Help designing a low-noise TIA

[John Larkin]

On Thu, 6 Jun 2019 07:13:36 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Thursday, June 6, 2019 at 9:38:14 AM UTC-4, John Larkin wrote:
On Thu, 06 Jun 2019 06:22:45 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Wed, 05 Jun 2019 15:56:17 -0700) it happened John Larkin
jjlarkin@highland_snip_technology.com> wrote in
jvhgfe9npk8bauedauu1582o2i8b6jeps0@4ax.com>:

On Wed, 05 Jun 2019 18:29:36 GMT, Jan Panteltje
pNaOnStPeAlMtje@yahoo.com> wrote:
It seems to me, if I was to tinker with that setup,
to make a 'ballistic' meter I would do something this

+9V +9V
| |
[ ] R1 o voltmeter +
|
|-------------------------o voltmeter -
| |
c |
coil + --- b NPN ===
e Q1 | C1
| |
/// ///

+9V
|
[ ] R2 5k6
|
coil - --------| +.7V
| |
| c
-- b NPN
e Q2
|
////

How it works:
A positive pulse on coil + results in base current in Q1
that is amplified and the collector current then discharges C1.
C1 will slowly recharge via R1,
The voltmeter indication is proportional to the current peak.
R1 sets the gain in a way, te hhigher teh moresensitve, 5k6 is a nice value.
Q2 creates a stable bias for Q1.

I think the expected signal is about 200 nV.


You are working with a very low impedance coil / source,
so can use a low impedance sensing system to actually get some current,
Using a high impedance sensor gives next to zero current.
Transistors are basically current amplifiers...
200E-9 * 900
= 0.000180

The physics creates voltage. Why would anyone want to measure current?

Is not that what a galvanometer does, measure current?
Beta can be as high as 900 for some trannies.

This will be a very difficult thing to measure.

Maybe in 1916 when the paper was published...

There are papers online of modern versions. It's still really hard.

I found this,
https://inis.iaea.org/collection/NCLCollectionStore/_Public/28/038/28038450.pdf?r=1&r=1

Which doesn't talk much about electronics.. but does talk about getting
leads to the rotating coil.

Right, that's the torsion-bar angular oscillator. That makes an
enormous amount of sense. Nothing spins. The leads don't wind up, no
slip rings, the signal is symmetric/AC, and you can signal-average for
days, essentially over millions of experiments.

I'm still confused how Matt is making connections.
It seems like the local B-field is a problem too...
If that's the case then doing measurements in a city /building can be
made more difficult by all the moving pieces of iron... Cars,
trucks, elevators, etc. Which means the local B-field changes
in magnitude and direction by 1-2%. Maybe things are better late at night.
(when all good data is taken in my experience. :^)

George H.

--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Jan Panteltje]

On a sunny day (Thu, 06 Jun 2019 07:55:37 -0700) it happened John Larkin
<jjlarkin@highlandtechnology.com> wrote in
<haaifel0altjh9qc85krgfhjdtfngok8tf@4ax.com>:

Collector current For an Rb of about 100 Ohm:
-> 900 * (200E-9 / 100) = 1.8e-06 is 1.8uA

1.8 uA in a 500 k collector resistor:
-> (900 * (200E-9 / 100)) * 500e3 = 0.9 Volt

Is not Linux wcalc cool

Fits nice in my storage scope ADC range using PIC 1.024V internal reference...
Or any multimeter of this age,

You can always use a darlington to get an other factor 900 ...
Zin does not go up the same ratio.


Using high impedances is often asking for interference trouble.



Math right?

You have computed a voltage gain of 4 million in a single transistor.

Yes trannies are good!

Try it.

Trannies are good!

My old favorite universal NPN BC109C
hFE max 900 (good species)
Zin 6kOhm (best species)

voltage gain:
((900 * (200E-9 / 6000)) * 500e3) / 200E-9 = 75000 for a 500 k collector resistor

But how far are we leakage limited for that value collector resistor?
The collector cut off current at room temperature is 1 nA
1e-9 * 500e3 = 0.0005 = 500 uV

Your 200 nV amplified by 75000 is:
-> 200e-9 * 75000 = 0.015 = 15 mV

So we have .5 mV offset

OK than we increase the collector resistor to 10 M, a factor 20.
Then we have 20 x 15 mV = 300 mV signal, usable.
The offset is now 20 * .5 mV = 10 mV no problem on a 9V supply.
The actual gain is 20 x 75000 = 1.5e6.


I am sure there are better trannies than that one.
And you can always go darlington.


But I give you modern opamps can do a bit better,
but so can adding an extra stage.

Max beta found 1000:
https://electronics.stackexchange.com/questions/270393/industry-values-for-beta-\u03b2-in-bjt-current-amplification


Anyways. just for the sake of math,,
I no longer have BC109-C or even any BC109 here.

We do not have a speed limitation I think, signals are slow.

Just dreaming....

 

[John Larkin]

On Thu, 06 Jun 2019 15:51:23 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Thu, 06 Jun 2019 07:55:37 -0700) it happened John Larkin
jjlarkin@highlandtechnology.com> wrote in
haaifel0altjh9qc85krgfhjdtfngok8tf@4ax.com>:

Collector current For an Rb of about 100 Ohm:
-> 900 * (200E-9 / 100) = 1.8e-06 is 1.8uA

1.8 uA in a 500 k collector resistor:
-> (900 * (200E-9 / 100)) * 500e3 = 0.9 Volt

Is not Linux wcalc cool

Fits nice in my storage scope ADC range using PIC 1.024V internal reference...
Or any multimeter of this age,

You can always use a darlington to get an other factor 900 ...
Zin does not go up the same ratio.


Using high impedances is often asking for interference trouble.



Math right?

You have computed a voltage gain of 4 million in a single transistor.

Yes trannies are good!

Try it.

Trannies are good!

My old favorite universal NPN BC109C
hFE max 900 (good species)
Zin 6kOhm (best species)

voltage gain:
((900 * (200E-9 / 6000)) * 500e3) / 200E-9 = 75000 for a 500 k collector resistor

But how far are we leakage limited for that value collector resistor?
The collector cut off current at room temperature is 1 nA
1e-9 * 500e3 = 0.0005 = 500 uV

Your 200 nV amplified by 75000 is:
-> 200e-9 * 75000 = 0.015 = 15 mV

So we have .5 mV offset

OK than we increase the collector resistor to 10 M, a factor 20.
Then we have 20 x 15 mV = 300 mV signal, usable.
The offset is now 20 * .5 mV = 10 mV no problem on a 9V supply.
The actual gain is 20 x 75000 = 1.5e6.

Sorry, Jan, but that's all silly.

Build one. Or even Spice one.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

Hi all,
Again, thanks for the great ideas. I will try and keep up with them all. The original Tolman-Stewart used this rotating coil and braking; they re-did their experiment about a decade later with a constantly rotating system and then everything moved to ac and was easier. The recent papers from the 80s used the ac system. The problem is, the physics is harder, and we want this to be for undergrads. So we like the braking experiment better.

Re: noise reduction. We can try and reduce the noise by turning off monitors and running at night. Ideally, we’d like to be able to run this as part of our undergraduate advanced lab, so making it an extremely low-noise environment is not ideal. But we can try. We’ve actually tried mu-metal, and it is not great at keeping out the magnetic fields. We’re currently considering just adding a 60 Hz notch filter.

We haven’t yet seen the signal we want, precisely because right now the noise swamps the TIA, and brings it to the rails.

George, The dc offset when it is just sitting there usually sits around a volt or so. We’re working with low-noise op-amps, and so they are not trimmable. We were using the LT1792 and that one we could trim and that worked well. We can and have tested the circuit with an actual honest-to-goodness current source, and with two coils, where we send a pulse into one coil and pick it up in the second coil. The first experiment works well, the second is inconclusive so far.

As for connections to the coils, to contact the rotating coil we use a rotary feedthrough that one of my students found on the web: http://www.mercotac.com/html/110.html . The stationary coil, we just bring the contacts out and onto the bench. None of the electronics is attached to the coils, it is all on the bench. We connect our two coils in series when the wires come to the breadboard.

Jan Panteltje, I am intrigued by your suggestion, as we initially wanted to integrate the current pulse. Our thought was that integrating would allow us to integrate over the noise and be left with only the slower 0.5 s pulse. But in order to get a large current, the input impedance of the measurement system has to be small. This is why the TIA, with its virtual ground at the negative input is nice, acts as zero input impedance for the current pulse. What would be the input impedance of your transistor circuit be?

Whit3rd, We started first using a charge amplifier exactly as you described, but without the switch. So to prevent pinning the op-amp we added a feedback resistor also. The system we had was very unstable, even for short periods of time. But we did not try it with a switch to keep the capacitor for charging. You are right that the math is easy if we can integrate the current to get the charge. Then we only need to know the starting speed and the total charge.

John Larkin, I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?

 

[piglet]

On 06/06/2019 17:48, ithacacollegephysics@gmail.com wrote:
> ... We’re working with low-noise op-amps, and so they are not trimmable....

I am puzzled by that. Have I missed something. I thought op-amps without
offset adjust trim pins could usually still be trimmed by injecting a
small trim bias into one of the inputs?

piglet

 

... We’re working with low-noise op-amps, and so they are not trimmable....

I am puzzled by that. Have I missed something. I thought op-amps without
offset adjust trim pins could usually still be trimmed by injecting a
small trim bias into one of the inputs?

piglet

We have not tried that. The LT1792 uses one of its extra pins with a variable resistor between one rail and ground to trim the offset. I haven't tried it into the input.

 

[whit3rd]

On Thursday, June 6, 2019 at 9:48:53 AM UTC-7, ithacacoll...@gmail.com wrote:

> I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?

Yes, the second coil co-located with the moving one can be differenced and the only output
effect will be low (because of common-mode rejection ratio); if it's KNOWN to be 60 Hz noise,
you can also gate your outputs at some multiple of 1/60 second (for 50 and 60 Hz, a multiple
of 1/10 second is preferable) which should reject a large portion of that pickup. Your
signal is DC, but the pickup goes positive and negative so an integer number of cycles
sums to... zero.

Common-mode rejection, like matching of the coils, has errors (resistor value errors, in
the case of the diff amp), but unlike coils-in-series connection, it is possible to trim the
coil difference if you feed the two coils to different input pins. It might be reasonable
to gate-alike two charge amplifiers, one for the driven coil and one for the static coil,
and difference-amplify the outputs. Charge amplifier gain depends on capacitor value,
that'd be an obvious place to gain-change for balance of the two coils.
results,

Your apparatus also has to reject AM radio input, as well as the more audible 60 Hz artifacts.
I'm guessing that any nearby switchmode power (cellphones?) is deadly, too.

 

[Joseph Gwinn]

On Jun 6, 2019, ithacacollegephysics@gmail.com wrote
(in article<0b997183-7742-482d-8891-c646d5f288b7@googlegroups.com&gt:

Hi all,
Again, thanks for the great ideas. I will try and keep up with them all. The
original Tolman-Stewart used this rotating coil and braking; they re-did
their experiment about a decade later with a constantly rotating system and
then everything moved to ac and was easier. The recent papers from the 80s
used the ac system. The problem is, the physics is harder, and we want this
to be for undergrads. So we like the braking experiment better.

Re: noise reduction. We can try and reduce the noise by turning off monitors
and running at night. Ideally, we´d like to be able to run this as part
of
our undergraduate advanced lab, so making it an extremely low-noise
environment is not ideal. But we can try. We´ve actually tried
mu-metal,
and it is not great at keeping out the magnetic fields. We´re
currently
considering just adding a 60 Hz notch filter.

We haven´t yet seen the signal we want, precisely because right now
the
noise swamps the TIA, and brings it to the rails.

George, The dc offset when it is just sitting there usually sits around a
volt or so. We´re working with low-noise op-amps, and so they are not
trimmable. We were using the LT1792 and that one we could trim and that
worked well. We can and have tested the circuit with an actual
honest-to-goodness current source, and with two coils, where we send a pulse
into one coil and pick it up in the second coil. The first experiment works
well, the second is inconclusive so far.

There is a standard trick for compensating integrators for opamp offset
induced drift:

"Integrating Fluxmeter with Input Current Compensation to Cancel Drift",
Thomas G. Chadbourne, US Patent 3,978,399 issued 31 August 1976.

This may be useful.

Joe Gwinn .

As for connections to the coils, to contact the rotating coil we use a rotary
feedthrough that one of my students found on the web:
http://www.mercotac.com/html/110.html . The stationary coil, we just bring
the contacts out and onto the bench. None of the electronics is attached to
the coils, it is all on the bench. We connect our two coils in series when
the wires come to the breadboard.

Jan Panteltje, I am intrigued by your suggestion, as we initially wanted to
integrate the current pulse. Our thought was that integrating would allow us
to integrate over the noise and be left with only the slower 0.5 s pulse. But
in order to get a large current, the input impedance of the measurement
system has to be small. This is why the TIA, with its virtual ground at the
negative input is nice, acts as zero input impedance for the current pulse.
What would be the input impedance of your transistor circuit be?

Whit3rd, We started first using a charge amplifier exactly as you described,
but without the switch. So to prevent pinning the op-amp we added a feedback
resistor also. The system we had was very unstable, even for short periods of
time. But we did not try it with a switch to keep the capacitor for charging.
You are right that the math is easy if we can integrate the current to get
the charge. Then we only need to know the starting speed and the total charge.

John Larkin, I have limited experience with a differential amplifier. Can
those be built to reject the 60 Hz noise?

 

On Thursday, June 6, 2019 at 9:55:56 PM UTC-4, George Herold wrote:

On Thursday, June 6, 2019 at 4:28:34 PM UTC-4, whit3rd wrote:
On Thursday, June 6, 2019 at 9:48:53 AM UTC-7, ithacacoll...@gmail.com wrote:

I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?

Yes, the second coil co-located with the moving one can be differenced and the only output
effect will be low (because of common-mode rejection ratio); if it's KNOWN to be 60 Hz noise,
you can also gate your outputs at some multiple of 1/60 second (for 50 and 60 Hz, a multiple
of 1/10 second is preferable) which should reject a large portion of that pickup. Your
signal is DC, but the pickup goes positive and negative so an integer number of cycles
sums to... zero.

Common-mode rejection, like matching of the coils, has errors (resistor value errors, in
the case of the diff amp), but unlike coils-in-series connection, it is possible to trim the
coil difference if you feed the two coils to different input pins.

Oh, I always figured he (Matt) had the comp coil in series.
If subtracting afterwards then yeah you have to match both
signal chains. (10% capacitors might ruin your whole day.)
Our compensating coil is in series with our rotating (signal) coil. But we could easily take them apart and put the two signals into a difference amplifier to subtract the signals afterwards.

There's plenty of 60 Hz noise in the room, as commenters have pointed out, so if I can get a variable capacitor I can easily tune the compensating coil to zero out the signal from the rotating coil, to be left with only the signal.

I don't have a good feel for the electronics: would that be more feasible than the transistor method from Jan?

I should see how much of the 60 Hz and other crud is canceled
in our EF-NMR.

George H.
It might be reasonable
to gate-alike two charge amplifiers, one for the driven coil and one for the static coil,
and difference-amplify the outputs. Charge amplifier gain depends on capacitor value,
that'd be an obvious place to gain-change for balance of the two coils.
results,

Your apparatus also has to reject AM radio input, as well as the more audible 60 Hz artifacts.
I'm guessing that any nearby switchmode power (cellphones?) is deadly, too.

 

[George Herold]

On Thursday, June 6, 2019 at 4:28:34 PM UTC-4, whit3rd wrote:

On Thursday, June 6, 2019 at 9:48:53 AM UTC-7, ithacacoll...@gmail.com wrote:

I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?

Yes, the second coil co-located with the moving one can be differenced and the only output
effect will be low (because of common-mode rejection ratio); if it's KNOWN to be 60 Hz noise,
you can also gate your outputs at some multiple of 1/60 second (for 50 and 60 Hz, a multiple
of 1/10 second is preferable) which should reject a large portion of that pickup. Your
signal is DC, but the pickup goes positive and negative so an integer number of cycles
sums to... zero.

Common-mode rejection, like matching of the coils, has errors (resistor value errors, in
the case of the diff amp), but unlike coils-in-series connection, it is possible to trim the
coil difference if you feed the two coils to different input pins.

Oh, I always figured he (Matt) had the comp coil in series.
If subtracting afterwards then yeah you have to match both
signal chains. (10% capacitors might ruin your whole day.)

I should see how much of the 60 Hz and other crud is canceled
in our EF-NMR.

George H.
It might be reasonable

to gate-alike two charge amplifiers, one for the driven coil and one for the static coil,
and difference-amplify the outputs. Charge amplifier gain depends on capacitor value,
that'd be an obvious place to gain-change for balance of the two coils.
results,

Your apparatus also has to reject AM radio input, as well as the more audible 60 Hz artifacts.
I'm guessing that any nearby switchmode power (cellphones?) is deadly, too.

 

[George Herold]

On Thursday, June 6, 2019 at 12:48:53 PM UTC-4, ithacacoll...@gmail.com wrote:

Hi all,
Again, thanks for the great ideas. I will try and keep up with them all. The original Tolman-Stewart used this rotating coil and braking; they re-did their experiment about a decade later with a constantly rotating system and then everything moved to ac and was easier. The recent papers from the 80s used the ac system. The problem is, the physics is harder, and we want this to be for undergrads. So we like the braking experiment better.

Re: noise reduction. We can try and reduce the noise by turning off monitors and running at night. Ideally, we’d like to be able to run this as part of our undergraduate advanced lab, so making it an extremely low-noise environment is not ideal. But we can try. We’ve actually tried mu-metal, and it is not great at keeping out the magnetic fields. We’re currently considering just adding a 60 Hz notch filter.

We haven’t yet seen the signal we want, precisely because right now the noise swamps the TIA, and brings it to the rails.

George, The dc offset when it is just sitting there usually sits around a volt or so. We’re working with low-noise op-amps, and so they are not trimmable. We were using the LT1792 and that one we could trim and that worked well. We can and have tested the circuit with an actual honest-to-goodness current source, and with two coils, where we send a pulse into one coil and pick it up in the second coil. The first experiment works well, the second is inconclusive so far.

As for connections to the coils, to contact the rotating coil we use a rotary feedthrough that one of my students found on the web: http://www.mercotac.com/html/110.html . The stationary coil, we just bring the contacts out and onto the bench. None of the electronics is attached to the coils, it is all on the bench. We connect our two coils in series when the wires come to the breadboard.

Huh, have other people used these (rotating contacts) for low noise
measurements? Is there a spec? I'd want to try and measure the noise,
low currents might be different than high.. (mA) currents.

George H.

Jan Panteltje, I am intrigued by your suggestion, as we initially wanted to integrate the current pulse. Our thought was that integrating would allow us to integrate over the noise and be left with only the slower 0.5 s pulse. But in order to get a large current, the input impedance of the measurement system has to be small. This is why the TIA, with its virtual ground at the negative input is nice, acts as zero input impedance for the current pulse. What would be the input impedance of your transistor circuit be?

Whit3rd, We started first using a charge amplifier exactly as you described, but without the switch. So to prevent pinning the op-amp we added a feedback resistor also. The system we had was very unstable, even for short periods of time. But we did not try it with a switch to keep the capacitor for charging. You are right that the math is easy if we can integrate the current to get the charge. Then we only need to know the starting speed and the total charge.

John Larkin, I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?

 

[John Larkin]

On Thu, 6 Jun 2019 20:05:11 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

On Thursday, June 6, 2019 at 12:48:53 PM UTC-4, ithacacoll...@gmail.com wrote:
Hi all,
Again, thanks for the great ideas. I will try and keep up with them all. The original Tolman-Stewart used this rotating coil and braking; they re-did their experiment about a decade later with a constantly rotating system and then everything moved to ac and was easier. The recent papers from the 80s used the ac system. The problem is, the physics is harder, and we want this to be for undergrads. So we like the braking experiment better.

Re: noise reduction. We can try and reduce the noise by turning off monitors and running at night. Ideally, we’d like to be able to run this as part of our undergraduate advanced lab, so making it an extremely low-noise environment is not ideal. But we can try. We’ve actually tried mu-metal, and it is not great at keeping out the magnetic fields. We’re currently considering just adding a 60 Hz notch filter.

We haven’t yet seen the signal we want, precisely because right now the noise swamps the TIA, and brings it to the rails.

George, The dc offset when it is just sitting there usually sits around a volt or so. We’re working with low-noise op-amps, and so they are not trimmable. We were using the LT1792 and that one we could trim and that worked well. We can and have tested the circuit with an actual honest-to-goodness current source, and with two coils, where we send a pulse into one coil and pick it up in the second coil. The first experiment works well, the second is inconclusive so far.

As for connections to the coils, to contact the rotating coil we use a rotary feedthrough that one of my students found on the web: http://www.mercotac.com/html/110.html . The stationary coil, we just bring the contacts out and onto the bench. None of the electronics is attached to the coils, it is all on the bench. We connect our two coils in series when the wires come to the breadboard.

Breadboard? At 200 nV?

Huh, have other people used these (rotating contacts) for low noise
measurements? Is there a spec? I'd want to try and measure the noise,
low currents might be different than high.. (mA) currents.

The mercotac things use "liquid metal" contacts. What metal is
liquid??

I'd expect some healthy thermoelectrics.

George H.


Jan Panteltje, I am intrigued by your suggestion, as we initially wanted to integrate the current pulse. Our thought was that integrating would allow us to integrate over the noise and be left with only the slower 0.5 s pulse. But in order to get a large current, the input impedance of the measurement system has to be small. This is why the TIA, with its virtual ground at the negative input is nice, acts as zero input impedance for the current pulse. What would be the input impedance of your transistor circuit be?

Whit3rd, We started first using a charge amplifier exactly as you described, but without the switch. So to prevent pinning the op-amp we added a feedback resistor also. The system we had was very unstable, even for short periods of time. But we did not try it with a switch to keep the capacitor for charging. You are right that the math is easy if we can integrate the current to get the charge. Then we only need to know the starting speed and the total charge.

John Larkin, I have limited experience with a differential amplifier. Can those be built to reject the 60 Hz noise?

If the 60 Hz is from a real magnetic field thing, no. If it's
electrostatic common-mode voltage, yes. 60 Hz can be filtered out as
long as it's not so big as to drive things nonlinear.

I think your rig needs an electrostatic shield. I've seen foamy sheet
stuff covered with aluminum foil.

In most rooms, the 60 Hz e-field is huge. Plus lots of other stuff.

Why not amplify and digitize the outputs of both coils?


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Jan Panteltje]

On a sunny day (Thu, 6 Jun 2019 09:48:48 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
<0b997183-7742-482d-8891-c646d5f288b7@googlegroups.com>:

Jan Panteltje, I am intrigued by your suggestion, as we initially wanted to
integrate the current pulse. Our thought was that integrating would allow
us to integrate over the noise and be left with only the slower 0.5 s pulse.
But in order to get a large current, the input impedance of the measurement
system has to be small. This is why the TIA, with its virtual ground at
the negative input is nice, acts as zero input impedance for the current
pulse. What would be the input impedance of your transistor circuit be?

It looks like at least several kOhm, bit higher than I expected.
Still it would experiment with it.
But others may have more advanced^H^H^H^H^H^H^H^Hcomplicated solutions.

Not sure about TIA, TIA makes sense if the current response of your coil is more
linear than the voltage response:
https://en.wikipedia.org/wiki/Transimpedance_amplifier
Because you are basically shifting some electrons does this make sense in this case?
And after all it is an artificial created low impedance created by feedback.

Some philosophy...
Storm coming ;-)

Anyways the suggestion of a deserted island with .. should help against the mains hum and cars.

 

[whit3rd]

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

> Why not amplify and digitize the outputs of both coils?

Wrong question. Amplify, certainly, but... digitize? To what end?

Or, do you mean apply a digital voltmeter?

If only a computer screen can display the results, the physical
effect hasn't really been convincingly demonstrated to the classroom.

 

Hi all again.

Joseph Gwinn, I will take a look at the patent to see how they compensated for the op-amp drift.

Regarding the rotary connectors, there is not a lot of noise information on their website. We moved to a liquid metal rotary feedthrough (the liquid metal is mercury) to remove problems with noise that comes from brushes. I don’t know what the noise specs are. We are set up to run it all using wire that twist up, but that is hard to run tests like that. You get one try, and then have to replace the wires.

Jan Panteltje, several kOhm of input impedance will reduce our expected current by a factor of ten or more. So I am not sure your design will work. We’d drop our expected current down to 100 pA, or less.

John Larkin, we did have the rig inside mu-metal for a while, but that was hard to maintain and did very little for the static (Earth’s) field that we also have to get rid of. (If you don’t get rid of the Earth’s field, when the coil is spinning the centrifugal force makes the coil slightly bigger, and then when you brake it it gets smaller, leading to a signal that looks like the one we are trying to see.) So we abandoned that. But we haven’t tried electrostatically shielding the entire setup. We put our breadboard into a metal box to reduce noise – are you suggesting we box up the whole thing into an electrostatic box? We had a real one of those when I was in grad school, a screened room. Is that necessary for the coils as well?

Ideally, I’d like to be able to see the integrated pulse or the actual V vs t of the pulse on the oscilloscope screen for the students to be able to see it before digitizing it. But if I have to digitize and then play with the signal, I will.

 

[John Larkin]

On Thu, 6 Jun 2019 22:49:01 -0700 (PDT), whit3rd <whit3rd@gmail.com>
wrote:

On Thursday, June 6, 2019 at 8:27:28 PM UTC-7, John Larkin wrote:

Why not amplify and digitize the outputs of both coils?

Wrong question. Amplify, certainly, but... digitize? To what end?

To see if the signal is real and not an artifact. To *see* what the
artifacts are. To filter and signal process with matlab or something.
To signal-average many runs. To subtract out the stationary coil
signal with tunable math.

Or, do you mean apply a digital voltmeter?

No, too slow to show the waveforms.

Don't integrate! Don't use a charge amp!

If only a computer screen can display the results, the physical
effect hasn't really been convincingly demonstrated to the classroom.

And to demonstrate a deceleration-rate-dependant pulse. And to
illustrate the basics of signal processing, which does tend to use
computers nowadays.

Why would a properly shaped pulse on a screen be less convincing than
a static voltage at an opamp output? I wouldn't accept that in this
case.

This is a great demo experiment, because there are so many hazards.
It's not going to be a great demo if it doesn't ever work.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[Jan Panteltje]

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
<85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

There seems to be a basic misunderstanding about how opamps work,
opamp output is the result of an input voltage (or current) being amplified,
in the TIA case that voltage is then feedback via a resistor
to bring the input back to zero,
ALMOST zero.
This gives the illusion of a low input impedance.
In fact the real opamp impedance may be very high.
There is a delay, so overshoot at the output, as it takes TIME to amplify the signal.
What did you gain? speed ? But speed in this context is very very low....
The bit of overshoot gives better speed impression.

It is the same as the old logic feedback counter that as kid I could not
figure out, until I realized it can only work because logic gates have a delay.

sigh

This is apart from issues such as linearity caused by the nice omhs feedback resistor of course.

 

[John Larkin]

On Fri, 7 Jun 2019 03:59:16 -0700 (PDT),
ithacacollegephysics@gmail.com wrote:

Hi all again.

Joseph Gwinn, I will take a look at the patent to see how they compensated for the op-amp drift.

Regarding the rotary connectors, there is not a lot of noise information on their website. We moved to a liquid metal rotary feedthrough (the liquid metal is mercury) to remove problems with noise that comes from brushes. I don’t know what the noise specs are. We are set up to run it all using wire that twist up, but that is hard to run tests like that. You get one try, and then have to replace the wires.

Jan Panteltje, several kOhm of input impedance will reduce our expected current by a factor of ten or more. So I am not sure your design will work. We’d drop our expected current down to 100 pA, or less.

John Larkin, we did have the rig inside mu-metal for a while, but that was hard to maintain and did very little for the static (Earth’s) field that we also have to get rid of. (If you don’t get rid of the Earth’s field, when the coil is spinning the centrifugal force makes the coil slightly bigger, and then when you brake it it gets smaller, leading to a signal that looks like the one we are trying to see.) So we abandoned that. But we haven’t tried electrostatically shielding the entire setup. We put our breadboard into a metal box to reduce noise – are you suggesting we box up the whole thing into an electrostatic box? We had a real one of those when I was in grad school, a screened room. Is that necessary for the coils as well?

Ideally, I’d like to be able to see the integrated pulse or the actual V vs t of the pulse on the oscilloscope screen for the students to be able to see it before digitizing it. But if I have to digitize and then play with the signal, I will.

This looks to be an extraordinally difficult experiment.

Magnetic shielding is probably not practical here. The compensating
and helmholtz coils can help with mag fields. If electrostatic
coupling is squirting noise into the system, it's easy to test that:
temporarily shield the area with grounded aluminum foil or something
and see if it makes any difference. Also look at the amp output with a
scope when nothing is spinning.

I will say, for the last time, that the basic physics makes voltage,
and measusing current was an artifact of hundred-year-old
instrumentation limits, and the TIA is a bad idea.

Good luck getting this to work.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Fri, 07 Jun 2019 15:49:26 +0200, Jeroen Belleman
<jeroen@nospam.please> wrote:

Jan Panteltje wrote:
On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).


Yeah, well, you forgot about the h_oe output conductance,
which isn't zero. There is effectively a resistor of a
few 10 kOhm in parallel with your 10 MOhm collector resistor,
limiting the gain. Somebody already told you.

Jeroen Belleman

Spice does transistors pretty well.


--

John Larkin Highland Technology, Inc

lunatic fringe electronics

 

[John Larkin]

On Fri, 07 Jun 2019 13:13:11 GMT, Jan Panteltje
<pNaOnStPeAlMtje@yahoo.com> wrote:

On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
ithacacollegephysics@gmail.com wrote in
85e803ec-c2d7-4b6a-a752-ce08625c0a32@googlegroups.com>:

Jan Panteltje, several kOhm of input impedance will reduce our expected current
by a factor of ten or more. So I am not sure your design will work. We'd
drop our expected current down to 100 pA, or less.

This is the interesting part.
That 100 pA (1e-10) will flow into the base junction of that transistor and if it is a high beta type
here for the simplicity of numbers not hFE 900 but 1000, result in a collector current of 1e-7,
or .1 uA.
In a 10 MOhm collector resistor that will give 1 V (1e-7 * 1e7).

There seems to be a basic misunderstanding about how opamps work,

And some basic misunderstandings of how transistors work. Higher beta
does not increase the voltage gain of the usual transistor amplifier.

200 nV will not push 100 pA into the base of a transistor whose
collector current is around 1 uA. The impedance looking into the base
will be 10s of megohms.




--

John Larkin Highland Technology, Inc

lunatic fringe electronics