This very interesting tread contains a lot of hot air
but not very many builds (except from the ready built unit shown above).
The tread imo also contains quite heated arguments
arising from different views on what really constitutes necessary elements in actually building a working reference. Some of the things mentioned in this tread are: Enclosure, PCB, Thermal Gradients and EMF.
It amazes me however that some posters use all this energy on discussing the merits or lack of such for details of building without first checking datasheets (& more) from manufacturer(s). {I might be wrong, but I cannot find any references to the text below}.
Here is a "copy and paste" from
http://cds.linear.com/docs/en/datasheet/1000afd.pdf :
(Please forgive me the lack of decent formatting - blame "Control-C"
).
---
LTZ1000/LTZ1000A
4
1000afd
a
pplica
T
ions
i
n
F
or
M
a
T
ion
LT Z
1000 and
LT Z
1000A are capable of providing ultimate
voltage
reference
performance. Temperature
drifts
of
better
than
0.03
ppm/°C and long-term stability on the order of
1?V per month can be achieved. Noise of about
0.15
ppm
can also be obtained. This performance is at the expense
of circuit complexity, since external influences can easily
cause output voltage shifts of more than 1ppm.
Thermocouple effects are one of the worst problems and
can give apparent drifts of many ppm/°C as well as cause
low frequency noise. The kovar input leads of the TO-5
package form thermocouples when connected to copper
PC boards. These thermocouples generate outputs of
35?V/°C. It is mandatory to keep the zener and transistor
leads at the same temperature, otherwise
1
ppm to
5
ppm
shifts in the output voltage can easily be expected from
these thermocouples.
Air currents blowing across the leads can also cause small
temperature variations, especially since the package is
heated. This will look like
1
ppm to
5
ppm of low frequency
noise occurring over a several minute period. For best
results, the device should be located in an enclosed area
and well shielded from air currents.
Certainly, any temperature gradient externally
generated
,
say
from a power supply, should not appear across the
critical circuitry. The leads to the transistor and zener
should be connected to equal size PC traces to equalize
the heat loss and maintain them at similar temperatures.
The bottom portion of the PC board should be shielded
against air currents as well.
Resistors, as well as having resistance temperature coef-
ficients, can generate thermocouple effects. Some types of
resistors can generate hundreds of microvolts of thermo-
couple voltage. These thermocouple effects in the resistor
can also interfere with the output voltage. Wire wound
resistors usually have the lowest thermocouple voltage,
while tin oxide type resistors have very high thermocouple
voltage. Film resistors, especially Vishay precision film
resistors, can have low thermocouple voltage.
Ordinary breadboarding techniques are not good enough
to give stable output voltage with the
LT Z
1000 family
devices. For breadboarding, it is suggested that a small
printed circuit board be made up using the reference, the
amplifier and wire wound resistors. Care must be taken to
ensure that heater current does not flow through the same
ground lead as the negative side of the reference
(
emitter
of Q1). Current changes in the heater could add to, or
subtract
from, the reference voltage causing errors with
temperature. Single point grounding using low resistance
wiring is suggested.
---
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