LTZ1000/LTZ1000A

4

1000afd

applicaTions inForMaTion

LTZ1000 and LTZ1000A are capable of providing ultimate

voltagereferenceperformance.Temperaturedriftsofbetter

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 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

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

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,cangeneratethermocoupleeffects.Sometypesof

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 LTZ1000 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.

block DiagraM

1

8

Q2

Q1

3

5

6

2

4

7

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*SUBSTRATE DEVICES–DO NOT FORWARD BIAS

1000 TA07