Project Analog - January 2008 - (Page 7) provide protection for the sensor circuit. Ferrite beads, capacitive feedthrough filters and RC filters function as filters and only limit the slew rate of a transient-input voltage. A voltage-clamping device (such as a TVS zener diode) is required to limit the input voltage to a safe value that will not damage the IC amplifier. Though a TVS device is similar to a standard zener diode, they are designed to turn on fast and dissipate a short duration, high-peak energy voltage transient. In contrast, a zener diode is designed to clamp a steady-state voltage for a long duration. In many applications, combinations of different EMI/ ESD filter devices are often used. One option is to use a capacitive feed-through filter that is located inside the connector, in addition to TVS and RC filters which are placed on the PCB board, as shown in Figure 3. Feed-through capacitors are typically mounted on a conductive chassis, with the mechanical mounting forming the ground connection. The noise signal is filtered at the connector before the signal reaches the PCB. The effectiveness of the filter is usually very good because the inductance associated with the ground connection is minimized. The TVS diodes on the PCB ensure that the transient voltage is limited to a safe value, while the RC filters provide additional filtering to the instrumentation amplifier. Figure 4 provides the design equations for a RC filter which can be used with differential and instrumentation amplifiers. The RC combinations of R1/C1 and R2/C2 are used to form common mode filters and reduce the noise which is common to both input lines. The common- mode resistors and capacitors should be matched as close as possible and the resistors should have a tolerance of 1% or better, while the capacitors should be at least 5%. Capacitor C3 forms a differential mode filter that attenuates the signal with respect to the difference in the voltage potentials of the two inputs. C3 also compensates for any mismatch of R1/C1 and R2/C2, which is important because the difference in the R/C combinations degrades an amplifier’s CMRR. Contents Viewpoint Wireless sensor networks Temperature sensing technology Analog news fig. 3 rEmotE sEnsors may rEquirE multiPlE Emi filtEr dEvicEs. Microchip analog page Thermal management overview fig. 4 Providing Emi/Esd ovErvoltagE ProtEction with rEsistors and caPacitors. Sample center microchipDIRECT Reference designs/ app notes Technical training • 7 · projeCt analog · jan 08 http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2543¶m=en021419&pageId=79 http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=2543¶m=en021419&pageId=79 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
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