11

LT1021

1021fc

Effects of Air Movement on Low Frequency Noise

The LT1021 has very low noise because of the buried zener

used in its design. In the 0.1Hz to 10Hz band, peak-to-peak

noise is about 0.5ppm of the DC output. To achieve this

low noise, however, care must be taken to shield the

reference from ambient air turbulence. Air movement can

create noise because of thermoelectric differences

between IC package leads (especially kovar lead TO-5) and

printed circuit board materials and/or sockets. Power

dissipation in the reference, even though it rarely exceeds

20mW, is enough to cause small temperature gradients in

the package leads. Variations in thermal resistance, caused

by uneven air flow, create differential lead temperatures,

thereby causing thermoelectric voltage noise at the output

of the reference. The following XY plotter trace dramati-

cally illustrates this effect. The first half of the plot was

done with the LT1021 shielded from ambient air with a

small foam cup. The cup was then removed for the second

half of the trace. Ambient in both cases was a lab environ-

ment with no excessive air turbulence from air condition-

ers, opening/closing doors, etc. Removing the foam cup

increases the output noise by almost an order of magni-

tude in the 0.01Hz to 1Hz band! The kovar leads of the

TO-5 (H) package are the primary culprit. Alloy 42 and

copper lead frames used on dual-in-line packages are not

nearly as sensitive to thermally generated noise because

they are intrinsically matched.

There is nothing magical about foam cups—any enclo-

sure which blocks air flow from the reference will do.

Smaller enclosures are better since they do not allow the

build-up of internally generated air movement. Naturally,

heat generating components external to the reference

itself should not be included inside the enclosure.

Noise Induced By Air Turbulence (TO-5 Package)

TIME (MINUTES)

06

10

1021 AI07

24

812

20

µV

FOAM CUP

REMOVED

LT1021-7 (TO-5 PACKAGE)

f = 0.01Hz TO 10Hz

Restricted Trim Range for Improved

Resolution, 10V, “C” Version Only

LT1021C-10

OUT

IN

10.000V

TRIM RANGE

≈ ±10mV

GND

TRIM

R2

50k

1021 TA11

R1

75k

1021 TA03

LT1021-10

OUT

IN

GND

TRIM

R1*

10k

*CAN BE RAISED TO 20k FOR

LESS CRITICAL APPLICATIONS

LT1021-10

OUT

IN

GND

LT1021 TA04

D1

15V

R1

4.7k

–10V AT 50mA

R2

4.7k

–15V

15V

Q1

2N2905

Negative Series Reference

LT1021-10 Full Trim Range (

±0.7%)