Project Analog - November 2008 - (Page 7)

Secondly, the amplifier must have enough open-loop gain to provide the required amplification, while still maintaining performance. For a typical amplifier, the open-loop gain remains constant over a certain frequency range and then begins to roll off at 20 dB per decade. Most general-purpose amplifiers have an open- loop gain of 100 dB to 120 dB. Such an amplifier can handle a gain of 500, as mentioned above, but would probably limit the overall performance of the system. fiG 1 oPEN looP GaiN vS. frEquENcy by definition, “strain” is an alteration due to an external force. A strain gauge uses electrical resistance in order to quantify the amount of strain caused by an external force.” For example, assume an amplifier has an open-loop gain of 120 dB. We will also assume that the bandwidth of the amplifier is sufficient to not be a factor for the frequency of interest, at a gain of 500. A gain of 500 is approximately 54 dB, so 54 dB of the amplifier’s open-loop gain of the amplifier is dedicated to providing gain. The remaining 66 dB is the open-loop-gain margin that is available to ensure linearity, as shown in Figure 1. This implies that the amplifier is linear to approximately one part in 2000, or about 11 bits. To get around this limitation, a system designer can select an amplifier with more open-loop gain. Although such amplifiers exist, they are not widely available and may not be cost effective. Another solution is to use multiple-gain stages to achieve the overall desired gain, such as a traditional two or three amplifier instrumentation topology. Perhaps a better solution would be to use an instrumentation amplifier, which is specifically designed to provide high gain and good performance, and generally has better voltage offset and drift performance relative to a discrete instrumentation amplifier circuit. amplIfIer noISe Another important parameter that must be considered when evaluating amplifiers for use with strain gauges is the amplifier noise. It is important to note that an amplifier’s voltage noise is input referred, so any gain or attenuation in the circuit will also affect the amplifier noise. Also, keep in mind that an amplifier’s noise may not be constant over frequency, Contents 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 7 · 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=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

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.