Project Analog - November 2008 - (Page 8)

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 especially at a low frequency where 1/f noise can become the dominant noise source. This low-frequency noise is critical in strain-gauge applications, as the input signal is near dc. Input-BIaS current Input-bias current is a third amplifier parameter that is critical in a signal- conditioning application. Bias current is the amount of current flow into the inputs of the amplifier to bias the input transistors. The magnitude of this current can vary from µA down to pA, and is strongly dependent upon the architecture of the amplifier-input circuitry. This parameter becomes extremely important when connecting a highimpedance sensor to the input of an amplifier. As the bias current flows through this high impedance, a voltage drop occurs across the impedance, resulting in a voltage error. For these applications, a lowbias current is required. Strain gauges can vary in nominal resistance from hundreds of ohms up to a thousand ohms. The current running through the bridged sensors is relatively large when compared to the bias current of the amplifier. Unless additional impedance is added prior to the amplifier for filtering or ESD protection, bias current typically does not play a significant role. concluSIon Strain gauges are found in a wide variety of applications, from electronic scales to measuring the torque on a steel beam. Each application presents its own design requirements and challenges. When selecting an amplifier to provide signal conditioning, it is imperative to ensure that the amplifier can provide the required gain and still maintain acceptable linearity. Noise is also a critical specification, and must be examined across the frequency band of interest. Finally, the input-bias current of the amplifier may also affect system performance, especially for highimpedance inputs. mIcrochIp RESOURCES Contents AN1030, “Weigh Scale Applications for the MCP3551”: http://ww1. microchip.com/downloads/en/ AppNotes/01030a.pdf AN699, “Anti-Aliasing, Analog Filters for Data Acquisition Systems”: http://ww1. microchip.com/downloads/en/ AppNotes/00699b.pdf Signal Chain Design Guide: http://ww1.microchip. com/downloads/en/ AppNotes/21825c.pdf AN682, “Using Single Supply Op Amps in Embedded Systems”: http://ww1. microchip.com/downloads/en/ AppNotes/00682c.pdf 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 8 · PROjeCt ANAlOG · VOlUme 2 / NUmbeR 6 http://ww1.microchip.com/downloads/en/AppNotes/01030a.pdf http://ww1.microchip.com/downloads/en/AppNotes/01030a.pdf http://ww1.microchip.com/downloads/en/AppNotes/01030a.pdf http://ww1.microchip.com/downloads/en/AppNotes/00699b.pdf http://ww1.microchip.com/downloads/en/AppNotes/00699b.pdf http://ww1.microchip.com/downloads/en/AppNotes/00699b.pdf http://ww1.microchip.com/downloads/en/AppNotes/21825c.pdf http://ww1.microchip.com/downloads/en/AppNotes/21825c.pdf http://ww1.microchip.com/downloads/en/AppNotes/21825c.pdf http://www.microchip.com http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://ww1.microchip.com/downloads/en/AppNotes/00682c.pdf http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=79&redirects=analog http://ww1.microchip.com/downloads/en/AppNotes/00682c.pdf http://sample.microchip.com/Default.aspx?testCookies=true http://ww1.microchip.com/downloads/en/AppNotes/00682c.pdf 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

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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