Dr. Frank Stellmach
drfrank.stellmach at freenet.de
Tue Sep 14 05:28:22 UTC 2010
I would like to emphasize, what Bill wrote, i.e. use a 2nd inner shield,
completely around the LTZ circuitry.
This also helps to thermally stabilize the circuitry, just put some
foamed polstyrene inside the inner box.
Use separate grounds for supply and analogue signal.
I also could not implement that inner shield, because my layout with new
and very old ww resistors grew too big.
More compact metal foil resistors, (<1ppm/K typ.) as in the HP 3458A
design, would be preferred.
I replaced the diode at the output of the OpAmp (OP07) of the Linear
Device schematic by an ordinary npn, to reduce the load on the OpAmp and
to have a low impedance output stage.
If you did not yet order the LTZ, I suggest to get the LTZ1000, not the
This allows to use the lowest stabilization temperature, around 45..50°C
for lowest drift, see Pickering patent and description of the Fluke 7000
Most important is proper design and shielding of the supply circuitry,
stated also somewhere in the 7000 manual.
So, please design a separate PCB for this, so that this could be
shielded separately, and can be updated later, if you encounter
As I operate the circuitry from the mains, there is definitely a lot of
potential to improve my very simple supply circuit.
The LTZ circuitry seems to be prone to multiplexer spikes, caused by
DMMs. (Although the 3458A should be very quiet in that aspect). So I
thought about an RC at the output to block these.
Anyhow, after 3/4 year of permanent operation of both LTZs, if I run
them against the 5442 and the 3458A (pimped to 65°C operation), all 4
standards are within 1ppm or less. That gives me confidence that the
LTZs really have a drift of around 1ppm/year as stated in the data
sheet. So I ignore the glitches at the moment.
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