LT1027LS8

10

1027ls8f

For more information www.linear.com/LT1027LS8

Typical applicaTions

LS8

1027LS8 F08a

LS8

1027LS8 F08b

Figure 8. (a) 3-Sided PCB Tab Cutout, (b) 4-Sided PCB Cutout.

Lines Represent Cuts All the Way Through the PCB

(a)

(b)

Figure 9. Illustrates Drift of LT1027LS8 with Large Changes in

Humidity. Using Proper PCB Layout Techniques Limits This Drift

to a Few ppm

TIME (HRS)

0

20 40 60 80 100 120 140 160 180 200 220

–20

–10

0

10

20

30

40

50

60

70

80

–10

0

10

20

30

40

50

60

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90

1027LS8 F09

HUMIDITY

TEMPERATURE

Humidity Sensitivity

Plastic mold compounds absorb water. With changes in

relative humidity, plastic packaging materials change the

amount of pressure they apply to the die inside. These

pressure changes can cause slight changes in the output

of a voltage reference, usually on the order of 100ppm.

The LS8 package is hermetic, so it is not affected by

humidity, and is therefore more stable in environments

where humidity may be a concern. However, PC board

material may absorb water and apply mechanical stress

to the LT1027LS8. Proper board materials and layout are

essential.

For best stability, the PC board layout is critical. Change

in temperature and position of the PC board, as well as

aging, can alter the mechanical stress applied to compo-

nentssolderedtotheboard.FR4andsimilarmaterialsalso

absorb water, causing the board to swell. Even conformal

coating or potting of the board does not always eliminate

this effect, though it may delay the symptoms by reducing

the rate of absorption.

Power and ground planes should be omitted under the

voltagereferenceICforbeststability.Figure8ashowsatab

cut through the PC board on three sides of an LT1027LS8,

which significantly reduces stress on the IC, as described

in Application Note 82. For even better performance,

Figure 8b shows slots cut through the PC board on all

four sides. The slots should be as long as possible, and

the corners just large enough to accommodate routing of

traces. It has been shown that for PC boards designed in

this way, humidity sensitivity can be reduced to less than

35ppm for a change in relative humidity of approximately

60%. Mounting the reference near the center of the board,

with slots on four sides, can further reduce the sensitivity

to less than 10ppm.

An additional advantage of slotting the PC board is that

the LT1027LS8 is thermally isolated from surrounding

circuitry. This separation can help reduce thermocouple

effects and improve accuracy.