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Simple Automatic Gain Control amp for Geophone

Emanuele Annino
Prodigy 20 points
Other Parts Discussed in Thread: OPA350, OPA2188

Hi,
I'm studying the behavior of a Geophone for ground vibrations and seismic response, but I read in literature that the impulse signal tend to attenuate into the ground, so the deepest signals decrease in intensity and it's difficult their acquisition by an ADC.
The use of an Automatic Gain Control preamplifier should solve the problem, not only to increase the above signals but also to limit the spike signals.
So, my questions: Is it possible to design an AGC with an op-amp like OPA350 and few components?
Or in alternative: Is there a chip with built-in AGC functionality for the above target?

Thank you in advance.

  • 0
    TI__Guru* 93105 points

    Hello Emanuele,

    I discussed your geophone application with one of my colleagues who has a more thorough understanding about them and their application requirements. Geophones typically are usable from a few hertz, to a few hundred hertz. Therefore, any amplifier used for this application must have low 1/f noise. Otherwise, low level signals from the geophone may be masked by low frequency noise. The best performers in that regard, by design, is a chopper operational amplifier. The OPA350 is a high performance operational amplifier, but it is not a chopper. Also very important is the THD performance so that negligible distortion is added to the waveforms produced by the geophone. An operational amplifier that would easily meet these requirements is the OPA2188.

    http://www.ti.com/product/opa2188

    When it comes to AGC, that may be difficult to apply and still be able to achieve very good THD performance. Most of the circuits I am finding use a JFET as a voltage controlled resistor. Here are two Electronic Design articles that might serve as a starting place. I would employ the OPA2188 instead of the amplifiers listed:

    http://electronicdesign.com/analog/effective-agc-amplifier-can-be-built-nominal-cost

    http://electronicdesign.com/analog/op-amp-and-two-jfets-form-voltage-controlled-amplifier

    I hope this helps you get started.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 20 points

    Hello Thomas,
    many thanks, I think your response is really helpful.
    I will connect the AGC amplifier at the beginning of other modules (filters/amplifier).
    I designed the following circuit (starting form the ones you suggested me) that I
    attach to this post.
    These are the main topics:
    1) A dual power source +/- 2.5 Volts for the op-amp (according to the preamp and filters connected at the output);
    2) two resistors of 4.7K in place of the potentiometer, before the output;
    3) a gain of about 100 defined by the two resistors R7 (47K) and R8 (470);
    4) the substitution of electrolythic capacitors with not polarized capacitors of 1uF;
    5) the use of BC547 NPN transistor and BF245C JFET transistor.

    After the simulations I obtained a constant gain of about 40 milliVolts at the output any time I changed the input amplitude signal (from few millivolts to Volts) so I think the circuit does its job.
    As i mentioned before, the constant output of this AGC scheme will be directed to other stages for the real amplification and filtering.
    Your reply to improve some details or correcting any errors will be welcome!

    I will need a lot of time to test the circuit in the reality, since at the moment I don't have all the components.
    Thank you again!

  • 0 in reply to Emanuele Annino
    TI__Guru* 93105 points

    Hi Emanuele,

    It looks like you are making good progress with your design. Thanks for including your schematic. Since I don't have your TINA simulation circuit file I trust the simulation results are as you described. Do let us know how the OPA2188 works in this AGC application, and if the circuit meets your performance requirements.

    Regards, Thomas

    PA- Linear Applications Engineering