Project Analog - November 2008 - (Page 6)

mcP4725: 12-BiT Dac WiTh EEProm iN SoT-23-6 in a change in the strain gauge’s electrical resistance. This change in resistance can then be measured and the magnitude of the applied force quantified. So, how does one actually go about measuring strain? Typically, one or more strain gauges are arranged in a Wheatstone-bridge configuration, due to the excellent sensitivity that this circuit offers. A Wheatstone bridge for a strain gauge application consists of four elements arranged in a diamond pattern, with each side consisting of a resistive element (either a strain gauge or a fixed resistor). An excitation voltage is then applied to the bridge, and the output voltage across the middle of the bridge is measured. A quarter bridge consists of only one variable resistor element — the strain gauge. A half bridge has two variable resistor elements, and a full bridge has all four elements as variable resistor elements — in this case, strain gauges. The advantage of having more strain gauges is an increase in sensitivity. All else being equal, a half-bridge configuration will have twice the sensitivity as a quarter bridge, while the full bridge will have four times the sensitivity as the quarter bridge. SIGnal condItIonInG Even when using multiple strain gauges in a Wheatstone-bridge configuration, the total change in output voltage is relatively small, typically in the millivolt range. Due to the small signal amplitude, a gain stage is usually required before converting the voltage to a digital signal via an analog-to-digital converter (ADC). There may also be specific noise sources that must be filtered out of the analog input signal. Some examples of noise sources include power-line hum (either 50 Hz or 60 Hz), mechanical scale resonance or external electrical interference. Finally, any signalconditioning circuitry must not add significant noise to the system, or else the small output voltage from the Wheatstone bridge will be lost. In summary, the signal-conditioning circuitry must have sufficient gain, provide proper filtering and be low noise. offSet VoltaGe and open-loop GaIn Depending upon the output swing of the bridged gauges and the full-scale input of the ADC, the analog signal may need amplification by 500 times or more. In addition, the circuitry must provide adequate headroom so that the signal does not saturate the amplifier or the ADC. Several considerations must be made when selecting an amplifier or amplifiers to use in a gain stage. First, due to the high amount of gain that may be required, the offset voltage of the amplifier becomes critical. Any voltage offset due to the amplifier will be multiplied by the gain. For example, an amplifier that has an offset of 1 mV and configuration for a gain of 500 times would produce an error voltage of 0.5V at the output of the amplifier. For a strain gauge bridge that produces 10 mV at full scale, the amplifier-offset error could result in a measurement error of 10 percent. Therefore, a low-offset, precision amplifier is required, such as the MCP606. Contents Viewpoint Selecting the Proper Amplifier Using a Digital Potentiometer to Optimize a Photo Detection Circuit Resistor Networks in Critical Applications Analog news The MCP4725 is a low-power, high-accuracy, single channel, 12-bit buffered voltage output Digital-to-Analog Converter (DAC) with non-volatile memory (EEPROM). Its on-board precision output amplifier allows it to achieve rail-torail analog output swing. The MCP4725 has a two-wire I2C™ compatible serial interface for standard (100-kHz), fast (400-kHz) or high-speed (3.4-MHz) mode. The MCP4725 is an ideal DAC device where design simplicity and small footprint is desired. Applications • Set Point or Offset Trimming • Sensor Calibration • Closed-Loop Servo Control • Low-Power Portable Instrumentation • PC Peripherals • Data Acquisition Systems Click here for more information on the MCP4725 Microchip analog page Sample center microchipDIRECT Reference designs/ app notes Technical training 6 · 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=1335&dDocName=en532229 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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