ltz1000a Linear Technology Corporation, ltz1000a Datasheet - Page 4

• Manufacturer Part Number: ltz1000a
• Description: Ultra Precision Reference
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LTZ1000/LTZ1000A
BLOCK DIAGRAM
LTZ1000 and LTZ1000A are capable of providing ultimate
voltage reference performance. Temperature drifts of better
than 0.03ppm/°C and long-term stability on the order of
1μV per month can be achieved. Noise of about 0.15ppm
can also be obtained. This performance is at the expense
of circuit complexity, since external infl uences 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
35μV/°C. It is mandatory to keep the zener and transistor
leads at the same temperature, otherwise 1ppm to 5ppm
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 1ppm to 5ppm 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
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*SUBSTRATE DEVICES–DO NOT FORWARD BIAS
1
2
*
6
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-
fi cients, 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 fi lm
resistors, can have low thermocouple voltage.
Ordinary breadboarding techniques are not good enough
to give stable output voltage with the LTZ1000 family
devices. For breadboarding, it is suggested that a small
printed circuit board be made up using the reference, the
amplifi er and wire wound resistors. Care must be taken to
ensure that heater current does not fl ow 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.
Q2
8
3
4
Q1
5
7
1000 TA07
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