Precision Function Generator: MAX038 goes SMD （2）

Published:2011/8/7 22:50:00 Author:Li xiao na From:SeekIC

By Klaus-Jiirgen Thiesler

MLC capacitors

Solid, leaded, capacitors in the microfarads range are every expensive, inaccurate and bulky components and it is not surprising to see they are rarely used these days. SMD multilayer ceramic chip capacitors (MLCCs) are much more accurate, tiny and cheap mainly because they are manufactured by the millions for use in mobile phones. Mind you, MLCCs are not a new development — in fact they have been around for at least ten years. The codes stated in the parts list indicate the dielectric (X5R...) and the case shape (1210).

For the low end of the frequency range a 100-uF MLCC with a working voltage of 6.3 V is used with excellent results. Its ESR (equivalent series resistance) is stunningly low at less than 10 mil. A 1 OO^F, 68-uF and 22-pF MLCC stacked to make o neat pile was measured and found to have an equivalent capacitance of216uF.

Large value ceramic capacitors in X5R technology have a typical tolerance of ±20%, as opposed to +80/-20% for Y5V types. RF ceramic capacitors like the 22-pF MLCC with COG structure and a maximum deviation of 0.5 pF are manufactured by, among others, Toiyo Yuden. Although the temperature coefficient of just ±30 ppm (Class 1) is a bit over the top for our application, it does demonstrate the technological advances made by passive component designers. The author found that Reichelt (www.reichelt.de) supplies an assortment of 0805 style COG ceramic chips at a reasonable price.

Adjustment requires a capacitance meter to be able to select the frequency determining capacitors for desired value, or combine a nmef of them to arrive at the desired values. These chip capacitors may be stacked onto each other and occupying just one pair of PCB solder pads. As you can see from Figure 3, the ’tower block’ method helps to keep copper tracks as short as possible. For a change, you’ll find that the trusty solder iron has the edge over industrial mounting methods like Row soldering or hot air! Although SMD components in 0805 and 1210 cases are really small, they can still be soldered manually if you have a steady hand.





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