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# Preamplifier for Geophone (Vibration Sensor)

Hi,

I have a Geophone which is giving min output 10mV and max 1V after observing on DSO.

Before giving it to an ADC I want to amplify the sensor output so that max output remains

under 5v and has enough gain and noise free output to an ADC. I'm not able to understand

how to start building the circuit. May anyone give me some starting tips on same?

Will using LM358 work?

• Hello Chetan,

It sounds like you need a system with a gain of 5V/V and a rail-to-rail output.    Without knowing more about your system, here is a very basic circuit that performs the operation you're trying to achieve.

The circuit uses the OPA376 in a standard non-inverting AC amplification circuit commonly employed for microphones, geophones, and other small signal AC sensors.  You will need to adjust the geophone bias resistor value ( R1) to match the specifications of your model.

Also depending on the required bandwidth the low-pass and high-pass filters can be modified.  The three low-pass filter R+C combinations are below, and the -3dB frequency can be calculated with f = (1/(2*pi*R*C))

R1+C3
R4+C6
R3+C1

Since the circuit is in a gain of only 5V/V the R4+C6 filter is not very effective so the dominate filtering will come from R3+C1 and R1+C3.

The high-pass filter R+C combinations are below:

(R5||R6)+C4
R2+C5

Here is the example circuit and AC results:

Have fun!

Regards,
Collin Wells
Precision Linear Applications

Regards,
Collin Wells

• In reply to Collin Wells:

Hi Collin,

Thanks for your detailed explanation.

However the Geophone sensor I've is closed one, it means no way I can adjust bias resistor of Geophone.

Also it's having 5 PIN output : 3 are X,Y and Z triaxial , 4th one is GND and 5th Common.

Why consider AC analysis?

As per my knowledge output of any sensor should be a DC value.

• In reply to Anonymous:

Hi Chetan,

My recommendation for the bias resistor was only if your model required one.

I was under the impression you were interested in the changing AC waveforms associated with the seismic activity.  Do you not care about the AC content?  I don't think you're really interested in the DC value alone unless I don't fully understand your application.  Do you know how fast the input signal in your application can change?

Regards,
Collin Wells
Precision Linear

Regards,
Collin Wells

• In reply to Collin Wells:

Hi Collin,

My application target for now is very simple : To fetch Geophone's sensor output and give it to ADC on
the development board I'm having. After going through some details I understood it will be good if I use a
preamp before giving it to ADC of the development board. Also same was confirmed via DSO. As this is
the first time I'm working on Geophone (http://www.instantel.com/pdf/720B0002.pdf only consider circular shaped
geophone not the kit) so I thought observing DC value is good.

Minimum DC I was getting around 10 mV that too after keeping Geophone stable.
Maximum DC I got was 1v that when I was shacking Geophone as much possible.
Frequency was varying  from Khz to Mhz.

Please do correct me if I've gone wrong and what should be my possible step.

• In reply to Anonymous:

Chetan;

We may have a problem with semantics-- Colin was talking about a geophone, which is used to measure ground vibration-- an AC output. However, your description of a triaxial sensor sounds more like an inertial sensor. Its output would be DC voltages in a static condition and AC + DC outputs under dynamic conditions.

To make any circuit recommendation it is necessary to also know the impedance of your transducer and the bandwidth that you need-- as well as the power supply voltage(s) that may be available.

Regards, Neil P. Albaugh   ex-Burr-Brown

• In reply to Neil Albaugh:

Hello Neil,

The Geophone show here is what I have : http://www.instantel.com/pdf/720B0002.pdf
However I don't have exact datasheet for Triaxial Geophone.

As per my understanding for Geophone I thought getting preamp to generate max 5v output
will be fine as, as said earlier I'll give that output to development board so safer voltage should
be 5 volt.

Impedance and bandwidth I can't comment.
Regarding power supply : 12 volt DC as I'm planning to have more circuitry once this phase is completed.

• In reply to Anonymous:

**** Note: I saw Neil's post just now before I was about to post this.  Chetan please clarify what your final application is.  From the datasheet for the sensor it looks like it would be seismography but if it is not this could be why I'm having some trouble understanding some of your application requirements.  ****

Hi Chetan,

Thanks for the additional information.  After reading the datasheet this type of sensor does appear to be a seimography related geophone and you will need an AC amplifier for this type of application.  If you are only concerned with the peak or max value then follow the amplification stage with a standard Peak-Detector or Envelope Detector circuit.  The sensor is listed for a flat-response over the frequency range of 2Hz to 250Hz.  I modfied the original circuit I sent for this lower bandwidth.  Again, this is a very basic circuit and additional filtering or conditioning may be required.

1643.OPA376 G = 5.TSC

Regards,
Collin Wells
Precision Linear

Regards,
Collin Wells

• In reply to Collin Wells:

Hi Collin,

Yes it's on seismography.

With X ,Y and Z as output for the sensor also means AC output?

I'm confused what to do.

• In reply to Anonymous:

Chetan;

From the looks of the data sheet, it is a complete system; no preamplifier is necessary.

Regards, Neil P. Albaugh   ex-Burr-Brown

• In reply to Neil Albaugh:

Hi Neil,

I think I should clarify it again. I only have the Geophone that is shown in the datasheet.

Do you mean to say that whatever is the output of Geophone that I can interface directly

to the development board without any peamp?

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