Datasheet LT1027LS8 (Analog Devices) - 10

link to page 10 link to page 10 LT1027LS8
Typical applicaTions 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. LS8 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 1027LS8 F08a material may absorb water and apply mechanical stress
(a)
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- nents soldered to the board. FR4 and similar materials also absorb water, causing the board to swell. Even conformal LS8 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 1027LS8 F08b voltage reference IC for best stability. Figure 8a shows a tab cut through the PC board on three sides of an LT1027LS8,
(b)
which significantly reduces stress on the IC, as described
Figure 8. (a) 3-Sided PCB Tab Cutout, (b) 4-Sided PCB Cutout.
in Application Note 82. For even better performance,
Lines Represent Cuts All the Way Through the PCB
Figure 8b shows slots cut through the PC board on all four sides. The slots should be as long as possible, and 80 90 V the corners just large enough to accommodate routing of 70 IN = 10V 80 traces. It has been shown that for PC boards designed in 60 HUMIDITY 70 this way, humidity sensitivity can be reduced to less than TURE (°C) 50 60 HUMIDIT 35ppm for a change in relative humidity of approximately 40 50 60%. Mounting the reference near the center of the board, 30 40 Y (%) with slots on four sides, can further reduce the sensitivity 20 TEMPERATURE 30 to less than 10ppm. 10 20 (PPM) AND TEMPERA 0 10 An additional advantage of slotting the PC board is that V OUT–10 0 the LT1027LS8 is thermally isolated from surrounding –20 –10 0 20 40 60 80 100 120 140 160 180 200 220 circuitry. This separation can help reduce thermocouple TIME (HRS) 1027LS8 F09 effects and improve accuracy.
Figure 9. Illustrates Drift of LT1027LS8 with Large Changes in Humidity. Using Proper PCB Layout Techniques Limits This Drift to a Few ppm
1027ls8f 10 For more information www.linear.com/LT1027LS8 Document Outline Features Applications Typical Application Description Absolute Maximum Ratings Order Information Pin Configuration Typical Performance Characteristics Pin Functions Block Diagram Applications Information Typical Applications Package Description Typical Application Related Parts