# Help designing a low-noise TIA

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Hi all,

I am posting to this forum at the suggestion of George Herold from Teachspi
n, he says you are the people to help me out.  I understand a decent amount
about circuits, but not enough to design the low-noise TIA I would like to
build.

We're trying to create a modern version of the Tolman-Stewart experiment ht
tps://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of t
he first proofs that electrons inside metals carry the current.

In this experiment, a coil of wire is spun to high speeds and then braked r
apidly.  The electrons keep moving and create a small pulse of current.  Or
iginally, Tolman and Stewart used a ballistic galvanometer to act as a char
ge amplifier and integrate the current to find the total charge.

I?d like to use a TIA to convert the small current pulse into a vol
tage, then record that voltage as a function of time.  The problem is that
the coil acts as a giant antenna and picks up all sorts of unwanted noise,
so I?d like to get rid of that noise.  In particular, it is really
good at finding 60 Hz signals in the room.

Right now we?re using an OPA 140 with 1 GOhm and 10 pF as a feedbac
k resistor and capacitor in parallel.  We attach the coil (about 200 Ohm re
sistance, 500 mH inductance) to one input and put the other input across 20
0 Ohms to ground.  The large amplification leads to huge amplification of t
he noise, and it is hard to see our signal.  We expect the current pulse to
be 1 nA of current, almost square wave in shape, and it should last the du
ration of the braking, about 0.5 seconds.

Any suggestions appreciated!

--Matt Sullivan
Ithaca College Physics

Re: Help designing a low-noise TIA
On 6/4/19 3:28 PM, snipped-for-privacy@gmail.com wrote:

One approach would be to put another winding around it,
electrostatically shielded inside and out using copper tape, and hang
the TIA on that (inside the shield).  You could get the coupling
coefficient up to probably 0.5, and it's easily measured, so it wouldn't
hurt the accuracy much.

The shield would look like a shorted turn , so you use two half-turn
shields with a nice big overlap , insulated with kapton tape.  Some
capacitors going across the gap would help too.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
We've slightly trimmed the long signature. Click to see the full one.
Re: Help designing a low-noise TIA
:

pin, he says you are the people to help me out.  I understand a decent amou
nt about circuits, but not enough to design the low-noise TIA I would like
to build.

https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one of
the first proofs that electrons inside metals carry the current.

rapidly.  The electrons keep moving and create a small pulse of current.
Originally, Tolman and Stewart used a ballistic galvanometer to act as a ch
arge amplifier and integrate the current to find the total charge.

oltage, then record that voltage as a function of time.  The problem is tha
t the coil acts as a giant antenna and picks up all sorts of unwanted noise
, so I?d like to get rid of that noise.  In particular, it is reall
y good at finding 60 Hz signals in the room.

ack resistor and capacitor in parallel.  We attach the coil (about 200 Ohm
resistance, 500 mH inductance) to one input and put the other input across
200 Ohms to ground.  The large amplification leads to huge amplification of
the noise, and it is hard to see our signal.  We expect the current pulse
to be 1 nA of current, almost square wave in shape, and it should last the
duration of the braking, about 0.5 seconds.

Hi Matt, Thanks for asking here... I guess I'm hoping that this will give

you more ideas, than just you and I going back and forth on email.

The OPA140 looks fine.  1G ohm and 10 pF is a 10 ms time constant..

But your big problem is the 60 Hz pickup from your coil?

How big is your coil (diameter)?  Do you see signals from the coil
spinning in the earth's B-field?  I was browsing this,
https://authors.library.caltech.edu/3372/1/TOLpr16b.pdf
(from your wiki link) and it looks to be a big coil.  But hard to tell.

We do a trick in our Earth's field nmr where there are two coils in
series (but wound in opposite directions.)  A many turn inner coil that
picks up the nmr signal.  and a much bigger outer coil, with the same
turns*area as the inner coil.  And this picks up the same emf from the
AC 60 Hz as the signal coil... and cancels it.   I think JR will take a tur
n
or two off the inner coil to try and match the cancellation.

George H.

Re: Help designing a low-noise TIA
On Tuesday, June 4, 2019 at 1:46:13 PM UTC-7, George Herold wrote:

Great paper.  It really gives you a feel for how physics was done
back in the days before physics was "done."

I wonder what was responsible for the rapid variations in the local
geomagnetic field that they had to compensate for.  They say that they
could detect the effect of cars driving by, but they also say that
separating the compensating coil(s) from the experiment had little
effect, suggesting that the fluctuations were coming from a significant
distance.

I guess they could have been seeing the effects of solar activity,
measuring the K index a couple of decades before the term caught on.

-- john, KE5FX

Re: Help designing a low-noise TIA
On Tuesday, June 4, 2019 at 5:24:53 PM UTC-4, John Miles, KE5FX wrote:

Huh.. I'm going to have to read it.

I've got optical pumping coils/ apparatus, that I take
'down home on the farm' to test.... In the living room
I can see the fish tank pump spinning, it's a permanent magnet.

I test 'em in a bedroom; I can totally see a car pull into the nearby
drive-way.

There's some daily variation in the B-field... I never tried to measure
it.
gotten worse since then.

George H.

Re: Help designing a low-noise TIA
On 2019-06-04 13:46, George Herold wrote:

If the 60Hz getting in is E-field there is also the trick of winding
with coax and grounding one side (and only one).

However, much of it will be magnetic and then Matt can only notch it
out. If the detection is done in software notch filters are easy. If
analog you'd almost have to use a switched-capacitor filter for that.
Very likely several notch filters are required because when 60Hz is gone
Matt will discover that there's also a lot of 180Hz and 300Hz. Also,
make sure there is absolutely no 60Hz gear that turns on and off at
randon, such as electric cooktops, because that's next to impossible to
notch out.

--
Regards, Joerg

http://www.analogconsultants.com/

Re: Help designing a low-noise TIA
On 6/4/2019 4:48 PM, Joerg wrote:

I have no idea about any of this, but wonder if a rotary transformer
is of any use, and my source would be from a VCR, always thought they
were a bit magical.
Mikek
PS, not sure it won't blow apart stopping 1/2 second.

Re: Help designing a low-noise TIA
wrote:

A serious lowpass filter would take all the HF stuff out; his signal
is ballpark 1 Hz. But that won't work if anything rails ahead of the
filter.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

Re: Help designing a low-noise TIA
On 2019-06-08 07:02, John Larkin wrote:

I might have misunderstood, I though he needs to observe a sharp end of
some otherwise slow electron effect. If you need only a few Hz BW, yes,
then lowpass is the ticket.

At 1e9 current-to-voltage transfer ratio I am afraid that can happen
easily. Worst case in spurts and that really messes up any signal.

--
Regards, Joerg

http://www.analogconsultants.com/

Re: Help designing a low-noise TIA
On 2019-06-08 07:16, Joerg wrote:

Old rule in RF engineering: Make the gain in the first stage only as
immediately, and only then amplify more.

--
Regards, Joerg

http://www.analogconsultants.com/

Re: Help designing a low-noise TIA
On 6/8/2019 9:02 AM, John Larkin wrote:

You didn't directly respond to me, but I'm afraid a VCR rotary
transformer is more of a high pass filter and wouldn't be much use at 1 HZ.
Mikek

Re: Help designing a low-noise TIA

A magnetic or optical coupler would probably need some sort of
modulation, which would be a lot of electronics to spin.

One could use mag coupling into the stationary coil which is already
there. Then there would only be one signal to digitize.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

Re: Help designing a low-noise TIA
On Tue, 4 Jun 2019 12:28:49 -0700 (PDT),
snipped-for-privacy@gmail.com wrote:

The source impedance of the voltage pulse will be low, so you may not
want a high-impedance TIA. The coil is more of a 200 ohm voltage

If the braking lasts a half second, a lot of the noise could be
filtered out. Or brake faster and get more signal.

I don't understand your circuit, where the 200 ohm things go. Can you
post the schematic?

You'll have two sources of noise, electrostatic and magnetic. The
electrostatic can be shielded.

Is it possible to split the coil into two sections, wired so that the
voltages add for the deceleration but cancel for external fields?

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

We've slightly trimmed the long signature. Click to see the full one.
Re: Help designing a low-noise TIA
On Tuesday, June 4, 2019 at 6:18:41 PM UTC-4, John Larkin wrote:

spin, he says you are the people to help me out.  I understand a decent amo
unt about circuits, but not enough to design the low-noise TIA I would like
to build.

https://en.wikipedia.org/wiki/Stewart%E2%80%93Tolman_effect that was one o
f the first proofs that electrons inside metals carry the current.

d rapidly.  The electrons keep moving and create a small pulse of current.
Originally, Tolman and Stewart used a ballistic galvanometer to act as a c
harge amplifier and integrate the current to find the total charge.

voltage, then record that voltage as a function of time.  The problem is th
at the coil acts as a giant antenna and picks up all sorts of unwanted nois
e, so I?d like to get rid of that noise.  In particular, it is real
ly good at finding 60 Hz signals in the room.

back resistor and capacitor in parallel.  We attach the coil (about 200 Ohm
resistance, 500 mH inductance) to one input and put the other input across
200 Ohms to ground.  The large amplification leads to huge amplification o
f the noise, and it is hard to see our signal.  We expect the current pulse
to be 1 nA of current, almost square wave in shape, and it should last the
duration of the braking, about 0.5 seconds.

Yeah, Matt wants to measure current, but I'm not sure a TIA is the
answer.  (I think he has 2 x 200 ohms, but I'm not sure.)

I was thinking of a gated integrator... which I haven't used
in years.  You could also trigger a DSO and average.
(which is like a gated integrator, with more time information.)

George H.

Re: Help designing a low-noise TIA
On Tue, 4 Jun 2019 17:38:39 -0700 (PDT), George Herold

A ballistic galvonmeter measured current, but that doesn't mean we
have to. I think the physics produces voltage.

--

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement

We've slightly trimmed the long signature. Click to see the full one.
Re: Help designing a low-noise TIA
On 05/06/19 02:05, John Larkin wrote:

At school we only ever used them to measure charge. They
integrated current over time of a current pulse, where
the pulse's duration was a short fraction of the
ballistic galvanometer's response time.

Never saw them being used to measure current; ordinary
galvanometers were sufficient for that.

Re: Help designing a low-noise TIA
On Tuesday, June 4, 2019 at 6:06:22 PM UTC-7, John Larkin wrote:

A ballistic galvanometer is very slow; it measures integral of current,
i.e. charge.

A transimpedance amp might work, but so might a zero-ed active capacitor divider
(capacitor negative feedback becomes a hold capacitor).   You could reverse the
polarity and run the experiment twice, to get rid of the inevitable offset voltage.

It just has to integrate for a half second during the deceleration, then hold for long
enough to get a reading.   Relays for that kind of timing are easily arranged.

Re: Help designing a low-noise TIA
On Wed, 5 Jun 2019 06:15:05 -0700 (PDT),
snipped-for-privacy@gmail.com wrote:

WOrks.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics

Re: Help designing a low-noise TIA
Hi all,

Thanks for the replies!  A lot to think about.

If you want to see our setup, here is a picture:
https://www.dropbox.com/s/is3vdxlcz24ohml/TS-expt.jpg?dl=0

Here is the simple circuit we are working with now:
https://www.dropbox.com/s/f9aik177qqellcj/TS-ckt.jpg?dl=0
In the simulated circuit, we've added a current source to act as the electr
on motion upon braking the coil.

Phil, I am not sure I follow your suggestions, sorry.  We'd put another coi
l outside the rotating coil to measure the signal from the rotating coil?
Elaborations appreciated!

George, our coil is about 8" in diameter.  I remember the Earth NMR from th
e Food Truck when it came to Ithaca College.  If you look at our setup, we
have something very similar: we have a second, stationary, coil directly be
low the rotating coil, and this coil is counter-wound.  This definitely hel
ps to reduce the 60 Hz noise (if there is other noise, it is drowned out by
60 Hz noise).  At the time I thought our methods were essentially identica
l.  Do you think a larger stationary coil surrounding the rotating coil wou
ld work better than a stationary coil of the same diameter beneath the rota
ting coil?

Jeorg, right now we reduce the remaining 60 Hz noise by simply averaging ou
r voltage signal over 17 ms, so we get one data point for every 60 Hz cycle
.  This does reduce our noise.  I planned to try to remove the 60 Hz noise
via FFT and then fiter out the high frequencies, but I wanted to try and fi
nd an experimental solution first.

John Larkin, braking faster would be hard.  The coil has about a pound of c
opper wire 8 inches from the rotation axis, and we spin it up to 6000 or 70
00 rpm.  It has a lot of angular momentum!  Half a second is the best we've
been able to do.  And I am afraid we've worked pretty hard to get it spinn
ing down even that quickly.  As for the split coil, we have essentially don
e that with the rotating coil and the stationary coil.  They can't both be
rotating, or the signal we are hoping for cancels out (the electrons in the
two coils are counter-rotating, so you get two pulses of current in opposi
te directions).

George, we did try a charge amplifier at first, which as far as I understan
d is the electronic analog of the ballistic galvanometer.  But it was hard
to get the op-amps to be stable and still be able to integrate over a whole
second.  So I figured we have much faster electronics now, so to get the t
otal charge I can just integrate the current, so that's why we are trying a
TIA now.

Re: Help designing a low-noise TIA
On Wed, 5 Jun 2019 07:42:37 -0700 (PDT),
snipped-for-privacy@gmail.com wrote:

R2 just adds Johnson noise. That opamp is fairly noisy too.

The DC gain of that circuit is about 4e6, and the typical input offset
of that opamp is 30 uV. Multiply those!

Is the amplifier and its batteries rotating on the coil? It would be a
lot easier to export volts than nanovolts.

Optical coupling would be fun to get the signal off the spinning coil.

I still think it would be better to use a voltage amplifier rather
than a TIA. Low noise diffamp.

--

John Larkin         Highland Technology, Inc

lunatic fringe electronics