LT1021 LINER [Linear Technology], LT1021 Datasheet - Page 11

• Manufacturer Part Number: LT1021
• Description: Precision Reference
• Manufacturer: LINER [Linear Technology]

APPLICATIONS
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
TYPICAL APPLICATIONS
Restricted Trim Range for Improved
V
Resolution, 10V, “C” Version Only
IN
IN
GND
LT1021C-10
TRIM RANGE
TRIM
OUT
75k
R1
U
10mV
INFORMATION
R2
50k
1021 TA11
U
10.000V
U
W
LT1021-10 Full Trim Range ( 0.7%)
V
IN
U
*CAN BE RAISED TO 20k FOR
IN
LESS CRITICAL APPLICATIONS
GND
LT1021-10
TRIM
OUT
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.
R1*
10k
1021 TA03
V
OUT
Noise Induced By Air Turbulence (TO-5 Package)
0
LT1021-7 (TO-5 PACKAGE)
f = 0.01Hz TO 10Hz
2
–15V
20 V
4
TIME (MINUTES)
4.7k
R2
6
Negative Series Reference
2N2905
FOAM CUP
REMOVED
15V
Q1
8
R1
4.7k
D1
15V
10
1021 AI07
IN
LT1021-10
LT1021 TA04
12
GND
LT1021
OUT
–10V AT 50mA
11