Hi, I have a 2Hz. natural frequency Geophone HS, which will give a signal of the vibration velocity of 1 volt/inch per second, and wish to measure the vibration velocity down1 Hz. I saw there is a German company has a similar solution but not complete the same. please see the web page https://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/t/161153 . Can some one show me how it can be done?
Darren
Hi Darren,
We're not Geophone experts here, just amplifier experts, but I'll give it a try. What about using an amplifier with increasing gain at low frequencies such at the resulting transfer function of both the amplifier and the Geophone is flat:
In that example I show a geophone with a corner frequency at 10Hz (-3dB) and a single-pole role off implemented by R1, C1, VG1, and VCVS1. The gain of the amplifier increases at low frequencies to counteract the decline in signal output from the geophone. I chose a low frequency corner of 0.1Hz (implemented by R5 and C2) and a low frequency gain of 40dB.
This circuit only restores the original geophone output at frequencies below 10Hz, so additional gain may be required to raise the geophone output to the necessary signal levels at all frequencies.
Implementing this in reality may be somewhat difficult. Capactior C2 is very large and so it may exhibit unexpected behavior due to dielectric absorption. The OPA188 was selected because of its excellent low frequency noise and thermal drift, which will be very important in this application.
Darren,
.TSC is the extension for a TINA-TI simulation file. TINA-TI is a free circuit simulation program provided by TI, that you should consider downloading as you work on this project. It will help us share circuit ideas back and forth and can possibly reveal problems with the circuit before it is built. Here is an image of the circuit in my previous post:
Darren,
That's correct, the slow of the shelving low-pass will continue on above 15 Hz. When I did this design I hoped that would be acceptable since I thought higher frequency seismic events are local, rather than teleseismic. The pre-amplifier is there only the reduce the gain requirements in the shelving low-pass filter and provide the proper termination impedance for the geophone. It is NOT intended to correct the transfer function, the feedback capacitor value is only selected to attenuate high frequency noise from this stage without affecting the overall transfer function. If you need to preserve the high frequency information as well, then we need a bypass pathway around the lowpass filter for those high frequency signals. In the below schematic I inserted op amp U3 to act as a bypass pathway around the lowpass filter. The two outputs are summed through the 100 ohm resistors.
By matching the corner frequencies and flat transfer function over a wide bandwidth can be produced:
