Project Analog - February 2008 - (Page 7) W W A B C2 Pot1 A C1 + A VOUT B R1 R2 VBRG1 Rheo2 W Rheo1 mcP355x tiny aPPlication sEnsor dEmo board The MCP3550/1/3 devices are 2.7V to 5.5V low power, 22-bit Delta-Sigma Analog-to-Digital Converters (ADCs). The MCP355X Tiny Application Sensor Demo Board is used to demonstrate the most basic application of the devices using a ratiometric connection with the system power supply as the voltage reference. The extreme common mode rejection capability of the MCP355X devices, along with their excellent normal mode power supply rejection at 50 and 60 Hz, allows for excellent system performance. Click here for more information on the MCP355XDM-TAS VIN B Pot2 fig. 4 ProgrammablE filtEr VBRG2 fig. 5 whEatstonE bridgE trimming RSENSOR conditions on the sensor change, the resistance of the sensor will change, causing the VBRG2 voltage to change. The delta voltage between VBRG1 and VBRG2 can then be used to determine the state of the system (temperature, weight, etc.). Rheo 1 is used in a rheostat mode to limit the current or trim the current through the Wheatstone Bridge. As the size, power consumption, and cost of digital potentiometers continue to decrease, more and more applications will be able to benefit from the additional flexibility that digital potentiometers have to offer. Some of these applications include mechanical potentiometer replacement, amplifiers with offset and trimming, band pass filtering with offset and gain trimming, programmable filters, Wheatstone bridge trimming and general embedded system designs. Contents Viewpoint Digital potentiometer application circuits Smart ADC architecture Layout techniques for high accuracy and resolution ADCs Analog news Pot and the capacitor value (Cx). Each additional stage of the RC filter is used to enhance the rolloff characteristics for the filter. The capacitors Cx should be the same, while the wiper values of the Pots should be similar. The differences would be to compensate for the slight variations of the RAB values of each Pot and the variations of the capacitors. The following circuit (figure 5) shows a Wheatstone Bridge with current limiting. In a Wheatstone Bridge, there are four resistive elements. In this example, two are fixed value (R1 and R2), there is a resistive sensor (RSENSOR) and then there is the digital potentiometer in rheostat configuration to calibrate the circuit due to variations of the resistive sensor. This sensor could be for temperature or weight measurement. At a default condition, the sensor should be a given value, but this value will change from device to device. To compensate for the resistive changes in the R1 plus Rsensor leg of the bridge, the Rheo2 would be modified for the R2 plus Rheo2 leg of the bridge. This would be done so that the voltages of VBRG1 and VBRG2 are at their desired levels. Many times this is VBRG1 = VBRG2. Now as the Microchip analog page Mixed-signal overview Sample center microchipDIRECT Reference designs/ app notes Technical training • 7 · ProjeCT ANALog · feb 08 http://www.microchip.com http://www.microchip.com/analog http://www.microchip.com/analog http://www.microchip.com/mixedsignal http://sample.microchip.com/Default.aspx?testCookies=true http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en026682 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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