LT1021DCS8-10PBF Datasheet, PDF (11/16 Pages) Linear Technology – Precision Reference

LT1021
APPLICATIONS INFORMATION
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)
LT1021-7 (TO-5 PACKAGE)
f = 0.01Hz TO 10Hz
20µV
FOAM CUP
REMOVED
02
4
6
8
TIME (MINUTES)
10 12
1021 AI07
TYPICAL APPLICATIONS
Restricted Trim Range for Improved
Resolution, 10V, “C” Version Only
LT1021-10 Full Trim Range (±0.7%)
LT1021C-10
V_IN IN
OUT
GND TRIM
R1
75k
10.000V
R2
50k
1021 TA11
TRIM RANGE ≈ ±10mV
LT1021-10
V_IN IN
OUT
GND TRIM
V_OUT
R1*
10k
1021 TA03
*CAN BE RAISED TO 20k FOR
LESS CRITICAL APPLICATIONS
Negative Series Reference
15V
R1
4.7k
LT1021-10
IN
OUT
D1
R2
15V
GND
4.7k
–15V
Q1
2N2905
–10V AT 50mA
LT1021 TA04
1021fc
11