Simpler PI...

[Carl-NC]

The low frequency amplifier noise will be filtered out by the demodulator stage (acting like a high-pass filter). For the pre-amp, only the noise density above the pulses per second rate (PPS) is relevant.
Aziz

Be careful, our demodulators are not like analog mixers which do frequency translation. Rather, they are more like sample-and-holds which fold all the spectra into the Nyquist band. The target response gets dropped right on top of 1/f noise.

[green]

The low frequency amplifier noise will be filtered out by the demodulator stage (acting like a high-pass filter). For the pre-amp, only the noise density above the pulses per second rate (PPS) is relevant.
Aziz

Thinking after my reply, The example wasn't random. I think I need to understand random noise better.

[Aziz]

[We know, that the low frequency and the 1/f noise contribution is much (20 Hz .. 1kHz). If we neglect this low frequency noise (we are doing the PI at PPS rates and won't really see this low frequency noise]

If I replace the amplifier with a signal generator, (10hz, 1mv) output and read the voltage with an A-D or a sampling integrator at 1khz sample rate what should I expect see?

The low frequency amplifier noise will be filtered out by the demodulator stage (acting like a high-pass filter). For the pre-amp, only the noise density above the pulses per second rate (PPS) is relevant.
Aziz

[Aziz]

You need to divide by 177, not 141, for a single-pole noise BW. This results in 3.2nV/rtHz.

Thanks Carl, the noise density figure becomes more attractive now.
Aziz

[Carl-NC]

the specified 0.56 µV noise energy is the integrated noise energy density for 20 Hz to 20 kHz.
Divide this by 141 and you get approx. the voltage noise density in nV.
en = 0.56 µV/sqrt(20000 Hz - 20 Hz) = 0.56 µV/141 = almost 4 nV/sqrt(Hz)

You need to divide by 177, not 141, for a single-pole noise BW. This results in 3.2nV/rtHz.

[green]

[We know, that the low frequency and the 1/f noise contribution is much (20 Hz .. 1kHz). If we neglect this low frequency noise (we are doing the PI at PPS rates and won't really see this low frequency noise]

If I replace the amplifier with a signal generator, (10hz, 1mv) output and read the voltage with an A-D or a sampling integrator at 1khz sample rate what should I expect see?

[Waikiki_Sweep]

Thanks! That is really simple explanation.

NJM2068 where to buy:

Tayda Electronics - $0.40
Mouser - $0.60
Digikey - only wholesale 2000 :-(
eBay - crazy shipping charges
Also more offers are here: http://octopart.com/parts/search?q=NJM2068

[Aziz]

In any official datasheet you can see only that statement:

Equivalent Input Noise Voltage for NJM2068 - 0.56 μV

May be it is equal 3 nV/sqrt(Hz) may be not.
Too much work for me to check it anyway.

If you have your calculator ready:

http://www.nikhef.nl/~jds/vlsi/noise/Amplifiers.PDF

Hi Waikiki_Sweep,

the specified 0.56 µV noise energy is the integrated noise energy density for 20 Hz to 20 kHz.
Divide this by 141 and you get approx. the voltage noise density in nV.
en = 0.56 µV/sqrt(20000 Hz - 20 Hz) = 0.56 µV/141 = almost 4 nV/sqrt(Hz)

We know, that the low frequency and the 1/f noise contribution is much (20 Hz .. 1kHz). If we neglect this low frequency noise (we are doing the PI at PPS rates and won't really see this low frequency noise), then we are at approx. 3 nV/sqrt(Hz) region for f > 1kHz.
The typical measured noise is even better than the maximum specifed noise voltage in the data sheet.

I know that the NJM2068 is an excellent device. I have tested and seen its noise spectrum compared against several op-amps (NE5532 for instance). It's really a bargain.

Cheers,
Aziz

[Waikiki_Sweep]

The NJM2068 is around 3 nV/sqrt(Hz) input referred voltage noise density.

In any official datasheet you can see only that statement:

Equivalent Input Noise Voltage for NJM2068 - 0.56 μV

May be it is equal 3 nV/sqrt(Hz) may be not.
Too much work for me to check it anyway.

If you have your calculator ready:

http://www.nikhef.nl/~jds/vlsi/noise/Amplifiers.PDF

[Beenthereuk]

Oh wow!, the fishing must be excellent there too. You're too lucky

[Waikiki_Sweep]

You're waddling in the Black Sea this time of year ?!

No. I am not that crazy. Last eight years I live in Honolulu.
And weather here looks much better especially for waddling at winter. Google "Waikiki".
But also a competitive place - one detectorist on ten tourists probably.

[Beenthereuk]

You're waddling in the Black Sea this time of year ?!

[Waikiki_Sweep]

Yep. Video is slow. But it detects! Very good for first try!
Probably some improvements will make it detecting a coin from same distance.
Also it is good to build a pinpointer.

Also move your targets side by side not up to down because it imitates sweeping coil over ground and keep targets in horizontal position because it gives strong signal. Detecting coin or ring that stays on edge reducing range 2-3 times with any detector.

I have P-MOS with very low threshold J176 so I will try. Not tried it yet because weather getting better and I am sitting in water with my SurfPI all the day.

Audio volume is not much important for using headphones.
Headphones are better than speaker because often if you go outdoor with your detector noise is too high to use speaker anyway.

[Beenthereuk]

http://videobam.com/jITBK

Waikiki, my circuit uses an NMOS FET with a VGSth = 2V. It needs a threshold that is 1V or more above ground. BUT, the incoming pulse is positive in mine and hence it is inverted and is negative when it hits the transistor's gate - if your pulse is in the opposite direction, it will not pass through the threshold and trigger the nFET. Either way, it is so simple that you should be able get it to work by using a PMOS FET at the output if you have to.

This is a really crap video I made of mine working. Detects small bars of Al and Cu at upto 12 cm or so. Adding another tranny for a speaker driver will seriously boost the sound..

[Aziz]

NJM2068

Hi all,

have a look at the Japanese op-amp comparison table:

http://www.oocities.org/de/c_schirp/thenoiselevelcomparison.pdf

The NJM2068 is around 3 nV/sqrt(Hz) input referred voltage noise density.
You can forget your NE5532/NE5534!

I also have found the SPICE models!
See here: http://semicon.njr.co.jp/eng/macro/index.html

Cheers,
Aziz