----- Original Message ----- 
From: "Blair lade" 
To: 
Sent: Tuesday, July 12, 2011 2:24 PM
Subject: RE: Noise


Geoff,

 I see you have updated the schematic on your web site,
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.
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’t 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,
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

Hello Blair,

I always put the largest capacitors on the input side of the regulators,
This way when power is cut and the voltage dies, the input should always be a higher
voltage than the output so i do not worry about loss or upsets
to power. I have not yet in many years lost an Opamp
to lightning so being worried about RFI like always
i do not want any diodes near the input.
Because you use caps like this should have fuses
but since these wall converters are protected internally
(so i understand) I do not worry about shorts.
caps have a nasty tendency to short when they go bad.

I too would like to experiment with the negative legs
as an input but cant afford to anymore.
Electronics parts are no longer pennies, now they
are dollars.
I live on SSI disability and since 1980s never has the
increases kept up with inflation.

I am quite happy with what i see in this circuit
even tho it is not a true velocity result.

If i try anything else it will be overdamping
or one chip instrumentation amp.

I think the distance of 50ft is not bad.
The signal is a damping current loop within a 100%
shieldid cable. The signal should outshine
the noise with ease.

I do not see any improvements to make
at this moment.

keep an eye on my website from time
to time and tell me if you see unwanted artifacts
no one else can.

I will save these emails and review them
from time to time to see if it helps me understand
more than I already do.

Regards,
geoff 

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