Comments needed on this circuit

its obviously a Hi-Z problem due to the nature of your app. Microphone wire is crap most of the time from what i have seen. you could try some tightly shielded RG174..

in any case, after looking at your circuit i think moving it all to the location of the sensor and having a low Z output. or at least move one stage to give you a unity gain with low Z, make sure you use Large by pass caps in the sensor end for the +/- lines to keep the EMF out of that. your basicly picking up the EMF and your cable is the secondary.

Hello,

The circuit above has been used by me for the last year to receive regional and teleseismic signals but I have been plagued by a problem that requires me to seek help.

The lead in cable is a shielded, twisted pair, microphone cable. Its shield stops maybe 2 feet from the sensor. The sensor cable itself has no shield. Its entire length is approximately 50 feet and parallels a major airconditioner power supply line about 4 or 5 feet away for maybe 30 of the

Every time the Air conditioner comes on a tremendous electrical disturbance is generated in the input of this circuit causing the class "A" baseline to follow a nonsymmetrical cycle of waveform maybe 30 to 60 seconds in period.

I am seeking recommendations to stop or significantly reduce this electrical disturbance. The offending device is actually a heat pump and the signature of the disturbance is quite different depending upon whether the device is heating or cooling.

I am unable to relocate the sensor so other solutions are necessary.

Any help here is appreciated.

gmv

Why does the shield stop 2 feet from the sensor?

How about putting some/all of the amplification at the sensor end?

I read in sci.electronics.design that gmv wrote (in ) about 'Comments needed on this circuit', on Sun, 16 Jan 2005:

The unbalanced input stage is wide open to every disturbance under the sun. You need a balanced input stage and a twisted-pair shielded cable to your sensor. With that huge gain in the first stage, I suspect the whole design, frankly.

You are probably right. I am trying to go as cheap as absolutely possible. Is it possible to use a diff amp on the input if there is no center tap from the sensor ?

Surprisingly this circuit produces fairly good results so long as there is no electrical disturbances in the vicinity. I have found using electrolytic condensors works quite well so long as you do not use them in a feedback type filter.

I read in sci.electronics.design that gmv wrote (in ) about 'Comments needed on this circuit', on Sun, 16 Jan 2005:

Centre-tapping is not necessary unless your source and amplifier input both have very high common-mode impedance. In that case, putting a pair of resistors (preferably closely matched in value) in series across the sensor and grounding their junction may help.

The geophone came with out a shielded cable it only has a single pair going to the coil/magnet assy inside. Or so it seems. I was trying to get away as cheap as possible and could not find the fancy shielded wire you guys speak of at the local radio shack store. Yes I think a preamp at the sensor might be the way to go. Or possibly an instrumentation amplifier but it all increases the cost. Is it possible to create a three wire system from the two wire sensor so I can use a Diff amp as the input ?

The first stage should be a differential input. This will eliminate most of your problems. Try to connect the shield from the cable to ground of your circuit where the power supply is connected to your circuit. I also noticed you are doing a lot of low pass filtering. You could look into a 'Sallen-key' filter ('Google' for it). This is an easy filter to construct with an opamp and will probably save you several opamps.

And lose the twisted pair? Bad idea. With a differential input amplifier and shielded microphone wire, you can have several hundred feet of wire between the microphone and the amplifier without any problems. I see no problem in the setup of the OP.

Is that 432 ohm at the sensor something you added or is it internal to the geophone? Your input lines are not "balanced", a terminology which simply means each input line from the sensor has identical impedance to your circuit common over all frequencies in this case. Your interference problem runs deeper than just electrical. From your description, it is clear that the heat pump assembly is transmitting shock and/or acoustic wave components into the ground that are within the frequency range of the geophone.

That circuit would reject common-mode noise better if the two branches consisting of 2.2k+120k resistors both had equal capacitors across the 120k resistors.

You should also be aware that using 3 op-amps for an instrumentation amplifier can be done with better common mode rejection using the popular circuit diagrammed below:

|\ ----------|+\ | >-------o---------. .-|-/ | | | |/ .-. | 100k 100k | | | | ___ ___ | | | 20k '--|___|--o---|___|- | '-' | | | | | | | | | | '------------o | | | | |\| | .-. '--|-\ | | | | >---' | | 2k .--|+/ '-' | |/| | | .------------o | | | | | | 100k | 100k | .-. ___ | ___ | | | .--|___|--o---|___|-. | | | 20k | | | |\| '-' | | '-|-\ | | === | >-------o---------' GND ---------|+/ |/|

(created by AACircuit v1.28.4 beta 13/12/04

Thanks to input from you guys I tried an instrumentation amplifier on my input and that seems to have eliminated the 60HZ noise I had in my first stage.

circuita.gif is the noisy devil. circuitb.gif is the one I am now using.

gmv

this file is now circuita.gif

That's a lie if I ever heard one.