Project Analog - January 2008 - (Page 6) fig. 1 accuratE mEasurEmEnts rEquirE carEful grounding. The grounding methods of circuits A and C provide a single-ended input measurement that should only be used with a local sensor. The separate sensor and amplifier grounds of circuit A can produce an offset voltage due to the difference in the voltage potentials of the two grounds. In local sensor applications, the magnitude of VGND is small and either an inverting or non-inverting op amp can be used. The configuration of circuit C should be used with caution in low signal output sensors such as thermocouples. Noise can be induced into the measurement via the thermocouple wires and the magnitude of the sensor voltage will be affected by ground bounce or switching noise at the amplifier’s ground. Shielded cableS A shielded cable is an effective tool to prevent radiated interference from introducing a noise voltage on the signal wires. Shielded twisted-pair cables reduce the loop area of the victim signal and minimize the voltage induced on the sensor signal lines. The noise signals on each wire in the cable will be essentially equal, which is the assumption needed for the high CMRR feature of a differential amplifier to cancel the interference. For most applications, it is recommended that the shield be connected to ground only at one point. In applications using a metal enclosure, the shield’s connection to chassis ground can occur through the mechanical connection of the cable connector to the box. Figure 2 provides an example of a remote Resistance Temperature Detector (RTD) measurement using a shielded cable. eMi/eSd Protection EMI and ESD filters function as both a noise filter and a protective device to the circuit on the PCB. An IC input pin should never be connected directly to a remote sensor because EMI or ESD overvoltage failures will likely occur. Ferrite beads, capacitive feedthrough filters, Resistor-Capacitor (RC) filter and Transient-VoltageSuppressor (TVS) zener diodes are popular devices that can be used to Contents Viewpoint Wireless sensor networks Temperature sensing technology Analog news Microchip analog page Thermal management overview Sample center microchipDIRECT Reference designs/ app notes Technical training fig. 2 usE shiEldEd cablEs and Emi filtErs for rEmotE mEasurEmEnts. 6 · 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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