PI noise

[6666]

Put as much as possible in the box and shield it
you may have to buy a family size pizza, and just use alfoil that you use in the kitchen to cover it, its cheap.

You could also do some experiments for your own interest and progressively shield a section at a time, which would be very easy to do, just wrap a part of the project with plastic kitchen wrap so no shorts and then alfoil , srunch the foil to make a cocoon and ground the foil.

[Orbit]

It is still a big job before thee ! All the best !

[6666]

Shield just the preamp or which parts to include in the shield?

Put as much as possible in the box and shield it
you may have to buy a family size pizza, and just use alfoil that you use in the kitchen to cover it, its cheap.

[green]

Shield just the preamp or which parts to include in the shield?

[6666]

A quick and dirty shield for testing
Attach ground wire to project
put project in pizza box (cardboard) extend ground wire and probes etc, some pizza boxs already have holes in sides
close box, cover box in aluminium foil, srunch foil making sure all foil is electrically touching, connect ground wire to foil and test
eat pizza.

[moodz]

...paint can ?? ... not very ergonomic. If you must use household items then use a biscuit tin or cake tin that has a lid. Square biscuit tin is best ...solder wire from lid to box and to ground ... cut or punch holes for wires to come out.line the tin with plastic so shorts wont happen. Winner winner chicken dinner.☺

[Chet]

Been reading on measuring amplifier noise. One option using a scope. Measure peak-peak noise/6/gain/Fc^.5. Another suggestion was to put the circuits in an empty metal paint can. Including some measurements, does anyone see something that I should do different before I try the paint can? Inputs jumpered. Changed C to vary the cutoff frequency for A2(PI_noise_4)

Hi Green


One time at work I lined the inside of a wooden box with screen wire. The inside of the hinged lid was covered also. I cut a few notches below the lid for the test leads. I grounded it to the scope cabinet ground. It worked great.


Have a good day,
Chet

[green]

Been reading on measuring amplifier noise. One option using a scope. Measure peak-peak noise/6/gain/Fc^.5. Another suggestion was to put the circuits in an empty metal paint can. Including some measurements, does anyone see something that I should do different before I try the paint can? Inputs jumpered. Changed C to vary the cutoff frequency for A2(PI_noise_4)

[moodz]

... yup ... just like the USS Enterprise ... shields up !



I have reused this shielding maybe over 100 times. Just use very short tinned wire to fix "lid" on. Soldered the connectors to the PCB also. Cut the PCB using a $15 paper guillotine.

[green]

Thanks for the replies. I have a magnifying lamp with a cfl bulb and a cheap lamp dimmer to control power for the soldering iron. Both need to be turned off when a coil is connected to the preamp. See no difference in the signal at post amplifier out (E) if they are on or off if the preamp input is shorted. I was assuming I was looking at amplifier noise with the input shorted. Maybe not.

[moodz]

...hmm yes ... I would have to agree with Eric. I would recommend a good ground plane in addition to any shielding .. there practically no other option for low noise amplifier work.

[Ferric Toes]

The circuit was put together as a learning platform. Didn't have SMD adapters for the control circuit and coil driver. They have made it a lot easier to wire the circuit.

From looking at the picture, your setup is much too open and you are bound to pick up noise from external sources. Here in UK, it is surprising how much radio noise there is a low frequencies, even a few broadcast stations still exist between 200kHz and 170kHz. And then there is the radio clock control signals at 100kHz. All of this can create havoc with noise measurements, plus there is a huge amount of impulse noise.

At the very least put the preamp in a screened box with coax for the input from coil driver and/or coil. Also use a small figure 8 coil, again shielded if possible in a plastic project box that has been sprayed with an emc graphite coating. Any long leads from power supplies should have rf filters where they enter screened boxes housing electronics. All long leads act as antennas.

I have found that computers need to be off and also the power supply for my temperature controlled soldering iron. Wall plug switch mode psu's also often radiate noise.

Without taking these precautions you will be forever chasing your tail.

Eric.

[green]

..would you be able to post a picture of the physical circuit ( as built ) ? Thanks.

The circuit was put together as a learning platform. Didn't have SMD adapters for the control circuit and coil driver. They have made it a lot easier to wire the circuit.

[moodz]

..would you be able to post a picture of the physical circuit ( as built ) ? Thanks.

[green]

Hi Mick, Thanks for the reply. Some more scope pictures. Sampling seems to be the problem. I'm thinking sampling with an A to D would be the same. Maybe I'm missing something or doing something wrong. The AD797 specs look real good. Need to try a SFT1440 isolation switch suggested by mickstv to get the input resistance low. Every thing I've tried hasn't worked. The reason I'm using a diff amp with an IB coil. I still think aliasing is causing the problem but hope it's not and there is a way to lower the noise I get from sampling.

The second scope picture PI_noise_4, integrator bypassed doesn't look right. I connected amplifier out to the integrator 3k input resistor causing it to saturate the direct coupled portion, no 1/f signal. Including a trace connected right.