I'm trying to connect a geophone to a U3. I purchased InAmps thinking I need to amplify the device so I could see it using the stream software. The geophone puts out about 250 mV when you shake it, and you can see it with the stream software (when plugged directly into the labjack, but when I apply it to the Inamp i just see a straight line. I have tried 1x and 11x amplification with either offset voltage, but I still only see a straight 2.5 volts or something like that. I guess I don't know enough about geophones to know how they are supposed to interface with a labjack. Any basic starting advice would be helpful.
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Perhaps your signals are floating?
https://labjack.com/support/app-notes/differential-analog-inputs
I would expect your geophone to be connected to INA+ and INA-, with a jumper or resistor also from INA- to GND.
See basic troubleshooting tips here:
https://labjack.com/support/datasheets/accessories/ljtick-inamp/appendix-e
I have the Geophone hooked up and looking at the signal in LJlogud. How do I see the response below 0V? it bottoms out and I can't see the negative swing.
If you move it slow you can use LJLogUD to acquire a waveform, but likely you need to use LJStreamUD to scan faster.
See in the specs here that "Typical Output Range" is 0.01 to 4.5 volts:
https://labjack.com/support/datasheets/accessories/ljtick-inamp
So the LJTIA cannot output negative voltages. It can take in small negative voltages, add an offset, and then produce a positive output. See the signal range tables and note that your Vcm is Vin/2.
If you are talking about directly to the U3 inputs, the high-voltage inputs on the U3-HV can accept +/-10 volts. The low voltage inputs are 0-2.4 or 0-3.6, with the small detail that a differential channel can handle a negative voltage down to about -0.3. In general to handle a negative voltage on a low voltage input you use a shifting circuit like you get from the LJTick-Divider-BIP10V:
https://labjack.com/support/datasheets/u3/hardware-description/ain/analo...
I am now using a ljtick-inamp and I see my signal, both positive and negative. the original signal is about 500 mv peak to peak, and the newly amplified signal is about 2 V peak to peak. It is a very dirty sine wave. Basically the original circuitry that I am trying to duplicate vibrates a geophone at 8 hz with a deflection of 5 mils. An amplifier card takes this dirty signal, cleans it up and amplifies it to 1.2 V peak to peak and then full wave rectifies it, and then filters it to a fairly clean 450 mv dc signal. This is then digitally converted to a display that reads "5.00". Every mil seems to equal about 90 mv, so at this point I am assuming linearity. ie if the voltage of the signal is 90 mv, that would equal 1 mil on the display.
When I create a table with this signal it just seems to average the offset voltage of 1.25 which makes sense because it is a peak to peak voltage. I am at a loss how to proceed though because this is all new territory for me... I want to take that dirty signal and get that 5.00 to start with. Can I even do that with the labjack / inamp?
I'm not quite following. Are you saying you want to use a U3-LV (and LJTIA if needed) to acquire (digitize) an AC signal that varies from 0.0 to 0.5 volts (peak-to-peak), so you can do some math on it that spits out a DC value that is 10 times the peak-to-peak value?
The input would be the varying 500 mv peak-to-peak voltage. The output would be a number based on a DC value up to 450 mv (which would equal the 500 mv peak-to-peak voltage), with 90 mv =1.00, 180 mv=2.00, 270 mv=3.00, 360 mv=4.00, and 450 mv=5.00.
So you have a -250mV to +250mV sine wave signal and basically you want to know the peak value of this signal. Peak is one obvious value, but peak-to-peak or rms are other DC values related to waveform magnitude that sound like they would work just as well. You just need some singular value related to waveform magnitude that you can then apply your desired scaling to.
Hardware: You could make a rectifier plus filter circuit that produces a DC value of roughly 0-250mV that you can simply sample with the U3. Problem is that your signal is small so diode drop will be tough to overcome.
Software: If you connect the signal to the LJTIA with Gain=x1 and Offset=0.4V, you will get a signal to the U3 of about +0.15 to +0.65 volts. You need to acquire 1 or more cycles of waveform data, and then do the desired math. I suggest you simply find the max of the dataset and subtract the offset of your LJTIA channel (roughly 0.4V), then multiply by roughly 20 to get your desired value of 0-5.
Note that if the frequency of your signal is at least 6 Hz and less than say 5000 Hz, the T7 could acquire the waveform and do useful math for you with its rms AIN-EF ability:
https://labjack.com/support/datasheets/t7/ain/extended-features/rms
... or on the T7 you could write a little Lua script that simply finds the peak value over some time which you could make work with any frequency less than perhaps 2000 Hz.