Preamplifier for Geophone (Vibration Sensor) - Amplifiers forum

Hello Chetan,

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This circuit is quite different then what I've recommended and there are likely a number of reasons why it is not working for you.  What are your final goals?  Are you trying to simply detect a seismic (or other) activity and make note of it, or do you intend to try to capture the magnitude of the waveform to observe it? 

To start, the gain of this circuit is 3300000/6700 = 492V/V.  We determined earlier that you only require a gain of 5V/V.  With a gain of 492V/V any input signal over 10.16mV will create an output voltage over 5V.  This is why your circuit is always displaying the maximum output, you've saturated the output with even the smallest input signals. 

Also, by biasing your input to GND your are essentially cutting off the lower half of your input signal because the common-mode input range of this LM358 rail-to-rail input amplifier only includes GND, not below it.  Therefore, I would suggest either using a dual supply operational amplifier or biasing the input to mid-supply to use the optimal part of the common-mode input range of your amplifier.  This would require you to change your peak detector to be biased mid-supply as well instead of GND. 

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Keep in mind that you will not see a sine wave at the "Output to DSO/ADC" node on your schematic.  This output will display the peak output of the circuit because it is a peak-detector (envelope detector).  To view the amplified input signal please look at the output of the amplifier which is at pin 1.

I wish you the best in your efforts, but if you do not choose to follow the advice and/or circuits I provide then I'm not sure there is a reason to continue this thread.

Regards,
Collin Wells

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