to also decrease =
the s/n ratio.
it is the only way to easily get flat velocity response between two =
resulting frequency break points.


A single op amp is used for the overdamping followed by lots of gain and =
some equalising / filtering
1. It is the high gain which creates the noise.

blair





From: psnlist-request@.............. =
[mailto:psnlist-request@............... On Behalf Of Bob McClure
Sent: Thursday, 7 July 2011 1:14 AM
To: psnlist@..............
Subject: Re: Noise





Raw data acquisition should always be done at a sample rate more than =
twice the upper frequency passed by the amplifier. Sample=20
averaging to a lower sample rate is not the way to go, as simple sample =
averaging will reduce, but not eliminate, aliasing. Instead,=20
low-pass filter the raw data to an upper frequency less than one half =
the reduced sample rate. Reduction to the final rate can then=20
be accomplished by simple decimation, or by sample averaging.

I have done experiments to verify the above statements. I would =
recommend that the digital low-pass filter be of fourth to sixth=20
order.

Bob



On Wed, Jul 6, 2011 at 11:11 AM, Randall Pratt  =
wrote:



I=E2=80=99m following the discussion of Geoff=E2=80=99s noise with =
interest.  I would like to ask some questions on alias noise.



If one were to vary the sample rate and sample at say 60sps, 40 sps, 15 =
sps, 6 sps is there a formula to take the peaks from the=20
various ffts and arrive at a list of probable noise alias problems?   I =
have done this and can see some changes in noise peaks on=20
the spectrums but I have not attempted to calculate a relationship.



Secondly, if the AD runs at 120 sps and averages to 6 sps recording rate =
will frequencies between 3 and 60 hz alias into the record?



Randy







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