-----Original Message----- 
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’t 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’ve 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, 
typically , they need to be 0.01% tolerance resistors.  This is quite critical to the performance of the cancellation circuit. There 
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 
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 
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’s are normally use to provide a very high impedance (many meg ohms) to the signals 
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 
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’s not to say that the way you are using it wont work, it’s just not the normal method of using IA’s.
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 ‘normal’ way and have the +ve inputs grounded, ignore the need to have bias resistors to ground in 
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 
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’s 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 
setting resistor that is the virtual earth, any signal (noise) that gets introduced between the virtual earth and the –ve input will 
get amplified a very large amount. This includes voltages produced from temperature gradients and dissimilar metals (seebeck 
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’s that used  virtual earth mixing busses and they all suffered from poor noise 
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 
ground loops.

There is quite a lot written about extending the LF response of seismometers by applying overdamping using negative resistance, and 
I guess that’s what you are attempting, using an IA for this is probably not the way to go unless you are using precision resistors, 
it’s 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 
averaging to a lower sample rate is not the way to go, as simple sample averaging will reduce, but not eliminate, aliasing. Instead, 
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 
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 
order.

Bob



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



I’m following the discussion of Geoff’s 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 
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 
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







__________________________________________________________

Public Seismic Network Mailing List (PSNLIST)

To leave this list email PSNLIST-REQUEST@.............. with 
the body of the message (first line only): unsubscribe
See http://www.seismicnet.com/maillist.html for more information.