Preamp

[Chet]

Hi Green


This is very interesting. I am trying to get a better understanding of the raw frontend noise and signal levels that would be seen by direct measurement with and ADC.
In your measurements and charting do you have any charts that would show an average noise level figure that we may expect to see? And what target signal levels might be expected to be discernable or considered detected above that noise?


In processing would long time constant targets fall into a low Hz band width and short time constant targets fall into a higher band width range?
I know that you probably encountered considerable noise from in house EMI but it would still help to understand some of the noise and target levels from real measurements.
I am really interested in what noise and detectable target levels would be seen by an induction balanced (IB) receiver coil.


Have a great day,
Chet

[green]

Hi Green


This is very interesting. I am trying to get a better understanding of the raw frontend noise and signal levels that would be seen by direct measurement with and ADC.
In your measurements and charting do you have any charts that would show an average noise level figure that we may expect to see? And what target signal levels might be expected to be discernable or considered detected above that noise?


In processing would long time constant targets fall into a low Hz band width and short time constant targets fall into a higher band width range?
I know that you probably encountered considerable noise from in house EMI but it would still help to understand some of the noise and target levels from real measurements.
I am really interested in what noise and detectable target levels would be seen by an induction balanced (IB) receiver coil.


Have a great day,
Chet

Hi Chet

Including a scope shot from another thread and a new one I made awhile back. Test_2, preamp connected to post amp(no integrator) OPA1612 1k resistor connected to +input or shorted, -input less than 200 ohms to common. Both tests the circuit was running with the Tx coil not connected. They show what I'm thinking. I think the diff amp is noisier than the opa1612 with 1k to ground on each input because of higher bias current. Integrator sampling increased noise. Working on a new circuit to try. Thinking of labeling the threshold pot in uv at the coil(should show noise level referenced to coil volts). Was surprised the coil noise wasn't higher, some high frequency noise but didn't increase low frequency amplitude, maybe it would with the integrator connected. The large spikes happen when the fet is turned on, need better PS isolation? IB coil, two 8 inch Rx coils side by side(one inverted for opposite phase) surrounded by a race track Tx coil.

Don't think target time constant effects target band width, time for coil to pass target. Interested in how Teleno's amp is working with a mono coil.

[Old cart]

[If you for example want to acquire all of the data out of a preamp that has +/-5 volt supplies and a measured noise level of say 500uV then you would need 10V/500uV or a 20000 to 1 ratio.] I'm still missing why you give a noise level without referencing it to the input or include a gain. A preamp with a gain of 10 or 1000 would make a difference finding the nickel at 15 inches with a 15 bit a to d converter. The noise after filtering should to be referenced to a frequency band that includes the target signal. Analog or digital filtering. Some low noise op amps include a 10 second .1 to 10 Hz scope trace in there spec sheet. I'm thinking the target signal is less than 10 Hz so something similar would make sense, maybe not.

It was just an example. While what you say is true this example includes how you would select an a to d after measuring what the actual levels would be. A band limited measurement from a spec sheet can be helpful. If you measure at the point the a to d is placed you will be taking into account all the factors including gain, noise level at the conversion point, emi, crosstalk, everything. Some of those are not easily modeled, at least by me. However if you have not built the circuit then you wil have to simulate what the signals wil be and this is, I think, quite complicated. Let me know if this still does not make sense.

[baum7154]

When measuring noise it is a good idea to amplify the test amp with an external low noise amp. Oscilloscopes are not designed to make measurements on small signals at frequencies this low. Scope external preamps are available, but are rather expensive. For example the Tektronix ADA400 but it only works with modern TEK scopes.
Such a preamp should have very low noise and adjustable gain and bandwidth. Adjustable offset is also nice to have. This group could of course design such a preamp or we could just use the design of the ADA400 as a basis. The schematic is available.
A simpler, cheaper design is available here

http://tangentsoft.net/elec/lnmp/


This design is well documented and even includes gerbers so no real design work would be necessary. It does have one issue though and that is that the input impedance is quite low at 100ohms. To fix this would require a high impedance low noise buffer to be added before the first op amp.
It has several component options explained and should work well with scopes and true RMS multimeters.

Lastly last night I read a post by Eric Foster who said the coil, if used in such a noise test must be horizontal as most noise is polarized. Placing the coil vertical will allow more external noise to enter. If we use a pendulum with a long line, like 2 M, the arc that the target makes wil be very slight and should not affect the results.

----------------------------

I agree with your last statement regarding the coil being Horizontal. All of my air testing the past year or two was done in the basement on my pool table with the coil horizontal, placed on the end of a roll of paper towels. I have found this orientation gives the least interference I can manage and it results in very quiet performance of the detector. It also allows a plastic ruler to be stood near the center of the coil for distance verification.

Dan

[Teleno]

I'm still missing why you give a noise level without referencing it to the input or include a gain.

It's very simple, you asked about the resolution of and ADC that samples the output, therefore it's the noise at output what you must look at. The gain is inmaterial reagarding the resolution of the ADC.

[green]

It was just an example. While what you say is true this example includes how you would select an a to d after measuring what the actual levels would be. A band limited measurement from a spec sheet can be helpful. If you measure at the point the a to d is placed you will be taking into account all the factors including gain, noise level at the conversion point, emi, crosstalk, everything. Some of those are not easily modeled, at least by me. However if you have not built the circuit then you wil have to simulate what the signals wil be and this is, I think, quite complicated. Let me know if this still does not make sense.

I'm just trying to learn. Including a scope picture from reply#52 and a chart from reply #62. I think they define target shape and amplitude for sweeping a 300uH 8 inch diameter coil with 1 amp peak current over a US nickel. About 1 uv change at the coil with the target at 15 inches from the coil. I know the peak noise referenced to the coil has to be less than 1 uv(how much less?) You mentioned filtering reduced noise and reduced required bits. I still think the target defines resolution and noise level. It doesn't matter how many bits I have if the peak noise level referenced to the coil is 5 uv. If the noise level is zero I still need enough bits to define a 1 uv change with the defined target recording shape. Bits hardware + averaging?

[Chet]

Hi Green


The charts are a great help. And your chart of the coil sweeping over a nickel explains the 10 Hz response. Your two receiver coil arrangement appears to do a good job of eliminating common mode EMI.


Does the frequency of the target eddy current slope inside of a 10 usec target sample gate need to be considered in later processing?


Thank you,
Chet

[6666]

I'm just trying to learn. Including a scope picture from reply#52 and a chart from reply #62. I think they define target shape and amplitude for sweeping a 300uH 8 inch diameter coil with 1 amp peak current over a US nickel. About 1 uv change at the coil with the target at 15 inches from the coil. I know the peak noise referenced to the coil has to be less than 1 uv(how much less?) You mentioned filtering reduced noise and reduced required bits. I still think the target defines resolution and noise level. It doesn't matter how many bits I have if the peak noise level referenced to the coil is 5 uv. If the noise level is zero I still need enough bits to define a 1 uv change with the defined target recording shape. Bits hardware + averaging?

Thanks for the graphs.

[Old cart]

I'm just trying to learn. Including a scope picture from reply#52 and a chart from reply #62. I think they define target shape and amplitude for sweeping a 300uH 8 inch diameter coil with 1 amp peak current over a US nickel. About 1 uv change at the coil with the target at 15 inches from the coil. I know the peak noise referenced to the coil has to be less than 1 uv(how much less?) You mentioned filtering reduced noise and reduced required bits. I still think the target defines resolution and noise level. It doesn't matter how many bits I have if the peak noise level referenced to the coil is 5 uv. If the noise level is zero I still need enough bits to define a 1 uv change with the defined target recording shape. Bits hardware + averaging?

I will answer this backward. If there is zero noise then yes you would need a resolution on 1 uV or better to be able to see the small signal. Interestingly you are better off having some noise, particularly if you do signal averaging. This is because the noise sums with the signal and raises it above the minimum detectable limit. For example if you had a 1 uv with 1 uV p-p noise the combined signal would be varying from 0 to 2uV which would ensure an a to d with 2uv resolution would detect the signal, even if only occasionally. If you averaged the signal 2 times and the noise is not synchronized with the signal the noise would "melt away" and you would left with a signal of 1 uV and noise of about 0.6 uv. So by using averaging you can see a signal that is equal to and even below the noise. But it is the noise that allows you to digitize the signal in the first place. Classic averaging can only be used on signals that repeat. It would not be very useful on a low frequency signal like a coil passing over a target. Why? Becuase to average 8 times would require 8 identical passes over the target.
However there is a form of averaging called boxcar integration. Google it. It does not require a repetitive signal but does require closely spaced samples in time. It could be used to good effect in a metal detector to pull the signal from the noise. It not only increases the effective resolution, it has a secondary benefit of acting like a low pass filter.
How do you measure and define noise? You can measure it anywhere you want, it is arbitrary. As long as you define where and under what conditions you are good to go. I mentioned the signal and noise levels at the input to the a-d because that is all the converter sees. It neither knows, or cars, what is going on at the coil. Further if there are signal processing stages like gain, filtering, they will modify how much noise and signal is present. Gain stages general do not affect the signal to noise ratio, as long as they are quiet. Filtering can easily improve s/n as long as the signal and noise frequencies are different.

[Qiaozhi]

Sean - I have registered a request to join the Sonsivri forum, but it requires someone to actually invite me to join for free. Otherwise it costs $20.
Please can you go to the Invitation Request section of their site and click on "Invite this person"?
Thanks.

I would need the invite for Sonsivri too. Here's my request: http://www.sonsivri.to/forum/index.p...=61849.new#new

Alas it is one forum where the rules are VERY harsh. I havn't been on there for over a year so they booted me even thought I was a paying member.

BE CAREFUL, the admin there are VERY quick to punish for infgringements. DO direct downloads, NO files on the forum, links ONLY. They have all sorts on there, I've even seen Mentor somewhere!

I will try to get back on there, I was Sidley65.

Have just re-applied.

Hmmm ..... Sean is absolutely correct about Sonsivri.
In an attempt to get my post numbers to a level where I could invite both Teleno and Sean (30 posts), I replied to a post there yesterday in the Win10 GUI thread (which is in the General section) where others were comparing Linux to Windows 10. There were a few Linux supporters there, so I added some positive comments. Then today I got a warning about being involved in a pissing contest! And now I'm muted, so cannot even lodge a complaint.
As far as I can see from having read several of the threads there, the site is really only a conduit for illegally exchanging pirated material. The discussions lack any serious technical content, so I wouldn't cry too much if you never get access.

Hahah!
Sean - I also looked at all posts made by pickit2 (the Admin who muted me) and I found this ->
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Quote from: Sidley65 on January 27, 2016, 09:16:44 21:16

Country	UK
Note	Electronics Engineer specialising in, well everything really. Was previously a VIP member but it lapsed (sorry I was involved in other things).

You was not a VIP member: we look after VIP members:

don't forget we have a long memory too.. & system logs too.
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They've got some serious issues on that site. Probably best to avoid.

[green]

Hi Green


The charts are a great help. And your chart of the coil sweeping over a nickel explains the 10 Hz response. Your two receiver coil arrangement appears to do a good job of eliminating common mode EMI.


Does the frequency of the target eddy current slope inside of a 10 usec target sample gate need to be considered in later processing?


Thank you,
Chet

I think the answer to your question is no. I'm trying to understand what is happening. I've been using a sampling integrator. If I add the GEB sample the no target noise increases. Depending on GEB delay, the GEB sample is 8 to 15 times longer than the target sample which makes integrator gain 8 to 15 times higher during the GEB sample. The noise doesn't increase that much. Looking at the above charts sampling is causing some or a lot of the noise. Is it because noise isn't repeating in the short samples, sample time jitter or something else? Don't know yet.

[Qiaozhi]

I think the answer to your question is no. I'm trying to understand what is happening. I've been using a sampling integrator. If I add the GEB sample the no target noise increases. Depending on GEB delay, the GEB sample is 8 to 15 times longer than the target sample which makes integrator gain 8 to 15 times higher during the GEB sample. The noise doesn't increase that much. Looking at the above charts sampling is causing some or a lot of the noise. Is it because noise isn't repeating in the short samples, sample time jitter or something else? Don't know yet.

Could it be because your GB sample has a higher gain than the main sample to compensate for the loss of target signal?
Therefore higher gain equates to more noise. (Using a longer sample pulse is the same as increasing the gain.)

[Sean_Goddard]

Pickit2 just has proven himself. I WAS a member, my posts are on there regarding Proton ICD and Proteus, I can see and prove it. AND I was a VIP I've got the receipts for the payments, in fact I paid for their hosting one month, ALL of it!

Yes, it is mainly a site for hacked software, but the issue is, there are many like myself who would use Mentor for small projects and would love to be able ot teach oursleves so that we can better our job prospects but not all of us have got £40K to spend.

I have a philosophy, If I'm doing it for my hobby, then I'll happily used hacked software, if I'm going to make a project commercial then I will pay for the full version, it's the least the developer deserves. I bought Tsien Boardmaker, cost me £600 then they moved to windows and wanted £1K per year so I dropped them. I now use DIPTRACE and am going to buy the Pro version when I have the cash.

[Old cart]

Pickit2 just has proven himself. I WAS a member, my posts are on there regarding Proton ICD and Proteus, I can see and prove it. AND I was a VIP I've got the receipts for the payments, in fact I paid for their hosting one month, ALL of it!

Yes, it is mainly a site for hacked software, but the issue is, there are many like myself who would use Mentor for small projects and would love to be able ot teach oursleves so that we can better our job prospects but not all of us have got £40K to spend.

I have a philosophy, If I'm doing it for my hobby, then I'll happily used hacked software, if I'm going to make a project commercial then I will pay for the full version, it's the least the developer deserves. I bought Tsien Boardmaker, cost me £600 then they moved to windows and wanted £1K per year so I dropped them. I now use DIPTRACE and am going to buy the Pro version when I have the cash.

I do not claim to be expert PCB designer but DIPTRACE is a very nice package, has good usability and has a competent fully functional version with 500 pins free to any hobbiest. You just have to send them an email and they will send you a code. See their website for details.

[Teleno]

Going back to the preamp, I'm stuck with Eagle at the moment, other CADs won't install in my older Ubuntu and the one that did (ExpressPCB) sucks big time.

Before drawing a plausible schematic I'd like to brainstorm the Power source with you guys (and girls eventually though quite unlikley).

I favor going as low as possible, 5 x 1.2V NiMH = 6V. Eventually 2 x such packs in parallel.

Pros:

- Most MOSFETS have low Rds_on to work at this level. The peak current would be (6V - 0.8 (series diode)) / Rcoil. Up to 3 A with a 1.7 ohm coil.
- Power efficiency. W (power) = V * I. For the same I the power used is lower as the voltage goes lower.
- Weight efficiency. The same Ah (ampere - hour) weight less as the voltage goes lower.
- Lower source resistance. Less batteries in series = smaller series resistance.
- Easy attachment of a MCU control unit.

Cons:

- Longer on-period because the I at the coil rises more slowly. "I(t) = V / L * t"... A 300uH col would reach 3A after 170 us. or 1A after 58 us.
- Voltage converter (doubler) necessary to power the MOSFET gate.
- Voltage converter (negative doubler) to power most OpAmps (single-rail verions are rare, usually slower and more expensive).


What's your take on the power source for an MCU-driven PI?