My first thought was that these were rather high frequencies for seismic signals, but then I am not experienced in that area.a seismic acquisition system to grab signals in the range of 1kHz to 20kHz.
Yes, in a busy wired-up building there are usually mains frequency magnetic fields everywhereI've built an amplifier around the LT1115 to amplify the signal from accelerometers ...
My last major challenge is a 60Hz signal, which I'm assuming is induction from wall line from the other (commercial) amplifiers I'm using or something along those lines.
My questions are: does it make sense that I could be amplifying a 60Hz signal even though I believe I do not have any 60Hz signal electrically(physically?) connected to my system.
Lots of gain there but how is it disposed?The LT1115 circuit supposedly amplifies ~100 times (in reality it's more like 50) and an audio amplifier adds another 10 fold gain and by that point I'm partially blasting out of +/-10v. I've noticed when I have an oscilloscope hooked up to nothing in my lab I see a 60Hz signal in the range of 25mV. Nearly all of my wire for my amplifiers/accelerometers is shielded and twisted.
Accelerometer -----------long wires----------- LT1115--audio amp--display
It would be much better to build it like this:
Accelerometer--LT1115-----------long wires-------------audio amp--display
And are there better resistor capacitor pairs than others for just doing a standard high pass filter? I tried a 1uF cap and a 100ohm resistor in a high pass formation and it seems to get rid of 60 Hz noise, though I am a bit nervous about the other signal it might be killing off as the 1/2(pi)RC for the pair is 1.5kHz as well as some inconsistent phase shift it might produce if I use the same filter with components that have varying tolerances on 8 separate channels. Is there a better way to do a high pass filter where you don't have to worry about a phase shift?
It could be that the RC filter is loading the output of the accelerometer and attenuating the required signal.Last one, it seemed like when I used my high pass filter between the accelerometers at the amplifier, I got nothing (60Hz or 10kHz). I didn't test that in depth with other amplifiers, so there could have been something else wrong, but does that make sense at all?
Without actually looking at your setup it is difficult to advise further.
However, I have had a look at you LT1115 circuit and re drawn it a bit. Some of it may seem a bit pedantic but in its original form it was a bit obscure, going on bizarre in certain areas, notably the power supply. I am at a loss to understand what that actually represented!
I have tried to show how I would connect things up as a system to minimise noise.
JimB
if you have a high gain amp stage it will pick up the mains too, try putting your finger on a scope probe: you will see an almost perfect sine wave of exactly 50/60 Hz depending on which side of the continent you live on
I've noticed a rather ugly 60 hz sin wave when I put my finger on a side of the scope probe and even if I just put my hand around a probe (capacitance??). Does that serve as good evidence that I could easily be picking up some noise from the mains?
If you simply connect a capacitor to a resistor and another capacitor to another resistor then the sections affect each other and cause a very droopy response. Instead you should make a Sallen and Key Butterworth highpass filter with 2 or 3 sections. Look in Google. It uses one opamp which you already have.
My record player and tape deck (remember them?) use preamps with very high gain especially at frequencies down to 20Hz and are unbalanced like yours but have no hum. The cable is inexpensive and small shielded audio cable.
I think you mentioned early in the thread that creating a 2nd or 3rd order high pass filter where, as I understand it, have a filter then have a second filter filtering that filter, etc... should eliminate the high frequency noise and it seemed to work well. Do you think it would also have a strong response on higher frequencies (2 orders of magnitude higher)?
If you think it would just have a droopy response on the 60Hz that's fine. I was using values for 120Hz (if my math is right, 1kohm resistor and 1uF cap). If the 60 Hz still exists it's ok; it will be filtered out with computer processing, it just needs to be significantly diminished from what it is now because it is exceeding the maximum voltage of my amplifiers and effectively eliminating the signals I'm interested in.
It would be preferable to use high pass filters over building a complete circuit (granted it's an easy circuit to build and it's well documented). I think the circuit you recommend is only for 3dB as well? I probably need to knock it down quite a bit.
A highpass filter passes high frequencies and blocks low frequencies. It has no effect on high frequency noise.I think you mentioned early in the thread that creating a 2nd or 3rd order high pass filter where, as I understand it, have a filter then have a second filter filtering that filter, etc... should eliminate the high frequency noise and it seemed to work well. Do you think it would also have a strong response on higher frequencies (2 orders of magnitude higher)?
Three RC sections connected together have a loss of -29dB at the calculated cutoff frequency. It is droopy so if the calculated frequency is 480Hz then 60Hz will not be reduced much but 4.8kHz and even 9.6kHz will also be reduced a little. A Sallen and Key Butterworth 3 section highpass filter calculated at 480Hz has 480Hz with a loss of only -3db and 60Hz will be reduced -54dB which is 1/500th. Frequencies above 1kHz will not be reduced.If you think it would just have a droopy response on the 60Hz that's fine.
A 3-section Sallen and Key Butterworth highpass filter is -3db at the calculated cutoff frequency then sharply drops at -18dB/octave. It uses three resistors and capacitors with the opamp you have now.It would be preferable to use high pass filters over building a complete circuit (granted it's an easy circuit to build and it's well documented). I think the circuit you recommend is only for 3dB as well?
My record player and tape deck had half an inch of wires but had no hum. Maybe yours is connected wrong. Maybe hum comes from your power supply.Ah, and for the record, we are using twisted shielded audio wire, but a 5/6 inches of the wire is exposed and is not twisted. Do you think trying to bring it down to a couple inches would make the difference?
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