Project Analog - November 2008 - (Page 14)

lookInG for StaBIlIty In high-reliability applications, customers need stable resistive elements. In most cases, customers require a resistor component that — after the calibration routine — they can “set and forget.” It is essential that once the resistor network is in place, the specified resistance values do not fluctuate. In medical and aerospace applications, a typical stability requirement is 10-ppm change annually. Absolute tolerances and tracking ratios are typically ±0.1% and ±0.05%, respectively. thIn-fIlm reSIStor networkS Thin-film resistors are the technology of choice for these applications. Furthermore, the thin-film resistor networks can be made more robust by sealing the resistor chip in hermetically sealed packages. This not only provides superior absolute and ratio tolerances, but also protects the network from atmospheric conditions that can cause device failures in molded and encapsulated packages. Substrates used for thin-film resistor networks also play an integral role in performance and reliability. These networks are typically fabricated on ceramic or silicon-based substrates, each producing various benefits. Many passive component manufacturers produce resistor networks and capacitors on silicon due to the decreased cost as compared to ceramic substrates. Ceramic material, however, is an ideal insulator for resistor networks and results in smaller parasitic effects than silicon. Standard VS. cuStom Standard resistor networks with E96 resistance values, for example, are commonly available in configurations such as bused and isolated circuits. Custom resistor networks can be configured for any resistance value and interconnect requirement. Such a custom resistor network usually entails a one-time engineering and tooling charge. The use of standard resistor networks, when possible, may result in lower costs due to the higher-volume usage these models see in the industry. when SIze matterS Many applications require smaller components to meet decreased size constraints. Medical equipment such as ultrasound machines is becoming increasingly smaller and lighter for portable field use. Aerospace applications typically restrict overall weight and size. Although the push for miniaturization is not as advanced as in the medicaldevice industry, in general, avionics and aerospace devices are trending toward smaller, lighter systems with reduced component counts. other applIcatIonS Instrumentation applications often require precision resistor networks with custom specifications for stability, accuracy, tolerance, and tracking ratios. One such example is precision geophones — devices that convert ground movement into voltage, which is then measured for the tiniest fluctuations. Such an instrument requires precisely matched resistor pairs to achieve the high common-mode rejection ratio needed to measure the extremely small signals with no input errors. Most geophones require custom resistor networks due to the precision specifications required. Similarly, instrumentation amps need internal precision-matched resistor pairs with extremely tight tolerances and tracking ratios, usually to detect small changes in analog signals, or to provide a stable voltage reference for amplifier gain setting. Contents Viewpoint Selecting the Proper Amplifier Using a Digital Potentiometer to Optimize a Photo Detection Circuit Resistor Networks in Critical Applications Analog news Microchip analog page Sample center microchipDIRECT Reference designs/ app notes Technical training 14 · PROjeCt ANAlOG · VOlUme 2 / NUmbeR 6 http://www.microchip.com http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchipdirect.com/catalogselection.aspx?returnURL=default.aspx http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1469&filter1=function&redirects=appnotes http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1423

Table of Contents Feed for the Digital Edition of Project Analog - September 2008

Project Analog - September 2008 Contents Viewpoint About Project Analog Sponsor Selecting the Proper Amplifier for Strain Gauge Applications An Overview of Analog Sensor Conditioning Circuits Using a Digital Potentiometer to Optimize a Photo Detection Circuit Resistor Networks in Critical Applications Selecting the Right Op Amp Analog News Contact Microchip New Microchip Products Treelink MINDI™ Active Filter Designer Microchip Advanced Parts Selector (MAPS)

Project Analog - September 2008

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