piclist 2008\07\27\171900a >
Thread: Low Noise, High Gain Amplifier
face picon face BY : Sean Breheny email (remove spam text)

Hi Fred,

Thanks for the explanation. Unfortunately, I don't have the time to
design the circuit you are asking for. It sounds like it is probably
doable for a determined mid-level newbie. I would suggest taking a
look at the article Dave mentioned and also looking at some of the
ultra low-noise op-amps available currently, like the LMV772 and the
AD797 (this has an internal noise level of 1nV per root Hz at
1kHz!!!). I would be tempted to make the input stage of my amplifier
differential (to help avoid 60Hz pickup when you run it over a cable
to the actual sensor), with a gain of about 10, and then feed the
output to a small chain of typical op-amps as a gain block. After a
gain of perhaps 1000, I would run it through either a switched
capacitor filter or state-variable filter block to narrow down the BW
to exactly the range you want. Finally, after a little more gain, I'd
probably digitize the signal and do a software PLL (or use a hardware
PLL) to pick out the signal frequency.


On Sun, Jul 27, 2008 at 1:08 AM, fred jones <@spam@boattow.....spamhotmail.com> wrote:
> Hi Sean,
> I appreciate you taking the time to help me.  I wanted to take this offline.  This is a proton precession magnetometer.  A sensor is made by winding a coil around a core that will contain a proton rich liquid.  For example, you might use distilled water.  The mag first applies the battery across the coil for a user selectable time, maybe 1 to 6 seconds.  Obviously there are tradeoffs for the amount of polarization time as well as the liquid used.  Water takes longer to polarize, utilizing more battery power, but the signal decay time is longer.  For kerosene or alcohol, the polarizing time is shorter using less battery but the signal decays faster so reading the pulses and figuring out the frequency has to be done much quicker before the signal gets too weak.  The protons are precessing at a frequency that is proportional to the strength of the earth's magnetic field.  They are precessing out of synch so the minute signal they generate cancel each other.  By polarizing the
>  iquid, the protons stop and line up.  When the battery voltage is discontinued, and the magnetic field from the coil is removed, the protons begin precessing at a frequency that is proportional to the magnetic field strength but they are now in synch and a signal of low level is available, at least for a short time until they will get out of synch and must be polarized again for the next reading.  I have fed a signal generator into an LM393 and converted the 2128hz sine wave into a square wave and fed that into a PIC.  It is able to use period measurement and calculate the equivalent magnetic field value of 50,000nT and display it on an LCD.  All I think I need is an amplifier and filter ciruits to feed the LM393 and turn it into a real mag.  I have built a sensor which by my calculations should generate somewhere around .7uV.  If you think this is going to be something I can handle, I appreciate the help.  If you think you could design the circuits, I would be willing to
>  ay you.  I hope this is more clear and again thanks.
> FJ> Date: Sat, 26 Jul 2008 23:57:03 -0400> From: shb7spam_OUTspam.....cornell.edu> To: spamBeGonepiclistspamBeGonespam@spam@mit.edu> Subject: Re: [EE] Low Noise, High Gain Amplifier> > Hi Fred,> > What kind of magnetometer are you considering? I'm not aware of any of> them which indicate field strength by changing the output frequency of> a sinusoid. Did I understand you correctly?> > Sean> > > On Sat, Jul 26, 2008 at 10:32 PM, fred jones <.....boattowspamspamhotmail.com> wrote:> >> >> >> >> > Thanks for the responses, I was out all day and just got back in town. Strange thing is I see I had at least 3 responses however this is the only one I actually received. Luckily it has several responses still attached. I wonder how many responses I didn't receive that also didn't make it here. I'm hoping to build a magnetometer. This would be used primarily in the woods or wilderness. Roughly one hertz to the next equals something like 25nT field strength (I have a formula that can be fairly easily implemented in the PIC code to calcualte
>  his). Therefore it is imperative that it be extremely stable and noise free. A slight amount of jitter would give innacurate data. I know this is possible because there are commercial units out there that yield .1nT accuracy, I'm just not experienced in this area. I will try the suggestions here and see how successful I am. I appreciate all the help you all have offered!Best wishes,FJ> From: spamBeGonepetertwitchEraseMEspamspam_OUThotmail.com> To: piclist> @!> > mit.edu> CC: EraseMEboattowspamBeGonespamKILLspamhotmail.com> Date: Sat, 26 Jul 2008 16:49:59 +0000> Subject: [EE] Low Noise, High Gain Amplifier> > What about a single hi-gain op-amp feeding a band-pass filter with a high Q inductance... Feed the output to a 555 timer arranged as a monostable, period set to half the period of the slowest frequency. That'll give you your sqw output thats pretty much frequency & phase-locked, & fairly noise-immune...> P> > {Original Message removed}
<e726f69f0807271418q282a6183w3b9ff3b8cb379d6e@mail.gmail.com> 7bit

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