How to keep voltage stable in the sub-100nV range?
In message 5818975F.8000603@yandex.com, Charles Steinmetz writes:
Of course, having to wait for settling to within 24 bits will put a real
limit on how fast you can change the voltage (how long you have to wait
after commanding a change before you can rely on the output voltage).
You can speed it up a LOT by intentionally overshooting the DAC for a
short period, provided you have very carefully calibrated the time
constants of the LP filters after it.
BTW: A good place to get some inspiration is the HP3245A service manual.
--
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phk@FreeBSD.ORG | TCP/IP since RFC 956
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Never attribute to malice what can adequately be explained by incompetence.
In message 1c4e84e7-4bfe-4e7d-dc2a-1771310efd74@mounty.de, Andreas Bergmann writes:
Hello David,
On 01.11.2016 15:50, David wrote:
avoid Mylar/polyester/PET and high dielectric constant ceramic
capacitors.
Do you have some specific recommendations/suggestions what to use?
Polypropylene? PTFE? Wet Tantalum capacitors (extreme expensive)?
Mica or Teflon
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
David wrote:
avoid Mylar/polyester/PET and high dielectric
constant ceramic capacitors.
Andreas wrote:
Do you have some specific recommendations/suggestions what to use?
PTFE, polystyrene, or polypropylene (in descending order of desirability
for this application).
In some cases, the lowest permittivity ceramics (NP0/C0G) might be OK,
but they would not be my first choice due to higher dielectric
absorption and susceptability to microphonics. Mica has significantly
higher dielectric absorption, so it is not a good choice when settling
to a very small tolerance is required.
Best regards,
Charles
On Wed, 2 Nov 2016 06:56:46 +0100, you wrote:
Hello David,
On 01.11.2016 15:50, David wrote:
avoid Mylar/polyester/PET and high dielectric constant ceramic
capacitors.
Do you have some specific recommendations/suggestions what to use?
Polypropylene? PTFE? Wet Tantalum capacitors (extreme expensive)?
Thanks,
Andreas
Wet tantalums might be the best for very long time constants but are
useless for settling times below hours to days. Some low leakage
aluminum electrolytics are just as good.
Charles pretty much covered it but I am not sure that polypropylene is
always worse than polystyrene. Some of the other plastic films are
pretty good also but polypropylene is the most common high performance
option. Teflon is the best but is also expensive and has poor
availablity.
When I looked into this a couple months ago in connection with
building a long time constant integrator for an analog only GPSDO, the
primary limitation was insulation resistance of the capacitor. Even
the best low dielectric constant C0G/NP0 capacitors were much worse
than polypropylene film which was itself was much better than
polyester.
In message dfoj1c5tqhjtsj8mjgkfo95fvs8gf8ml69@4ax.com, David writes:
When I looked into this a couple months ago in connection with
building a long time constant integrator for an analog only GPSDO, the
primary limitation was insulation resistance of the capacitor. Even
the best low dielectric constant C0G/NP0 capacitors were much worse
than polypropylene film which was itself was much better than
polyester.
We should add that there is a LOT of good wisdom about a lot of relevant
electronic details in r3 of Art of Electronics.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
On Wed, 02 Nov 2016 08:44:18 -0500
David davidwhess@gmail.com wrote:
Charles pretty much covered it but I am not sure that polypropylene is
always worse than polystyrene. Some of the other plastic films are
pretty good also but polypropylene is the most common high performance
option. Teflon is the best but is also expensive and has poor
availablity.
Polyphenyle sulfide (PPS) seems to be the new kid on the block
with quite good performance values (dielectric absorbtion, moisture
and temperature dependence) and quite readily available.
As with all film capacitors: be carefull and quick when handsoldering.
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
On 02.11.2016 14:44, David wrote:
Teflon is the best but is also expensive and has poor
availablity.
Quick check on ebay:
http://www.ebay.com/bhp/teflon-capacitor
One pair for $150.-? wohooo ....
Is this a part of the audiophoolery community?
Andreas
In message f3847ee8-a5ec-099c-91b1-d7c0b17599d4@mounty.de, Andreas Bergmann writes:
On 02.11.2016 14:44, David wrote:
Teflon is the best but is also expensive and has poor
availablity.
Quick check on ebay:
http://www.ebay.com/bhp/teflon-capacitor
One pair for $150.-? wohooo ....
Is this a part of the audiophoolery community?
Yes, they are very much in demand for audiohomoepathy, up to the
point where people actually roll their own.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
On Wed, 2 Nov 2016 00:33:32 +0100, you wrote:
On Tue, 01 Nov 2016 09:50:38 -0500
David davidwhess@gmail.com wrote:
- Thermocouples and temperature gradients - this is a huge problem
and special attention will need to be directed toward the layout and
maintaining an isothermal environment. Careful design is required to
get the specified drift performance out of chopper stabilized (10nV/C)
and low drift operational amplifiers (100nV/C before trimming or
grading).
Hmm... I guess I will have to put everything into a machined block of
aluminium or even copper/brass/bronze to keep the temperature gradients
low and temperature variations slow.
It is both more complicated and simpler than that. A thermal baffle
(Dixie cup) placed over sensitive circuits to prevent air flow and
convection can be enough. Check out Linear Technology application
note 9 written by Jim Williams of course for a discussion on this:
- Pink Noise - 1/f noise increases as frequency decreases. Chopper
amplifiers have flat 1/f noise so are invaluable below about 1 Hz.
I was thinking about using low noise opamps (probably LT1128 or LT1677)
with offset compensation using a LT2057 on each of them. This
should at least kill the 1/f noise and I would guess also most of
the opamp induced temperature variation. Does that make sense?
Or should I skip the low noise opamp and use the LT2057 directly
with some second order LP filter with 100Hz-1kHz bandwidth?
Attila Kinali
The LTC2057 will cancel the 1/f noise, drift, and offset but I think a
composite amplifier is only needed if wideband noise needs to be lower
than an LTC2057 will provide. Simple filtering will remove the
wideband noise on all except for the last stage.
The LT1028/LT1128 and LT1677 have high input bias current with its
associated high input current noise so are not suitable for buffering
high impedance (long time constant) filters. I might consider the
LT1793 if an LTC2057 by itself was not good enough.
I wonder if a pair of LTC2057's could be used in parallel for lower
noise. The LTC2057 has no clock pin though so intermodulation might
be the result.
Depending on whether or not you care about DC, aluminum electrolytics can
be made to work by bootstraping. There is a Jim Williams app note for a 0.1
to 10 Hz preamp which recommends a $400 wet slug tantalum. Which is a tad
pricey, so I gave Bootstaping a 4700 uF electrolytic a try. It ended up
that impact of leakage noise/drift was below the input referred noise of
the auto-zero amp at 6nV/rtHz over 0.1 to 10 Hz.
On Wed, Nov 2, 2016 at 9:44 AM, David davidwhess@gmail.com wrote:
On Wed, 2 Nov 2016 06:56:46 +0100, you wrote:
Hello David,
On 01.11.2016 15:50, David wrote:
avoid Mylar/polyester/PET and high dielectric constant ceramic
capacitors.
Do you have some specific recommendations/suggestions what to use?
Polypropylene? PTFE? Wet Tantalum capacitors (extreme expensive)?
Thanks,
Andreas
Wet tantalums might be the best for very long time constants but are
useless for settling times below hours to days. Some low leakage
aluminum electrolytics are just as good.
Charles pretty much covered it but I am not sure that polypropylene is
always worse than polystyrene. Some of the other plastic films are
pretty good also but polypropylene is the most common high performance
option. Teflon is the best but is also expensive and has poor
availablity.
When I looked into this a couple months ago in connection with
building a long time constant integrator for an analog only GPSDO, the
primary limitation was insulation resistance of the capacitor. Even
the best low dielectric constant C0G/NP0 capacitors were much worse
than polypropylene film which was itself was much better than
polyester.
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