"A" voltage? Is that 1mV, 1V, 10V?

How fast does it need to be updated?

⁣Met vriendelijke groet,
Rob Klein

verstuurd vanaf mijn smartphone​

Op 1 nov. 2016 13:14, bij 13:14, Attila Kinali attila@kinali.ch schreef:

In message 20161101131416.b99ff94ae493bb993f841968@kinali.ch, Attila Kinali writes:

What range of voltages are we talking about ?

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On Tue, 01 Nov 2016 12:35:39 +0000
"Poul-Henning Kamp" phk@phk.freebsd.dk wrote:

The DAC will probably output 0-3V or 0-5V, final output will be 0-5V.

		Attila Kinali

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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 Tue, 01 Nov 2016 13:25:00 +0100
Rob Klein rob.klein@smalldesign.nl wrote:

0-5V

That constraint has not been fully worked out yet.
It will probably end up as "as fast as the electronics can support" :-)
But the signal will be slowly moving, so settling time will not be
that much of an issue, as long as the signal level can be kept stable.

		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

Attila wrote:

Noise is just one of the concerns, and potentially not the hardest one
to deal with.  Settling time will be a real issue.  If the full-scale
output is 1v, you need to wait for the system to settle to within nearly
24 bits.

First, the LP filters will have a prolonged settling tail to 100nV
(meaning that the basic regulation must show 100nV stability for longer
-- probably much longer -- than you are contemplating), so drift may be
much more of an issue than you anticipate.

Second, everything -- the reference, the DAC, the buffer amps, the
resistors, and even the capacitors -- will exhibit temperature effects.
You will need an extremely stable thermal environment.  You will also
need to deal with changes in the self-heating of the DAC, the
amplifiers, and the resistors when you change the voltage, which will
likely produce considerably longer settling tails than the LP filters.
The wider the range of output voltages, the worse the self-heating
effects will be.

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).
And what is the downstream circuitry going to be doing during this
settling time?  Are you going to need a S&H that can hold to within
100nV for seconds at a time?

This isn't quite so much of a problem if each step is small, but
settling to within 24 bits is a pretty big deal even in that case.  And
probably much slower than you are expecting.

Best regards,

Charles

Is that a peak-peak requirement, meaning you would like 100/6 nV
approximately rms?

Do you Just need Precision, with little accuracy say 5%?

On Tuesday, 1 November 2016, Attila Kinali attila@kinali.ch wrote:

Read the various datasheets and application notes for chopper
stabilized amplifiers, the highest precision bipolar operational
amplifiers, and low input bias current operational amplifiers.  Linear
Technology and Analog Devices are great sources for this information.
The major issues in rough order of importance are:

1. 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).

2. Leakage - even at low impedance levels, minor amounts of leakage
   will cause significant errors.  100nV at 10k is only 10pA so this is a
   real problem for RC filters where long time constants require high
   impedance levels and capacitor leakage also needs to be considered;
   avoid Mylar/polyester/PET and high dielectric constant ceramic
   capacitors.  In the past, through-hole construction allowed "air
   wiring" sensitive nodes but that is not practical with surface mount
   parts.  Guards will need to be used in the layout which is impossible
   with the smaller surface mount packages.

3. Resistor self heating - even precision resistors become a problem
   at this level of stability because of self heating which also
   complicates the thermocouple problem.  High resistance values lower
   self heating but make errors from leakage current and current noise
   worse.

4. Dielectric absorption - this will be a problem if fast settling is
   required.  Avoid Mylar/polyester/PET and high dielectric constant
   ceramic capacitors.

5. Pink Noise - 1/f noise increases as frequency decreases.  Chopper
   amplifiers have flat 1/f noise so are invaluable below about 1 Hz.

On Tue, 1 Nov 2016 13:14:16 +0100, you wrote:

Moin,

Thanks a lot for all the answers!

On Tue, 01 Nov 2016 09:50:38 -0500
David davidwhess@gmail.com wrote:

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.

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

--
Malek's Law:
Any simple idea will be worded in the most complicated way.

Hello David,

On 01.11.2016 15:50, David wrote:

Do you have some specific recommendations/suggestions what to use?

Polypropylene? PTFE? Wet Tantalum capacitors (extreme expensive)?

Thanks,

Andreas