On Tue, 22 Aug 2017 10:59:13 -0700 (PDT), George Herold
<gherold@teachspin.com> wrote:

>On Tuesday, August 22, 2017 at 11:17:49 AM UTC-4, John Larkin wrote:
>> On Tue, 22 Aug 2017 15:54:23 +0100, David Nadlinger
>> <david@klickverbot.at> wrote:
>> 
>> >Hi Phil (and others),
>> >
>> >First of all, thank you very much for your message, and sorry for this 
>> >late reply &#4294967295; I was traveling abroad and had trouble accessing my news 
>> >server.
>> >
>> >On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
>> >> One approach is to switch the whole front end and not just the feedback resistor.
>> >
>> >I initially considered switching the entire frontend as a way of 
>> >tolerating more parasitic capacitance (vs. switching the TIA feedback 
>> >impedance). But after figuring out a workable configuration for that, it 
>> >seemed like sharing the frontend would be the less complex option (which 
>> >is handy given the small target PCB size).
>> >
>> >I suppose splitting up the frontends would allow me to trade off voltage 
>> >noise vs. current noise on the lower gain range, though, or go for a 
>> >higher 1/f knee but lower wideband noise, and would also make it easier 
>> >to add in an "ideal" output lowpass filter.
>> >
>> >> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the 
>> >FET up to about 100 MHz.
>> >
>> >Yep, this is what I was (probably in error) referring to as a 
>> >complementary feedback pair. In fact I was even going to try the BFT92, 
>> >as I have some BF{R,T}92s lying around. (They seem to be a good starting 
>> >point for low-noise, medium-speed things like this &#4294967295; any other 
>> >favourites I should be aware of?)
>> >
>> >[&#4294967295;]
>> >
>> >Okay, this part was supposed to be a further explanation of how I 
>> >couldn't get the loop stable in SPICE once I put the follower or 
>> >wraparound in. However, I had only tried follower designs that were 
>> >stable (and didn't peak beyond unity gain) in isolation before. Just 
>> >going with a "bare" PNP wraparound without any capacitors to roll off 
>> >the loop gain seems to work nicely in the full circuit in SPICE. Even 
>> >with a simple resistor load in the source, the performance looks to be 
>> >adequate. (For posterity, the circuit in SPICE: 
>> >http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)
>> >
>> >Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and 
>> >using active current sinks it looks like I could push the design past 
>> >200 MHz with 40 pF of input capacitance, but that's entirely unnecessary 
>> >for the application.
>> >
>> >I'll have to have a closer look at the biasing given device variations 
>> >and the drift performance once I've built the thing, but due to a 
>> >mess-up in our department's finance office I've been waiting for a 
>> >Digi-Key order to arrive for more than a month now&#4294967295;
>> >
>> >> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
>> >> 
>> >> The result is a pretty good follower out to a gigahertz or so.
>> >
>> >Although overkill for this application, I'll definitely keep that one in 
>> >my bag of tricks. I presume you prefer the BLM18BB beads because of 
>> >their wide resistive region to higher frequencies? I'm also not sure I 
>> >understand how to build a unity gain follower from it, but I'll look 
>> >into it more closely some other day.
>> >
>> >Cheers!
>> >
>> >  &#4294967295; David
>> 
>> 
>> Here's a circuit that switches a photodiode between two amps. It
>> uses's Phil's bootstrap cascode arrangement, and switches two cascode
>> transistors to steer the pd current.
>> 
>> https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1
>That's nice, Thanks.  
>The stuff down the bottom (labeled comp.) is to take care of the 
>bias current of the cascode?
>
>George H.     

R12 bleeds a little current in to the cascode transistor to keep it
on, low emitter impedance, at low photodiode currents. That creates a
DC offset error. R6 and Q1 create a simulated equivalent current that
is used to cancel the offset. Cute but overkill maybe.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Tuesday, August 22, 2017 at 8:17:49 AM UTC-7, John Larkin wrote:
> https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1

Nifty.  Are AD8034s OK driving 10 uf like that, though?  

I had one customer who used a setup like U3A/B to share a low-
noise reference with several ADCs.  It oscillated big-time, but 
they actually got away with it because the effect was common to 
all of the channels.

-- john, KE5FX

On Tuesday, August 22, 2017 at 11:17:49 AM UTC-4, John Larkin wrote:
> On Tue, 22 Aug 2017 15:54:23 +0100, David Nadlinger
> <david@klickverbot.at> wrote:
> 
> >Hi Phil (and others),
> >
> >First of all, thank you very much for your message, and sorry for this 
> >late reply &ndash; I was traveling abroad and had trouble accessing my news 
> >server.
> >
> >On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
> >> One approach is to switch the whole front end and not just the feedback resistor.
> >
> >I initially considered switching the entire frontend as a way of 
> >tolerating more parasitic capacitance (vs. switching the TIA feedback 
> >impedance). But after figuring out a workable configuration for that, it 
> >seemed like sharing the frontend would be the less complex option (which 
> >is handy given the small target PCB size).
> >
> >I suppose splitting up the frontends would allow me to trade off voltage 
> >noise vs. current noise on the lower gain range, though, or go for a 
> >higher 1/f knee but lower wideband noise, and would also make it easier 
> >to add in an "ideal" output lowpass filter.
> >
> >> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the 
> >FET up to about 100 MHz.
> >
> >Yep, this is what I was (probably in error) referring to as a 
> >complementary feedback pair. In fact I was even going to try the BFT92, 
> >as I have some BF{R,T}92s lying around. (They seem to be a good starting 
> >point for low-noise, medium-speed things like this &ndash; any other 
> >favourites I should be aware of?)
> >
> >[&hellip;]
> >
> >Okay, this part was supposed to be a further explanation of how I 
> >couldn't get the loop stable in SPICE once I put the follower or 
> >wraparound in. However, I had only tried follower designs that were 
> >stable (and didn't peak beyond unity gain) in isolation before. Just 
> >going with a "bare" PNP wraparound without any capacitors to roll off 
> >the loop gain seems to work nicely in the full circuit in SPICE. Even 
> >with a simple resistor load in the source, the performance looks to be 
> >adequate. (For posterity, the circuit in SPICE: 
> >http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)
> >
> >Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and 
> >using active current sinks it looks like I could push the design past 
> >200 MHz with 40 pF of input capacitance, but that's entirely unnecessary 
> >for the application.
> >
> >I'll have to have a closer look at the biasing given device variations 
> >and the drift performance once I've built the thing, but due to a 
> >mess-up in our department's finance office I've been waiting for a 
> >Digi-Key order to arrive for more than a month now&hellip;
> >
> >> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
> >> 
> >> The result is a pretty good follower out to a gigahertz or so.
> >
> >Although overkill for this application, I'll definitely keep that one in 
> >my bag of tricks. I presume you prefer the BLM18BB beads because of 
> >their wide resistive region to higher frequencies? I'm also not sure I 
> >understand how to build a unity gain follower from it, but I'll look 
> >into it more closely some other day.
> >
> >Cheers!
> >
> >  &mdash; David
> 
> 
> Here's a circuit that switches a photodiode between two amps. It
> uses's Phil's bootstrap cascode arrangement, and switches two cascode
> transistors to steer the pd current.
> 
> https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1
That's nice, Thanks.  
The stuff down the bottom (labeled comp.) is to take care of the 
bias current of the cascode?

George H.     
> 
> This worked fine; the real problem was the customer.
> 
> https://www.dropbox.com/s/4lo0wfz5xrsjn90/PT10.JPG?raw=1
> 
> 
> -- 
> 
> John Larkin         Highland Technology, Inc
> 
> lunatic fringe electronics

On Tue, 22 Aug 2017 15:54:23 +0100, David Nadlinger
<david@klickverbot.at> wrote:

>Hi Phil (and others),
>
>First of all, thank you very much for your message, and sorry for this 
>late reply &#4294967295; I was traveling abroad and had trouble accessing my news 
>server.
>
>On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
>> One approach is to switch the whole front end and not just the feedback resistor.
>
>I initially considered switching the entire frontend as a way of 
>tolerating more parasitic capacitance (vs. switching the TIA feedback 
>impedance). But after figuring out a workable configuration for that, it 
>seemed like sharing the frontend would be the less complex option (which 
>is handy given the small target PCB size).
>
>I suppose splitting up the frontends would allow me to trade off voltage 
>noise vs. current noise on the lower gain range, though, or go for a 
>higher 1/f knee but lower wideband noise, and would also make it easier 
>to add in an "ideal" output lowpass filter.
>
>> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the 
>FET up to about 100 MHz.
>
>Yep, this is what I was (probably in error) referring to as a 
>complementary feedback pair. In fact I was even going to try the BFT92, 
>as I have some BF{R,T}92s lying around. (They seem to be a good starting 
>point for low-noise, medium-speed things like this &#4294967295; any other 
>favourites I should be aware of?)
>
>[&#4294967295;]
>
>Okay, this part was supposed to be a further explanation of how I 
>couldn't get the loop stable in SPICE once I put the follower or 
>wraparound in. However, I had only tried follower designs that were 
>stable (and didn't peak beyond unity gain) in isolation before. Just 
>going with a "bare" PNP wraparound without any capacitors to roll off 
>the loop gain seems to work nicely in the full circuit in SPICE. Even 
>with a simple resistor load in the source, the performance looks to be 
>adequate. (For posterity, the circuit in SPICE: 
>http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)
>
>Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and 
>using active current sinks it looks like I could push the design past 
>200 MHz with 40 pF of input capacitance, but that's entirely unnecessary 
>for the application.
>
>I'll have to have a closer look at the biasing given device variations 
>and the drift performance once I've built the thing, but due to a 
>mess-up in our department's finance office I've been waiting for a 
>Digi-Key order to arrive for more than a month now&#4294967295;
>
>> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar.  As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
>> 
>> The result is a pretty good follower out to a gigahertz or so.
>
>Although overkill for this application, I'll definitely keep that one in 
>my bag of tricks. I presume you prefer the BLM18BB beads because of 
>their wide resistive region to higher frequencies? I'm also not sure I 
>understand how to build a unity gain follower from it, but I'll look 
>into it more closely some other day.
>
>Cheers!
>
>  &#4294967295; David


Here's a circuit that switches a photodiode between two amps. It
uses's Phil's bootstrap cascode arrangement, and switches two cascode
transistors to steer the pd current.

https://www.dropbox.com/s/xczb3j6td9vmo69/J712B.pdf?raw=1

This worked fine; the real problem was the customer.

https://www.dropbox.com/s/4lo0wfz5xrsjn90/PT10.JPG?raw=1


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics

Hi Phil (and others),

First of all, thank you very much for your message, and sorry for this
late reply &ndash; I was traveling abroad and had trouble accessing my news
server.

On 29.07.17 5:51 PM, pcdhobbs@gmail.com wrote:
> One approach is to switch the whole front end and not just the feedback resistor.

I initially considered switching the entire frontend as a way of
tolerating more parasitic capacitance (vs. switching the TIA feedback
impedance). But after figuring out a workable configuration for that, it
seemed like sharing the frontend would be the less complex option (which
is handy given the small target PCB size).

I suppose splitting up the frontends would allow me to trade off voltage
noise vs. current noise on the lower gain range, though, or go for a
higher 1/f knee but lower wideband noise, and would also make it easier
to add in an "ideal" output lowpass filter.

> The BF862 is slower than the op amp.I've had good luck with a BFT92 PNP wraparound (shunt feedback) on the
FET up to about 100 MHz.

Yep, this is what I was (probably in error) referring to as a
complementary feedback pair. In fact I was even going to try the BFT92,
as I have some BF{R,T}92s lying around. (They seem to be a good starting
point for low-noise, medium-speed things like this &ndash; any other
favourites I should be aware of?)

[&hellip;]

Okay, this part was supposed to be a further explanation of how I
couldn't get the loop stable in SPICE once I put the follower or
wraparound in. However, I had only tried follower designs that were
stable (and didn't peak beyond unity gain) in isolation before. Just
going with a "bare" PNP wraparound without any capacitors to roll off
the loop gain seems to work nicely in the full circuit in SPICE. Even
with a simple resistor load in the source, the performance looks to be
adequate. (For posterity, the circuit in SPICE:
http://klickverbot.at/science/sed/ada4817_bootstrap_wraparound_1.png.)

Bootstrapping the drain with a BFR92A from a BFR92A emitter follower and
using active current sinks it looks like I could push the design past
200 MHz with 40 pF of input capacitance, but that's entirely unnecessary
for the application.

I'll have to have a closer look at the biasing given device variations
and the drift performance once I've built the thing, but due to a
mess-up in our department's finance office I've been waiting for a
Digi-Key order to arrive for more than a month now&hellip;

> For faster stuff I like cascoding an ATF38143 pHEMT with a BFP640 SiGe:C bipolar. As a follower you'd need to bootstrap the drain, i.e. AC couple the BJT's base to the pHEMT's source. Use a BLM18BB-series 5- or 10-ohm bead in series with the base to keep the BJT from oscillating.
>
> The result is a pretty good follower out to a gigahertz or so.

Although overkill for this application, I'll definitely keep that one in
my bag of tricks. I presume you prefer the BLM18BB beads because of
their wide resistive region to higher frequencies? I'm also not sure I
understand how to build a unity gain follower from it, but I'll look
into it more closely some other day.

Cheers!

&mdash; David

On 08/03/2017 04:50 PM, Phil Hobbs wrote:
> On 08/03/2017 04:00 PM, Winfield Hill wrote:
>> David Nadlinger wrote...
>>>
>>> I am currently working on a range-switched photodiode design for use in 
>>> our laboratory [1]. It took me a while, but I've arrived at a design I'm 
>>> fairly happy with (in no small part thanks to Phil Hobbs' helpful 
>>> writings on the topic):
>>>
>>> My current draft uses a simple BF862 source follower at the summing node 
>>> of a ADA4817-based 2 M&Icirc;&copy;/2 k&Icirc;&copy; transimpedance amplifier to bootstrap the 
>>> 40 pF of photodiode capacitance (its output being AC-coupled into the PD 
>>> bias node). On paper and in SPICE, I am getting just over 2 MHz 
>>> bandwidth on the 2 M&Icirc;&copy; range at reasonable noise levels (limited by the 
>>> single BF862's e_n), assuming fairly realistic parasitics. On the lower 
>>> gain range, however, I'm only predicting ~75 MHz of bandwidth, as the 
>>> BF862 stage quickly loses steam driving the 40 pF load due to its output 
>>> impedance.
>>
>>  You want to add an emitter-follower to the BF862, to lower Zout
>>  and isolate it from the PD capacitance its driving.  You'll want
>>  a low e_n for that transistor.  For example, I use an FMMT718
>>  running at 2 mA.  I measure a 45MHz bandwidth for the bootstrap. 
>>
>>  Please check in AoE III, chapter 8, for details on all of the
>>  above, valuable e_n measurements, and other TIA amp tricks. 
>>
>>  BTW, I don't agree with Phil concerning these TIA bootstraps;
>>  one does fine for any gain over 0.95 or so, because you're
>>  reducing the PD's effective capacitance by 10 to 20x, and
>>  thus its en-C effects against the more noisy op-amp, which
>>  is already probably more than you need.
> 
> It's a lot easier to get a smooth, predictable transfer function and low
> phase  whoopdedoos with a super good bootstrap, though, particularly
> with high capacitance.  Otherwise you have to try cancelling the
> residual RC pole in the second stage, which generally works OK except
> that the cancellation is never perfect and hence you get late-time
> settling artifacts.
> 
> Plus I often like to use high slew (and thus noisy) amps such as the
> LM6171 as the TIA because it greatly reduces artifacts due to slew
> limiting.  In my designs the first couple of stages are much faster than
> the overall TIA, because you never know what sort of nasty sharp pulses
> the customer is going to send into the photodiode.

One other thing: I much prefer the PNP wraparound trick to adding an
emitter follower, because it doesn't degrade the noise the same way.

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 08/03/2017 04:00 PM, Winfield Hill wrote:
> David Nadlinger wrote...
>>
>> I am currently working on a range-switched photodiode design for use in 
>> our laboratory [1]. It took me a while, but I've arrived at a design I'm 
>> fairly happy with (in no small part thanks to Phil Hobbs' helpful 
>> writings on the topic):
>>
>> My current draft uses a simple BF862 source follower at the summing node 
>> of a ADA4817-based 2 M&Icirc;&copy;/2 k&Icirc;&copy; transimpedance amplifier to bootstrap the 
>> 40 pF of photodiode capacitance (its output being AC-coupled into the PD 
>> bias node). On paper and in SPICE, I am getting just over 2 MHz 
>> bandwidth on the 2 M&Icirc;&copy; range at reasonable noise levels (limited by the 
>> single BF862's e_n), assuming fairly realistic parasitics. On the lower 
>> gain range, however, I'm only predicting ~75 MHz of bandwidth, as the 
>> BF862 stage quickly loses steam driving the 40 pF load due to its output 
>> impedance.
> 
>  You want to add an emitter-follower to the BF862, to lower Zout
>  and isolate it from the PD capacitance its driving.  You'll want
>  a low e_n for that transistor.  For example, I use an FMMT718
>  running at 2 mA.  I measure a 45MHz bandwidth for the bootstrap. 
> 
>  Please check in AoE III, chapter 8, for details on all of the
>  above, valuable e_n measurements, and other TIA amp tricks. 
> 
>  BTW, I don't agree with Phil concerning these TIA bootstraps;
>  one does fine for any gain over 0.95 or so, because you're
>  reducing the PD's effective capacitance by 10 to 20x, and
>  thus its en-C effects against the more noisy op-amp, which
>  is already probably more than you need.

It's a lot easier to get a smooth, predictable transfer function and low
phase  whoopdedoos with a super good bootstrap, though, particularly
with high capacitance.  Otherwise you have to try cancelling the
residual RC pole in the second stage, which generally works OK except
that the cancellation is never perfect and hence you get late-time
settling artifacts.

Plus I often like to use high slew (and thus noisy) amps such as the
LM6171 as the TIA because it greatly reduces artifacts due to slew
limiting.  In my designs the first couple of stages are much faster than
the overall TIA, because you never know what sort of nasty sharp pulses
the customer is going to send into the photodiode.

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

David Nadlinger wrote...
>
> I am currently working on a range-switched photodiode design for use in 
> our laboratory [1]. It took me a while, but I've arrived at a design I'm 
> fairly happy with (in no small part thanks to Phil Hobbs' helpful 
> writings on the topic):
>
> My current draft uses a simple BF862 source follower at the summing node 
> of a ADA4817-based 2 M&Omega;/2 k&Omega; transimpedance amplifier to bootstrap the 
> 40 pF of photodiode capacitance (its output being AC-coupled into the PD 
> bias node). On paper and in SPICE, I am getting just over 2 MHz 
> bandwidth on the 2 M&Omega; range at reasonable noise levels (limited by the 
> single BF862's e_n), assuming fairly realistic parasitics. On the lower 
> gain range, however, I'm only predicting ~75 MHz of bandwidth, as the 
> BF862 stage quickly loses steam driving the 40 pF load due to its output 
> impedance.

 You want to add an emitter-follower to the BF862, to lower Zout
 and isolate it from the PD capacitance its driving.  You'll want
 a low e_n for that transistor.  For example, I use an FMMT718
 running at 2 mA.  I measure a 45MHz bandwidth for the bootstrap. 

 Please check in AoE III, chapter 8, for details on all of the
 above, valuable e_n measurements, and other TIA amp tricks. 

 BTW, I don't agree with Phil concerning these TIA bootstraps;
 one does fine for any gain over 0.95 or so, because you're
 reducing the PD's effective capacitance by 10 to 20x, and
 thus its en-C effects against the more noisy op-amp, which
 is already probably more than you need.


-- 
 Thanks,
    - Win

On Sunday, July 30, 2017 at 10:44:23 AM UTC-4, David Nadlinger wrote:
> On 28.07.17 4:42 PM, George Herold wrote:
> > Sounds nice.  (2MHz at 2 Meg ohm.)
> 
> It certainly looks like it will turn out nicer than what you can buy off 
> the people who bribe physicists with red snack boxes, yes. 1 MHz would 
> also be acceptable for the application, so I'm quite optimistic that 
> I'll be able to make it work.
> 
> > How are you switching the gain?
> 
> I'm using three semiconductor switches in a T configuration to switch 
> the low-impedance feedback path in and out (the middle leg dragging the 
> trace to ground when not used to reduce feedthrough). Getting the 
> parasitics low enough to not degrade the 2 M&Omega; range was a bit of a 
> challenge, but it turns out to be just about achievable without using 
> (comparatively) huge relays.
> 
> > Maybe three orders of magnitude is too much.
> 
> Perhaps I should have made this clearer in the original post, but it's 
> not actually the range switching that is the problem. I merely mentioned 
> it to provide some background for the design choices I made, as some of 
> the tradeoffs might be different if I only had a single 2 k&Omega; range to 
> worry about.

Oh I was asking for my own edification.  I've been recently trying to 
speed up my PD and the switch/stray/ capacitance (grayhill rotary 
switch) is enough such that I need no compensating cap. at 100k ohm
with a 3dB point of ~800kHz (? data book is at work).. big PD ~130 pF @10V... 


> 
> > Could you use a second ADA4817 to bootstrap the PD?
> > (I'm mostly a brute force type.)
> 
> I could, but this would increase the system noise by a factor of about 
> five &ndash; the ~0.8 nV/rtHz voltage noise of the BF862 across the 40 pF 
> photodiode capacitance already dominates the design, so the 4 nV/rtHz of 
> the ADA4817 look quite bad. The input capacitance of the op-amp is also 
> higher than that of a BF862 source follower, let alone a bootstrapped one.

Huh, (Thanks)
I was thinking it'd only double the noise of the opamp.   I hadn't 
thought about the bootstrap cutting down the v_noise*C_in term,
but only about more speed.  

That does mean (I think) that if I bootstrap my circuit with an opamp
it'll only increase the noise a bit.  

4nV/rtHz is not too bad... (my opamp is much worse, 
8nV.. four times as bad in my thinking... noise should be 
expressed as a density. V^2/Hz and not these weird EE units. :^)

George H.

George h.  
> 
>   &mdash; David