On Tuesday, May 5, 2015 at 9:38:08 AM UTC-4, George Herold wrote:
> On Monday, May 4, 2015 at 9:57:43 PM UTC-4, Winfield Hill wrote:
> > George Herold wrote...
> > > Winfield Hill wrote:
> > >> John Devereux wrote...
> > >>>
> > >>> Linear seem to have some new "interesting" high impedance,
> > >>> high-speed, low capacitance opamps LTC6268 and LTC6268-10.
> > >>> "500MHz" http://www.linear.com/product/LTC6268
> > >>> "4GHz" http://www.linear.com/product/LTC6268-10
> > >>>
> > >>> (but what's with the current noise plots?)
> > >> 
> > >>  LTC's LTC6268 competes with Burr-Brown / TI OPA656,
> > >>  and the decompensated LTC6268-10 with the OPA657.
> > >> 
> > >>  Those are especially useful to make trans-resistance
> > >>  amplifiers, TIA, used with high-speed photo-current
> > >>  signals, etc.  See AoE-III Chapter 8, pages 537-552.
> > >>  Or see Phil Hobbs' book, pages 693-714.
> > >> 
> > >>  The datasheet's current noise plot, which rises with
> > >>  frequency, shows e_n Cin noise:  i_n = 2pi f e_n Cin,
> > >>  see AoE-III sect 8.11.3 and E'qn 8.44.  The business
> > >>  of voltage-noise turning into a rather nasty current
> > >>  noise surprises people not familiar with the concept.
> > >> 
> > >>  For example, the amplifier's intrinsic low current-
> > >>  noise density of 7 fA/rt-Hz turns into a high noise
> > >>  of 4000 fA at 50MHz.  Their graph should have shown
> > >>  the node capacitance value.  We can back-calculate
> > >>  Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF.
> > >>  This is very low, probably without a PD connected.
> > >>  
> > >>  One thing that makes LTC's new amplifier ICs special
> > >>  are very low input capacitance, which is achieved by
> > >>  bootstrapping the protection diodes and other parts
> > >>  of the circuit to follow the input pins.
> > >> 
> > >>  This bootstrap scheme also reduces the input leakage
> > >>  current at high temperatures.  There are a few other
> > >>  amplifiers that can also do this, see AoE Figure 5.6.
> > >
> > > OK, If the current noise is due to e-sub-n-C, in the LT
> > > chip.  Then why is there nothing similar in the OPA656
> > > data sheet? http://www.ti.com/lit/ds/symlink/opa656.pdf
> > > It shows current noise as being absolutely flat.
> > >
> > > I must admit the idea that the bias current is a few fA
> > > and the current noise at ~1MHz is several pA strikes me
> > > as strange. 
> > 
> >  Hah, the reason the graph has a straight line: check out
> >  the draftman's ruler.  We give e_n-Cin-noise crossover
> >  frequency vs. feedback-resistor Johnson noise, E'qn 8.45,
> >  page 539.  The equivalent without a feedback resistor is
> >  fx = i_n / 2pi e_n Cin.  Assuming no external capacitance
> >  that's f_x = 1.3fA/rt-Hz / 2pi 7nV/rt-Hz 3.5pF = 8.5kHz.
> >  I'm sorry, but that's the reality.  Not everyone is aware
> >  of this, and the bench measurements aren't that easy.
> 
> Ahh... OK nothing to do with the bootstrap then.
> So If I plug numbers into my fav opa134 
> i_n= 3fA/rtHz, e_n=8nV/rtHz and C_in = 5 pf(common mode C_in?)
> I get fx = ~12 kHz... 

So I've been thinking about opamp current noise.
And looking at the noise plot for the opa134 pg 4 here. 
http://www.ti.com/lit/ds/symlink/opa134.pdf
(sorry for posting the same spec sheet again, 
I've measured the voltage noise in this opamp
a lot.. it's like an old friend... the voltage noise knee
is often past 1kHz.)

The current knee is not near the 12kHz number I calculated above.
but ~2.5-3kHz.  ~4 times worse, a capacitance of 20 pF! 

So if picking a new photodiode opamp I should pay attention 
to the current noise plot too?  

You know I think I measured this 20pF number.  
(Zero current noise in a PD TIA.)

George H.   


> (about what the data sheet shows...figure at bottom of page 4.)
> 
> 
> > 
> >  The explanation is easy enough: the voltage noise on the
> >  summing junction causes it to move up and down, and the
> >  capacitance on that node needs current to do this, which
> >  is supplied by the op-amp's output, and appears as signal.
> > 
> >  You should read our extensive discussion of this scene,
> >  AoE-III pages 538-552.  Figures 8.74 and 8.79 show some
> >  actual measurements of this troubling phenomena.  Note:
> >  Figure 8.74 is for the OPA655, Burr-Brown's precursor to
> >  the OPA656.  So we're talking apples and apples here.**
> > 
> >  The OPA655 datasheet properly shows the i_n = w e_n Cin
> >  effect, with a crossover frequency of about 5kHz, above
> >  which the current noise rises proportional to frequency.
> > 
> >  Sadly, this is with no external real-world capacitance.
> >  We show in Figure 8.74 the effect of this capacitance.
> >  ** OK, actually Figure 8.74 is with Rf = 1M (and Fig
> >  8.79 is with Rf = 10M).
> 
> Hmm.. OK I remember trying to come up with some means to measure the current noise 
> (of "my" opa134).  So you are measuring it by loading the inverting input with 
> a big Cap?  
> 
> I'll have to spend more time with your noise chapter.  
> I'm getting myself a little confused thinking about it.  
> If this HF current noise is really due to the voltage noise,
> then (it seems to me) the two noises are correlated!  
> (And then I worry about how they should add?) 
> 
> George H.
> > 
> > 
> > -- 
> >  Thanks,
> >     - Win

The real part of the refractive index is very nearly 1.0 for every material at that energy. Eberhard realized that the imaginary part (i.e. the loss) varied much more and could also be used to make thin film mirrors. 

IIRC he used a lot of thin layers of metal, alternating with much thinner layers of carbon. 

Cheers

Phil Hobbs

Den onsdag den 6. maj 2015 kl. 02.20.00 UTC+2 skrev DecadentLinuxUserNumeroUno:
> On Tue, 05 May 2015 19:46:28 -0400, Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> Gave us:
> 
> >On 5/5/2015 7:35 PM, whit3rd wrote:
> >> On Monday, May 4, 2015 at 6:31:14 PM UTC-7, Phil Hobbs wrote:
> >>
> >>> ... It's sad when those hard-earned photons disappear under a sea
> >>> of unnecessary noise.  Especially when your thesis is at stake.
> >>
> >> One solution: use bigger photons.  At 4 keV, there's a hundred ion
> >> pairs for each X-ray photon.   Electronic noise not generally a
> >> problem counting those puppies.
> >>
> >
> >Focusing them can be a bit of a problem, though. ;)
> >
> >(An acquaintance of mine at IBM, Eberhard Spiller, perfected the 
> >normal-incidence X-ray mirror, but it isn't for the faint of heart.)
> >
> >Cheers
> >
> >Phil Hobbs
> 
> 
>   Main feature:  First surface was NOT Aluminum, which X-rays pass
> through like visible light through clear glass.

not at 4keV

-Lasse

On Tue, 05 May 2015 19:46:28 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> Gave us:

>On 5/5/2015 7:35 PM, whit3rd wrote:
>> On Monday, May 4, 2015 at 6:31:14 PM UTC-7, Phil Hobbs wrote:
>>
>>> ... It's sad when those hard-earned photons disappear under a sea
>>> of unnecessary noise.  Especially when your thesis is at stake.
>>
>> One solution: use bigger photons.  At 4 keV, there's a hundred ion
>> pairs for each X-ray photon.   Electronic noise not generally a
>> problem counting those puppies.
>>
>
>Focusing them can be a bit of a problem, though. ;)
>
>(An acquaintance of mine at IBM, Eberhard Spiller, perfected the 
>normal-incidence X-ray mirror, but it isn't for the faint of heart.)
>
>Cheers
>
>Phil Hobbs


  Main feature:  First surface was NOT Aluminum, which X-rays pass
through like visible light through clear glass.

  Probably was Gold.

On 5/5/2015 7:35 PM, whit3rd wrote:
> On Monday, May 4, 2015 at 6:31:14 PM UTC-7, Phil Hobbs wrote:
>
>> ... It's sad when those hard-earned photons disappear under a sea
>> of unnecessary noise.  Especially when your thesis is at stake.
>
> One solution: use bigger photons.  At 4 keV, there's a hundred ion
> pairs for each X-ray photon.   Electronic noise not generally a
> problem counting those puppies.
>

Focusing them can be a bit of a problem, though. ;)

(An acquaintance of mine at IBM, Eberhard Spiller, perfected the 
normal-incidence X-ray mirror, but it isn't for the faint of heart.)

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

On Monday, May 4, 2015 at 6:31:14 PM UTC-7, Phil Hobbs wrote:

> ... It's sad when those hard-earned photons disappear 
> under a sea of unnecessary noise.  Especially when your thesis is at stake.

One solution: use bigger photons.  At 4 keV, there's a hundred ion
pairs for each X-ray photon.   Electronic noise not generally a problem counting
those puppies.

On 5/5/2015 11:55 AM, George Herold wrote:
> On Tuesday, May 5, 2015 at 10:15:39 AM UTC-4, Phil Hobbs wrote:
>> On 05/05/2015 09:38 AM, George Herold wrote:
>>> On Monday, May 4, 2015 at 9:57:43 PM UTC-4, Winfield Hill wrote:
>>>> George Herold wrote...
>>>>> Winfield Hill wrote:
>>>>>> John Devereux wrote...
>
> <snip>
>>> I'll have to spend more time with your noise chapter.
>>> I'm getting myself a little confused thinking about it.
>>> If this HF current noise is really due to the voltage noise,
>>> then (it seems to me) the two noises are correlated!
>>> (And then I worry about how they should add?)
>>
>> Well, ideally they'll be in quadrature at all frequencies, so their dot
>> product will be zero (sin x and cos x are uncorrelated).
>
> Ahh, OK.
> I must admit I find it a bit "mind expanding"
> to think about the two quadratures of the noise
> being uncorrelated.  I mean (taking shot noise as
> an example) both signals come from the random
> generation of one electron.

They do in the time domain, but the frequency-domain noise is from the 
whole ensemble.  Two electrons arriving at times differing by >> 1/BW 
make uncorrelated noise too.

Cheers

Phil Hobbs
>>
>> --
>> Dr Philip C D Hobbs
>> Principal Consultant
>> ElectroOptical Innovations LLC
>> Optics, Electro-optics, Photonics, Analog Electronics
>>
>> 160 North State Road #203
>> Briarcliff Manor NY 10510
>>
>> hobbs at electrooptical dot net
>> http://electrooptical.net
>


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

On Tuesday, May 5, 2015 at 10:15:39 AM UTC-4, Phil Hobbs wrote:
> On 05/05/2015 09:38 AM, George Herold wrote:
> > On Monday, May 4, 2015 at 9:57:43 PM UTC-4, Winfield Hill wrote:
> >> George Herold wrote...
> >>> Winfield Hill wrote:
> >>>> John Devereux wrote...

<snip>
> > I'll have to spend more time with your noise chapter.
> > I'm getting myself a little confused thinking about it.
> > If this HF current noise is really due to the voltage noise,
> > then (it seems to me) the two noises are correlated!
> > (And then I worry about how they should add?)
> 
> Well, ideally they'll be in quadrature at all frequencies, so their dot 
> product will be zero (sin x and cos x are uncorrelated).

Ahh, OK.  
I must admit I find it a bit "mind expanding" 
to think about the two quadratures of the noise 
being uncorrelated.  I mean (taking shot noise as 
an example) both signals come from the random
generation of one electron.  

George H.
> 
> Cheers
> 
> Phil Hobbs
> 
> 
> -- 
> Dr Philip C D Hobbs
> Principal Consultant
> ElectroOptical Innovations LLC
> Optics, Electro-optics, Photonics, Analog Electronics
> 
> 160 North State Road #203
> Briarcliff Manor NY 10510
> 
> hobbs at electrooptical dot net
> http://electrooptical.net

On 05/05/2015 09:38 AM, George Herold wrote:
> On Monday, May 4, 2015 at 9:57:43 PM UTC-4, Winfield Hill wrote:
>> George Herold wrote...
>>> Winfield Hill wrote:
>>>> John Devereux wrote...
>>>>>
>>>>> Linear seem to have some new "interesting" high impedance,
>>>>> high-speed, low capacitance opamps LTC6268 and LTC6268-10.
>>>>> "500MHz" http://www.linear.com/product/LTC6268
>>>>> "4GHz" http://www.linear.com/product/LTC6268-10
>>>>>
>>>>> (but what's with the current noise plots?)
>>>>
>>>>   LTC's LTC6268 competes with Burr-Brown / TI OPA656,
>>>>   and the decompensated LTC6268-10 with the OPA657.
>>>>
>>>>   Those are especially useful to make trans-resistance
>>>>   amplifiers, TIA, used with high-speed photo-current
>>>>   signals, etc.  See AoE-III Chapter 8, pages 537-552.
>>>>   Or see Phil Hobbs' book, pages 693-714.
>>>>
>>>>   The datasheet's current noise plot, which rises with
>>>>   frequency, shows e_n Cin noise:  i_n = 2pi f e_n Cin,
>>>>   see AoE-III sect 8.11.3 and E'qn 8.44.  The business
>>>>   of voltage-noise turning into a rather nasty current
>>>>   noise surprises people not familiar with the concept.
>>>>
>>>>   For example, the amplifier's intrinsic low current-
>>>>   noise density of 7 fA/rt-Hz turns into a high noise
>>>>   of 4000 fA at 50MHz.  Their graph should have shown
>>>>   the node capacitance value.  We can back-calculate
>>>>   Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF.
>>>>   This is very low, probably without a PD connected.
>>>>
>>>>   One thing that makes LTC's new amplifier ICs special
>>>>   are very low input capacitance, which is achieved by
>>>>   bootstrapping the protection diodes and other parts
>>>>   of the circuit to follow the input pins.
>>>>
>>>>   This bootstrap scheme also reduces the input leakage
>>>>   current at high temperatures.  There are a few other
>>>>   amplifiers that can also do this, see AoE Figure 5.6.
>>>
>>> OK, If the current noise is due to e-sub-n-C, in the LT
>>> chip.  Then why is there nothing similar in the OPA656
>>> data sheet? http://www.ti.com/lit/ds/symlink/opa656.pdf
>>> It shows current noise as being absolutely flat.
>>>
>>> I must admit the idea that the bias current is a few fA
>>> and the current noise at ~1MHz is several pA strikes me
>>> as strange.
>>
>>   Hah, the reason the graph has a straight line: check out
>>   the draftman's ruler.  We give e_n-Cin-noise crossover
>>   frequency vs. feedback-resistor Johnson noise, E'qn 8.45,
>>   page 539.  The equivalent without a feedback resistor is
>>   fx = i_n / 2pi e_n Cin.  Assuming no external capacitance
>>   that's f_x = 1.3fA/rt-Hz / 2pi 7nV/rt-Hz 3.5pF = 8.5kHz.
>>   I'm sorry, but that's the reality.  Not everyone is aware
>>   of this, and the bench measurements aren't that easy.
>
> Ahh... OK nothing to do with the bootstrap then.
> So If I plug numbers into my fav opa134
> i_n= 3fA/rtHz, e_n=8nV/rtHz and C_in = 5 pf(common mode C_in?)
> I get fx = ~12 kHz...
> http://www.ti.com/lit/ds/symlink/opa134.pdf
> (about what the data sheet shows...figure at bottom of page 4.)
>
>
>>
>>   The explanation is easy enough: the voltage noise on the
>>   summing junction causes it to move up and down, and the
>>   capacitance on that node needs current to do this, which
>>   is supplied by the op-amp's output, and appears as signal.
>>
>>   You should read our extensive discussion of this scene,
>>   AoE-III pages 538-552.  Figures 8.74 and 8.79 show some
>>   actual measurements of this troubling phenomena.  Note:
>>   Figure 8.74 is for the OPA655, Burr-Brown's precursor to
>>   the OPA656.  So we're talking apples and apples here.**
>>
>>   The OPA655 datasheet properly shows the i_n = w e_n Cin
>>   effect, with a crossover frequency of about 5kHz, above
>>   which the current noise rises proportional to frequency.
>>
>>   Sadly, this is with no external real-world capacitance.
>>   We show in Figure 8.74 the effect of this capacitance.
>>   ** OK, actually Figure 8.74 is with Rf = 1M (and Fig
>>   8.79 is with Rf = 10M).
>
> Hmm.. OK I remember trying to come up with some means to measure the current noise
> (of "my" opa134).  So you are measuring it by loading the inverting input with
> a big Cap?
>
> I'll have to spend more time with your noise chapter.
> I'm getting myself a little confused thinking about it.
> If this HF current noise is really due to the voltage noise,
> then (it seems to me) the two noises are correlated!
> (And then I worry about how they should add?)

Well, ideally they'll be in quadrature at all frequencies, so their dot 
product will be zero (sin x and cos x are uncorrelated).

Cheers

Phil Hobbs


-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

On Monday, May 4, 2015 at 9:57:43 PM UTC-4, Winfield Hill wrote:
> George Herold wrote...
> > Winfield Hill wrote:
> >> John Devereux wrote...
> >>>
> >>> Linear seem to have some new "interesting" high impedance,
> >>> high-speed, low capacitance opamps LTC6268 and LTC6268-10.
> >>> "500MHz" http://www.linear.com/product/LTC6268
> >>> "4GHz" http://www.linear.com/product/LTC6268-10
> >>>
> >>> (but what's with the current noise plots?)
> >> 
> >>  LTC's LTC6268 competes with Burr-Brown / TI OPA656,
> >>  and the decompensated LTC6268-10 with the OPA657.
> >> 
> >>  Those are especially useful to make trans-resistance
> >>  amplifiers, TIA, used with high-speed photo-current
> >>  signals, etc.  See AoE-III Chapter 8, pages 537-552.
> >>  Or see Phil Hobbs' book, pages 693-714.
> >> 
> >>  The datasheet's current noise plot, which rises with
> >>  frequency, shows e_n Cin noise:  i_n = 2pi f e_n Cin,
> >>  see AoE-III sect 8.11.3 and E'qn 8.44.  The business
> >>  of voltage-noise turning into a rather nasty current
> >>  noise surprises people not familiar with the concept.
> >> 
> >>  For example, the amplifier's intrinsic low current-
> >>  noise density of 7 fA/rt-Hz turns into a high noise
> >>  of 4000 fA at 50MHz.  Their graph should have shown
> >>  the node capacitance value.  We can back-calculate
> >>  Cin = i_n / 2pi f e_n = 4pA / 314M 4.3nV = 3 pF.
> >>  This is very low, probably without a PD connected.
> >>  
> >>  One thing that makes LTC's new amplifier ICs special
> >>  are very low input capacitance, which is achieved by
> >>  bootstrapping the protection diodes and other parts
> >>  of the circuit to follow the input pins.
> >> 
> >>  This bootstrap scheme also reduces the input leakage
> >>  current at high temperatures.  There are a few other
> >>  amplifiers that can also do this, see AoE Figure 5.6.
> >
> > OK, If the current noise is due to e-sub-n-C, in the LT
> > chip.  Then why is there nothing similar in the OPA656
> > data sheet? http://www.ti.com/lit/ds/symlink/opa656.pdf
> > It shows current noise as being absolutely flat.
> >
> > I must admit the idea that the bias current is a few fA
> > and the current noise at ~1MHz is several pA strikes me
> > as strange. 
> 
>  Hah, the reason the graph has a straight line: check out
>  the draftman's ruler.  We give e_n-Cin-noise crossover
>  frequency vs. feedback-resistor Johnson noise, E'qn 8.45,
>  page 539.  The equivalent without a feedback resistor is
>  fx = i_n / 2pi e_n Cin.  Assuming no external capacitance
>  that's f_x = 1.3fA/rt-Hz / 2pi 7nV/rt-Hz 3.5pF = 8.5kHz.
>  I'm sorry, but that's the reality.  Not everyone is aware
>  of this, and the bench measurements aren't that easy.

Ahh... OK nothing to do with the bootstrap then.
So If I plug numbers into my fav opa134 
i_n= 3fA/rtHz, e_n=8nV/rtHz and C_in = 5 pf(common mode C_in?)
I get fx = ~12 kHz... 
http://www.ti.com/lit/ds/symlink/opa134.pdf
(about what the data sheet shows...figure at bottom of page 4.)


> 
>  The explanation is easy enough: the voltage noise on the
>  summing junction causes it to move up and down, and the
>  capacitance on that node needs current to do this, which
>  is supplied by the op-amp's output, and appears as signal.
> 
>  You should read our extensive discussion of this scene,
>  AoE-III pages 538-552.  Figures 8.74 and 8.79 show some
>  actual measurements of this troubling phenomena.  Note:
>  Figure 8.74 is for the OPA655, Burr-Brown's precursor to
>  the OPA656.  So we're talking apples and apples here.**
> 
>  The OPA655 datasheet properly shows the i_n = w e_n Cin
>  effect, with a crossover frequency of about 5kHz, above
>  which the current noise rises proportional to frequency.
> 
>  Sadly, this is with no external real-world capacitance.
>  We show in Figure 8.74 the effect of this capacitance.
>  ** OK, actually Figure 8.74 is with Rf = 1M (and Fig
>  8.79 is with Rf = 10M).

Hmm.. OK I remember trying to come up with some means to measure the current noise 
(of "my" opa134).  So you are measuring it by loading the inverting input with 
a big Cap?  

I'll have to spend more time with your noise chapter.  
I'm getting myself a little confused thinking about it.  
If this HF current noise is really due to the voltage noise,
then (it seems to me) the two noises are correlated!  
(And then I worry about how they should add?) 

George H.
> 
> 
> -- 
>  Thanks,
>     - Win