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LT1021DCS8-10 Datasheet(PDF) 11 Page - Linear Technology |
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LT1021DCS8-10 Datasheet(HTML) 11 Page - Linear Technology |
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11 / 16 page  11 LT1021 1021fc 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) TIME (MINUTES) 06 10 1021 AI07 24 812 20 µV FOAM CUP REMOVED LT1021-7 (TO-5 PACKAGE) f = 0.01Hz TO 10Hz TYPICAL APPLICATIONS Restricted Trim Range for Improved Resolution, 10V, “C” Version Only LT1021C-10 OUT VIN IN 10.000V TRIM RANGE ≈ ±10mV GND TRIM R2 50k 1021 TA11 R1 75k 1021 TA03 LT1021-10 OUT IN GND TRIM R1* 10k VOUT *CAN BE RAISED TO 20k FOR LESS CRITICAL APPLICATIONS VIN 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%) |
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Similar Description - LT1021DCS8-10 |
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