Re: [volt-nuts] Tilt effect on Keithley 2000
Charles wrote:
What is the time constant? Slowish (i.e., could be thermal) or rapid
(presumably gravitational/mechanical stress)?
The 20-30uV on my LM399s should be thermal. Time constant
is about 10 seconds. Additionally the current consumption of
the reference cirquit changes by 3-5%. So I think that the
heater within LM399 has not a good thermal layout with respect
to the temperature sensor on the same chip.
For the Keithley differences I cannot tell the real time constant
since I had slow reading with averaging 10 or 50 measurements.
But since the effect is significantly larger than the reference alone
there might be a mixture of mechanical and thermal effects.
best regards
Andreas
Orientation has altered the point contact of my miniclips and the
test point. Its nearly eliminated by using alligators instead, this
is in measuring 10uV level changes.
With the Geller SVR board, the orientation of the board changes the
output voltage in the 10uV range too, but output stabilizes after
20-30 min to a new level.
As Charles already suggests, its noticeable that output is fast,
seconds, and over a wide range of uV when mechanical, and slow drift,
minutes, and eventually stabilized when it appears thermally related.
Joe Geller suggests the current draw of the LM399 heater is sensitive
to very small changes in ambient temperature, he exploits this in his
microgust anemometer circuit. When orienting an exposed chip, very
small temperature gradients exists between open air and a table,
particularly with fan or window making small air currents.
At 03:21 AM 9/5/2010, Andreas Jahn wrote:
Charles wrote:
What is the time constant? Slowish (i.e., could be thermal) or
rapid (presumably gravitational/mechanical stress)?
The 20-30uV on my LM399s should be thermal. Time constant
is about 10 seconds. Additionally the current consumption of the
reference cirquit changes by 3-5%. So I think that the
heater within LM399 has not a good thermal layout with respect
to the temperature sensor on the same chip.
For the Keithley differences I cannot tell the real time constant
since I had slow reading with averaging 10 or 50 measurements.
But since the effect is significantly larger than the reference alone
there might be a mixture of mechanical and thermal effects.
best regards
Andreas
Sincerely,
Marv Gozum
Philadelphia, PA
Marvin wrote
Joe Geller suggests the current draw of the LM399 heater is sensitive
to very small changes in ambient temperature, he exploits this in his
microgust anemometer circuit. When orienting an exposed chip, very
small temperature gradients exists between open air and a table,
particularly with fan or window making small air currents.
thanks for your contribution.
In my LM399 references I wanted to have a minimum power
consupmtion when using battery supply.
So I did not remove the foam cup of the device.
Instead I packaged the small PCB with the LM399 and the
temperature compensated current source into a housing of
polystyrene foam. This housing is packaged into a card box.
So there should not be much air current inside.
The most confusing fact is that the power consumption
is a minimum when turning the LM399s up side down
with the pins in upper direction. The foam cup has a
relative large hole for all 4 pins.
best regards
Andreas