11
LT1021
1021fc
APPLICATIONS INFORMATION
WUU
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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)
0610
1021 AI07
24
812
OUTPUT VOLTAGE NOISE (20µV/DIV)
20µV
FOAM CUP
REMOVED
LT1021-7 (TO-5 PACKAGE)
f = 0.01Hz TO 10Hz
TYPICAL APPLICATIONS
U
Restricted Trim Range for Improved
Resolution, 10V, “C” Version Only
LT1021C-10
OUT V
IN
IN 10.000V
TRIM RANGE ≈ ±10mV
GND TRIM
R2
50k
1021 TA11
R1
75k
1021 TA03
LT1021-10
OUT
IN
GND
TRIM
R1*
10k
V
OUT
*CAN BE RAISED TO 20k FOR
LESS CRITICAL APPLICATIONS
V
IN
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%)
