For those that insist on using switching power supplies
I have done this and it works great; the breakpoint between the
chopper amplifier and the low noise amplifier can be adjusted to
combine the wideband noise from the low noise amplifier and the 1/f
noise and drift of the chopper amplifier.
Jim Williams wrote a couple of different application notes where this
was used with both integrated and discrete amplifiers.
On Sat, 15 Oct 2016 00:53:25 +0200, you wrote:
...
The low frequency range is what we usually call the 1/f region,
although the long term stability also belongs to it. But unlike
the long term region you don't have to sacrifice a virgin to
get decent measurment data. Jim William's appnote[1] has lots of
details how to measure noise in this region. There are slightly
more modern circuits by Todd Owen/Amit Patel[2] and Gerhard Hoffmann[3].
I recently stumbled over a similar amplifier by Enrico Rubiola
and Franck Lardet-Vieudrin[4]. Both [4] and [5] explain why for
low impedance sources (like power supplies) a BJT input stage
would be a better choice than jFETs and also cover the influence
of temperature on the measurement. [6] gives some additional info
on how to design the differential input stage.
I wonder how an active offset voltage cancelation scheme for
the differential pair input stage using one of the chopper stabilized
opamps (eg LTC2057) would change the temperature dependence and long
term stability (aka 1/f^a noise) of the circuit, but I have not seen
any measurements of a system like this yet.
...
On Fri, 14 Oct 2016 22:25:55 -0500
David davidwhess@gmail.com wrote:
I have done this and it works great; the breakpoint between the
chopper amplifier and the low noise amplifier can be adjusted to
combine the wideband noise from the low noise amplifier and the 1/f
noise and drift of the chopper amplifier.
Jim Williams wrote a couple of different application notes where this
was used with both integrated and discrete amplifiers.
Yes, there are several appnotes and papers that list this method.
But I am not aware of any noise measurement below 0.1Hz for such
an amplifier setup.
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 Tue, 18 Oct 2016 11:27:05 +0200, you wrote:
On Fri, 14 Oct 2016 22:25:55 -0500
David davidwhess@gmail.com wrote:
I have done this and it works great; the breakpoint between the
chopper amplifier and the low noise amplifier can be adjusted to
combine the wideband noise from the low noise amplifier and the 1/f
noise and drift of the chopper amplifier.
Jim Williams wrote a couple of different application notes where this
was used with both integrated and discrete amplifiers.
Yes, there are several appnotes and papers that list this method.
But I am not aware of any noise measurement below 0.1Hz for such
an amplifier setup.
Attila Kinali
Below 0.1 Hz it is not all that interesting; the noise is just the
noise of the chopper amp and flat below the chopping frequency. I
think Jim Williams mentioned in one of his articles that at low
frequencies, noise and drift are effectively the same thing so thermal
EMF becomes a large if not the largest contributor.
When I did it, I extended the single ended design to a fully
differential gain of 1000 amplifier using a pair of LT1028s with a
pair of LTC1150s for correction. I used the noise curves to estimate
what the integrator gain should be and after adjusting it for minimum
noise from about 0.1 to 10 Hz, it was very close to the actual
crossover point in the datasheet specifications. RMS noise was
measured by taking the standard deviation of the DC values from a
Fluke 8505A over 10 seconds.
Hoi David,
On Tue, 18 Oct 2016 05:25:35 -0500
David davidwhess@gmail.com wrote:
Below 0.1 Hz it is not all that interesting;
Depends on what you are doing ;-)
the noise is just the
noise of the chopper amp and flat below the chopping frequency. I
think Jim Williams mentioned in one of his articles that at low
frequencies, noise and drift are effectively the same thing so
Yes, different words for the same thing.
thermal EMF becomes a large if not the largest contributor.
Ah.. good to know. Thanks!
Any guess what the other big factors are?
When I did it, I extended the single ended design to a fully
differential gain of 1000 amplifier using a pair of LT1028s with a
pair of LTC1150s for correction. I used the noise curves to estimate
what the integrator gain should be and after adjusting it for minimum
noise from about 0.1 to 10 Hz, it was very close to the actual
crossover point in the datasheet specifications. RMS noise was
measured by taking the standard deviation of the DC values from a
Fluke 8505A over 10 seconds.
You wouldn't have the schematics and the measurments available somewhere?
It would be interesting to have a look at them.
BTW: Should this discussion be moved over to volt-nuts?
I kind of feel we are getting too off-topic for time-nuts.
(though my interest comes from long term time measurment)
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 Tue, 18 Oct 2016 14:52:15 +0200, you wrote:
thermal EMF becomes a large if not the largest contributor.
Ah.. good to know. Thanks!
Any guess what the other big factors are?
Over time scales of 100s of milliseconds to seconds, the self heating
and temperature coefficient of the feedback network resistors causes
errors which extend settling time and look like low frequency noise.
At this level, self heating also contributes to non-linearity.
Intermodulation between frequency components of the signal close to
the chopping frequency can result in low frequency noise. Modern
integrated chopper amplifiers do various things to prevent this so I
do not think it is a problem now.
External thermal effects are the big problem though. Jim Williams
discusses this in Linear Technology application note 9 and includes
measurements showing noise down below 0.01 Hz. He also discusses
other sources of noise:
When I did it, I extended the single ended design to a fully
differential gain of 1000 amplifier using a pair of LT1028s with a
pair of LTC1150s for correction. I used the noise curves to estimate
what the integrator gain should be and after adjusting it for minimum
noise from about 0.1 to 10 Hz, it was very close to the actual
crossover point in the datasheet specifications. RMS noise was
measured by taking the standard deviation of the DC values from a
Fluke 8505A over 10 seconds.
You wouldn't have the schematics and the measurments available somewhere?
It would be interesting to have a look at them.
I may have them and my notes from more than 10 years ago but I could
redraw the schematic from memory and describe it well enough to
duplicate; the design is not complicated. I got the basic idea from
figure 14 on page 10 Linear Technology application note 21:
http://www.linear.com/docs/4116
Mirror the standard high input impedance non-inverting amplifier
configuration top to bottom to produce a 2 operational amplifier (4 in
this case) differential amplifier. The symmetry in the circuit helps
balance noise sources like thermocouples. I do not remember if
synchronizing the clocks of the chopper stabilized amplifiers improved
performance but if it did, the difference was not large at least with
LTC1050s and LT1028s.
The part I found amazing when I worked with this circuit is everything
worked just as theory predicted; the calculated integrated noise level
was just about right and the crossover frequency between the
amplifiers matched the datasheet specifications. The low frequency
noise was so low that I could measure resistance just from its low
frequency Johnson noise which scared me.
BTW: Should this discussion be moved over to volt-nuts?
I kind of feel we are getting too off-topic for time-nuts.
(though my interest comes from long term time measurment)
Attila Kinali
The original non-inverting circuit would be suitable for use in the
signal chain driving the voltage control input of a crystal oscillator
preserving low noise and low drift from the DAC. Replace the low
noise precision bipolar operational amplifier with a low noise low
input bias current operational amplifier and it might be useful for
implementing long time constant filters in an analog GPSDO design
although I suspect sacrificing input bias current (the chopper input
bias current is relatively high) for this level of precision and low
frequency noise may not be a worthwhile trade off.
Am Thu, 13 Oct 2016 17:57:02 -0500
schrieb "Graham / KE9H" ke9h.graham@gmail.com:
Actually, if they have the "CE" stamp on the product, then they have
very specific radio interference limits that they must test and meet.
It must have been tested, certified, and the certification package
available for inspection.
Umm, I guess most of us wish it was actually like that. Strictly
speaking, the CE sign legally is no more than a statement by the
manufacturer that he believes this very product conforms to the
applicable rules ans regulations. This does not imply any testing by
itself. It's more of an self assessment, which could also be based on
gut feeling and customer requirements ("must have CE sign").
Testing is not required to carry the CE mark. Yes, of course, the CE
mark implies that the product meets various requirements, out of which
some can only be guaranteed through testing, but this is often
overlooked.
Whether they actually met it, then pulled the interference supression
parts off the board as a "cost reduction" as is common in no-name
computer power supplies, or whether it never met it to begin with, is
for you to speculate. Some suppliers will explain to you that "CE"
means China Export, not that it meets the consolidated European
safety and electrical rules.
The more honest scumbags will just claim this very device was not
intended to be sold outside of China and they don't have the logistics
to produce enclosures with different labeling.
How much of this is considered close to truth depends on those who do
consider. As long as all those smallish shops in Shenzhen are not
faced by prosecution, this won't change, and China does not seem keen
on changing this.
Best regards,
Florian