PI noise

[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]

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)

[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

[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.☺

[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.

[green]

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

[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.

[Orbit]

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

[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.

[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)

Tried the paint can. Used a78L05 and a TLE2426 to make a +- 2.5 volt supply from a 9 volt battery so I could put power supply and amplifier in the can. Tried four different amplifiers with input shorted. Different p-p reading for each but couldn't see a difference in or out of the can for either of them. Back to trying to understand where the noise is coming from with the coil connected.

[green]

Wanted to see if lowering amplifier cut off frequency changed the noise. The diff amp is potted so I made a simple two stage amp with a LM4562. Recorded with the input shorted, 1k across the input and the TLC555 output across the input with two different cut off frequency's. Thought I would see a noise change but don't see much if any. Should I see a difference? PI_ noise_3(LM4562), PI_ noise_2(diff amp)reply#6.____ Sample rate 1kHz, target sample time 10usec, no EF sample, Coil command off.

Increasing the preamp fdbk TC from .2 to.6usec didn't lower post amplifier output noise signal with the preamp input shorted(PI_noise_3,first upper and lower picture). Tried some higher fdbk TC the other day. Wasn't thinking the fdbk TC would have to be increased as much to lower the noise. Had to reduce amplifier cutoff to 800Hz to get the same as the integrator bypassed with integrator gain=1 . Sample rate=1kHz, target sample=10usec, no EF sample. Still wondering if it's aliasing or something else causing the problem.

[green]

Tried the paint can. Used a78L05 and a TLE2426 to make a +- 2.5 volt supply from a 9 volt battery so I could put power supply and amplifier in the can. Tried four different amplifiers with input shorted. Different p-p reading for each but couldn't see a difference in or out of the can for either of them. Back to trying to understand where the noise is coming from with the coil connected.

Not saying shielding isn't necessary. I had a job recording strain on a large fork truck while operating at customer sites. We had a major noise problem at one site, stacking containers after being unloaded from the ship. The strain gage bridge was connected to an instrumentation amplifier(gain=100), could have been an AD620 or a Burr Brown followed by an adjustable gain post amplifier. We connected a speaker to post amp out. Just like listening to the station on a radio. The fork driver said there was a radio station near by. Wrapped the long cables going to the strain gages with some aluminum foil connected to ground, problem solved. I live in the country so maybe external noise other than what I generate isn't a big problem or I'm missing something.

[green]

[Forum activity can be sporadic, so don't be disappointed if your questions get few or no answers. Just try again. Remember, everyone here is a volunteer. from Thread : Basic Rules of the Forums]

Trying again. Some more scope pictures. This time two stage preamp A3, first stage(input, one side to common, other side to inverted input thru a 1k resistor, gain=20) second stage(gain=15). Coil on command disabled, 1kHz sample rate, 10usec target sample, no EF sample, input shorted. Capacitor across second stage fdbk resistor changed to adjust fdbk TC. Observations: first picture(integrator bypassed, two 16Hz low pass post amp stages, trace E is 1/f noise) second picture(integrator included gain=1, noise level about 4 times higher) third picture(fdbk TC increased to 1.5usec, same as second picture) forth picture(fdbk TC increased to 15usec, noise level about 1/2) fifth picture(fdbk TC increase to 150usec, noise same as first picture, 1/f noise). I've seen comments about 1/f noise being a problem and I have been concerned with it but the noise from sampling higher frequency noise looks to be higher. I'm thinking sampling the higher frequency noise is aliasing and causing higher amplitude noise at less than 16Hz. I wonder if sampling with an A-D with a 1usec sample might be worse. Are my observations correct? Am I missing something? I've tried three different amplifiers, different noise level but sampled noise is about four times higher than integrator bypassed noise. Any suggestions on how to reduce the sampled noise? Picture, PI_noise_4 included to show connection diagram.

[Qiaozhi]

[Forum activity can be sporadic, so don't be disappointed if your questions get few or no answers. Just try again. Remember, everyone here is a volunteer. from Thread : Basic Rules of the Forums]

Trying again. Some more scope pictures. This time two stage preamp A3, first stage(input, one side to common, other side to inverted input thru a 1k resistor, gain=20) second stage(gain=15). Coil on command disabled, 1kHz sample rate, 10usec target sample, no EF sample, input shorted. Capacitor across second stage fdbk resistor changed to adjust fdbk TC. Observations: first picture(integrator bypassed, two 16Hz low pass post amp stages, trace E is 1/f noise) second picture(integrator included gain=1, noise level about 4 times higher) third picture(fdbk TC increased to 1.5usec, same as second picture) forth picture(fdbk TC increased to 15usec, noise level about 1/2) fifth picture(fdbk TC increase to 150usec, noise same as first picture, 1/f noise). I've seen comments about 1/f noise being a problem and I have been concerned with it but the noise from sampling higher frequency noise looks to be higher. I'm thinking sampling the higher frequency noise is aliasing and causing higher amplitude noise at less than 16Hz. I wonder if sampling with an A-D with a 1usec sample might be worse. Are my observations correct? Am I missing something? I've tried three different amplifiers, different noise level but sampled noise is about four times higher than integrator bypassed noise. Any suggestions on how to reduce the sampled noise? Picture, PI_noise_4 included to show connection diagram.

I cannot help thinking that you're chasing a red herring here.
If you look at the first scope image (the noisy one), and then look at the third one (where the first stage of the preamp is removed), it looks like there may be a 20x difference in the noise level.
Can you do another scope measurement for the first (noisy) waveform, but with the scope set to 100mV/div?

[moodz]

...hmm you seem to be just connecting a chain of amplifiers and expecting the noise get lower ... well with any amplifier the noise just gets higher LOL. :-) An acceptable practice in the PI signal chain is to use a differential integrator ( see below ) .. maybe you could start with what works .. then make it better. Just a suggestion.