PI noise

[green]

You are powered off battery and have a bypass capacitor across the power pins of each opamp with very short leads ( preferably +VE is fed with 10 ohm or so resistor ?

The circuits are battery powered. I have at least a .1u and a 1u on each preamp power pin to common. Hadn't tried the series resistors so I added them this morning, no difference.

[6666]

This is interesting, is a 1C integrator also known as a single ended integrator ?
and how does it cancel EF ?

[Mechanic]

HI Eric,


I have tried several lme9990 and found that they all take quite a bit longer to settle(think it was up to 5 or 7us) than the AD797. Though a mate of mine was able to use the lme49990 and get it to settle earlier than I could with the same input impedance and gain, go figure! Though he was using a different coil...
When I did noise tests, I could not see any notable difference between the 797 and the lme. Same goes for using an lm394/5534 preamp. This settles as fast as the 797 and there seems to be no notable output noise difference. Though one thing I am still to try, from one of my initial tests, it looks as though the 797 is not as sensitive as the lm394/5534 preamp, however some other things were set up differently. Currently changing back those differences and will test on the weekend. I will know more after the weekend..

Cheers Mick

[Ferric Toes]

yup .. interesting about the 1C integrator .. I would have thought you could compensated for 'unbalance' by varying the sample duty cycles +/- x nanoseconds. The LME49990 is nice but apparently going end of life / production.

You can, but I don't think that is possible with my current pulse generator. I need to vary the sample pulse widths equally for other reasons. Is there a 'better' replacement for the LME49990 in view of it's going out of production?

Eric.

[moodz]

yup .. interesting about the 1C integrator .. I would have thought you could compensated for 'unbalance' by varying the sample duty cycles +/- x nanoseconds. The LME49990 is nice but apparently going end of life / production.

[Ferric Toes]

I started with a 2C integrator, #1 of the thread. The 2C integrator doesn't cancel EF as good as the 1C integrator for me so I have been using the 1C integrator. Had the same problem with the 2C integrator, noise higher than it is with the integrator bypassed. I'm not expecting the amplifiers to lower the noise, want to know why the integrator with a gain of one increases the noise.

I have found that a 2C integrator does not give as good an EF cancellation as a 1C integrator even if the capacitors are accurately matched and 1% resistors used on both sides. I also put a balancing 200R trimmer ahead of the gates to match the ON resistance. The best EF cancellation is to have a cross coupled differential output stage with a gain of 1x after the preamp which then feeds the two gates and then into a 1C integrator. The 4066 need to be upgraded to a device with better matching between gates otherwise the small differences in ON resistance will limit the cancellation. If you want to keep it, use a balancing trimmer again.
To make balancing easier, I inject a sine wave of about 4Hz into the preamp input with a summing resistor (220K in my case) and scope the integrator output. Adjust trimmer for zero movement of the base line. I use an analog scope 10mV/cm and dc coupled.

Use a NE5534A for the preamp and its dual, the NE5532, for the differential stage. The 'A' version of the 5534 has lower lf noise. Even better for the preamp is the LME49990.

Eric.

[moodz]

I started with a 2C integrator, #1 of the thread. The 2C integrator doesn't cancel EF as good as the 1C integrator for me so I have been using the 1C integrator. Had the same problem with the 2C integrator, noise higher than it is with the integrator bypassed. I'm not expecting the amplifiers to lower the noise, want to know why the integrator with a gain of one increases the noise.

You are powered off battery and have a bypass capacitor across the power pins of each opamp with very short leads ( preferably +VE is fed with 10 ohm or so resistor ?

[green]

...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.

[ATTACH]36572[/ATTACH]

I started with a 2C integrator, #1 of the thread. The 2C integrator doesn't cancel EF as good as the 1C integrator for me so I have been using the 1C integrator. Had the same problem with the 2C integrator, noise higher than it is with the integrator bypassed. I'm not expecting the amplifiers to lower the noise, want to know why the integrator with a gain of one increases the noise.

[green]

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?

You are looking at PI_noise_4. I added pictures to show schematic, when I did scope shots PI_noise_4 reply#9 I made an error on the second picture which I corrected in reply#10. Look at first two pictures in PI_noise_6, the first picture bypasses the sampling integrator with a gain of one. The second includes the sampling integrator with a gain of one, noise about four times higher. Why does sampling cause higher noise? Some of the pictures in PI_noise_4 reply#9 were included to answer reply#8

[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.

[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?

[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.

[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]

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]

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.