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Re: Help designing a low-noise TIA
On Fri, 7 Jun 2019 08:55:10 -0700 (PDT), George Herold

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"mercotac" is a subtle hint.

  
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One more hardly makes a difference.


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I have, several times.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Help designing a low-noise TIA
On Friday, June 7, 2019 at 8:55:17 AM UTC-7, George Herold wrote:
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If you put a battery and op amp into the spinning disk, the rotating
contacts can have the buffered integrator-capacitor voltage
on them, so a few microvolts won't matter.   The nanovolt signals don't
need to pass through the mystery metals.

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Noise only makes a single run uncertain, multiple runs will solve that.
PMI's old designs used SiN passivation with SiO2 overcoat, and had
very low 1/f noise; those OP27s, and other offerings, are available from
AD nowadays.   80nV in 0.1 to 10 Hz bandwidth, and you only really
care about 0.1 to 1 Hz if you look at the brake-pulse integration.   It might
hurt the take-a-thousand-readings schemes, however.

Re: Help designing a low-noise TIA

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For gaussian noise and the like, averaging does help. For 1/f noise,  


One large reason for going from the current "DC" scheme to an "AC"  
scheme is that 1/f noise falls off at higher frequencies.

The classic scheme would be some kind of torsionally vibrating coil feeding  
an AC coupled amplifier and a synchronous detector. This approach has already  
been mentioned and dismissed as being too complex for teaching, but has the  
advantage of being workable in exactly such a lab..

It occurs to me that a possible arrangement is two torsionally oscillating  
coils that are coaxial and almost touch, but vibrate opposite to one another.  
The coils would be wired such that the desired electron acceleration signals  
would add, while variation due to changes in coil radial size due to  
centrifugal effect interacting with the local magnetic field would largely  
cancel.

.
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Lower 1/f noise is always helpful.

Joe Gwinn



Re: Help designing a low-noise TIA
On Friday, June 7, 2019 at 2:52:34 PM UTC-7, Joseph Gwinn wrote:
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In what context?    If you null the capacitor charge, then do a few-seconds
 experiment,
there's both a lower and upper frequency limit to the whole experiment.

Then you can repeat the experiment a few dozen times, and expect a gaussian
distribution.   Nothing about the 'DC' experiment involves day-long
durations which would build up noise.   The deceleration/active sensing
is a few tenths of a second only.

Re: Help designing a low-noise TIA

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This is cancellation, not averaging.

There are lots of schemes to cancel 1/f noise (called _drift_ when the f is  
low enough), and taking two samples close in time, one with the other without  
the sought data, and differencing is the foundation.

Here is the grand-daddy of such schemes:

.<https://en.wikipedia.org/wiki/Microwave_radiometer

It is also the ancestor of lock-in amplifiers in general. Dicke founded  
Princeton Applied Research to commercialize the invention.

.<https://en.wikipedia.org/wiki/Lock-in_amplifier

.
Which brings me to a thought: If the signal of interest is a few Hertz, and  
drifts et al are a big deal, this may be a perfect application for a  
chopper-stabilized amplifier of some sort.

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Not quite. There is always some residual 1/f component, and this component  
does not average away, and so will come to dominate. So one always has to  
reduce the 1/f level.

Joe Gwinn



Re: Help designing a low-noise TIA
On a sunny day (Thu, 6 Jun 2019 09:48:48 -0700 (PDT)) it happened
snipped-for-privacy@gmail.com wrote in

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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.

Re: Help designing a low-noise TIA
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 cur
rent 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 Ear
th?s field, when the coil is spinning the centrifugal force makes t
he coil slightly bigger, and then when you brake it it gets smaller, leadin
g to a signal that looks like the one we are trying to see.)  So we abandon
ed that.  But we haven?t tried electrostatically shielding the enti
re 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?  W
e had a real one of those when I was in grad school, a screened room.  Is t
hat necessary for the coils as well?

Ideally, I?d like to be able to see the integrated pulse or the act
ual V vs t of the pulse on the oscilloscope screen for the students to be a
ble to see it before digitizing it.  But if I have to digitize and then pla
y with the signal, I will.



Re: Help designing a low-noise TIA
On a sunny day (Fri, 7 Jun 2019 03:59:16 -0700 (PDT)) it happened
snipped-for-privacy@gmail.com wrote in

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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.
:-)

Re: Help designing a low-noise TIA
Jan Panteltje wrote:
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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

Re: Help designing a low-noise TIA
On Fri, 07 Jun 2019 15:49:26 +0200, Jeroen Belleman

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Spice does transistors pretty well.


--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Help designing a low-noise TIA
On a sunny day (Fri, 07 Jun 2019 07:09:14 -0700) it happened John Larkin

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What spice?


Re: Help designing a low-noise TIA
On Fri, 07 Jun 2019 17:47:54 GMT, Jan Panteltje

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LT Spice. It's free and fairly easy to learn and use. It includes a
lot of transistor and mosfet and diode models.

I rarely do much of the old classic EE math any more... just guess
then Spice it. I even Spice voltage dividers and RC timers and such.

I just did an RLC to go between an opamp and a 40 Ms/s ADC input. The
ADC data sheet wants an RC, but the time constant was too slow for my
purposes. A little inductance snaps it right up.

And I had another case: we're redesigning a laser controller, for IC
lithography, that we did in 2002. We used (at the customer's request)
a Maxim tapped silicon delay line to generate some asynchronous
delays. The part is naturally long gone, so I'll use a triggered
oscillator and some FPGA logic to do the same function.

This took minutes to simulate:

https://www.dropbox.com/s/nezshpka0eush92/Tplus_Trig_Osc_2.jpg?dl=0



--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Help designing a low-noise TIA
On a sunny day (Fri, 07 Jun 2019 12:08:56 -0700) it happened John Larkin

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Very nice.
Yes I have LTspice,
it runs OK on an other PC, on this one it crashes the system,
probably a Linux windows emulator 'wine' fault.
It sometimes makes predictions that are not reality,
it predicted a video amplifier I designed would oscillate,
ten boards made and no oscillations...

Nice for filters and stuff that take a lot of time to calculate by hand.
There are also special filter programs though.


As to 'what spice' I was sort of joking a bit, my apologies,
French fries too much mayonnaise, should not have eaten almost the whole jar...


I may have a go at building that circuit I proposed,
and use a 10M 10x scope probe as collector resistor.
If it turns out the Ic is already too high at room temperature,
then I am wrong.
Maybe be after the weekend, some other project coming up now, garden fence repair..
We had oops (garden chair flies, goes outside stores stuff),
HAVE bad storms here.

Re: Help designing a low-noise TIA
On Sat, 08 Jun 2019 06:59:37 GMT, Jan Panteltje

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The world divides in to people who love mayonnaise (me and The Brat)
and people who are repulsed by it (her sister and my wife.)

When I'm using it I hide the jar behind the toaster so Mo isn't
grossed out.




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It's hard to get high voltage gain from a single transistor with a
resistive collector load. As the resistor goes up, you have to reduce
Ic, so that reduces Ib, and Zin goes up, so there less input signal
current, etc. A current source helps some.

I think the idealized transistor amplifier has a voltage gain that is
40 times the DC voltage drop in the collector resistor, something like
that.



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We had wonderful storms in New Orleans, but the weather in San
Francisco is lame. We don't even get lightning. June 8, 2019, 7AM, the
temperature is 58F.



--  

John Larkin         Highland Technology, Inc

lunatic fringe electronics  


Re: Help designing a low-noise TIA
On a sunny day (Fri, 07 Jun 2019 15:49:26 +0200) it happened Jeroen Belleman

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Yes Zout is not infinete, but depending on the tranny not as low as 10 k.


Re: Help designing a low-noise TIA
On Fri, 07 Jun 2019 13:13:11 GMT, Jan Panteltje

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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  


Re: Help designing a low-noise TIA
On a sunny day (Fri, 07 Jun 2019 06:52:55 -0700) it happened John Larkin

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He wrote:
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 >Jan Panteltje, several kOhm of input impedance will reduce our expected current
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<end quote>

He says 100 pA,  
dunno why u use volts.


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Really?

Did you try that?

Bad specimen? O/C?


Re: Help designing a low-noise TIA
On Friday, June 7, 2019 at 9:53:00 AM UTC-4, John Larkin wrote:
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I'm not sure how to calculate the input impedance..  
(I know, look in AoE.)  
But isn't the problem the emitter resistance.  r_e = 25 mV/Ic
At 1 uA of Ic, r_e = 25 k ohm and the gain is R_c/ r_e = 400.  

Jan, Do you know the Ebers-Moll model of the transistor?  

George H.  

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Re: Help designing a low-noise TIA
On Fri, 7 Jun 2019 11:43:20 -0700 (PDT), George Herold

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Dynamic resistance Re is 25K and Rb is beta times that.  


--  

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement  

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Re: Help designing a low-noise TIA
On Friday, June 7, 2019 at 3:13:09 PM UTC-4, John Larkin wrote:
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Oh, thanks.  That's fairly obvious, now that you've told me. :^)
GH
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