Project Analog - September 2008 - (Page 15)

Selecting the Right Op Amp By Gina Hann / Texas Instruments, Dallas TX http://www.ti.com Three of the most common design considerations prevent op-amp afterthought Contents Viewpoint Selecting the Proper Amplifier Using a Digital Potentiometer to Optimize a Photo Detection Circuit Resistor Networks in Critical Applications Analog news O perational amplifiers (op amps) play many different roles in optimizing the performance of mixed signal system design. Today’s mixed signal designer is challenged to understand the requirements of both the analog and digital portions of the signal chain. With the diverse range of op-amp functions available, choosing the right one for each niche element in a mixed-signal design can be tricky. In fact, very often the need for an op amp is not recognized until after board layout or even the first prototype, resulting in an expensive and time-consuming redesign. You can avoid this costly situation of “opamp afterthought” with a little bit of planning. Looking at a classic analog signal chain consisting of a input signalconditioning amplifier to digital converter to processor, there are some well-known functions for op amps — front-end signal conditioning and analog output signal conditioning. But there are a number of other opportunities where amplifiers can play a key role for improving the accuracy and performance of your mixedsignal design; and a number of opportunities for op-amp afterthought to occur. rule 1. know your sensor input before you choose an op amp or instrumentation amp, and know the output requirements of your conditioning amplifier. One of the most disappointing cases of op-amp afterthought occurs upon realizing that the wrong input amplifier was selected for the job, or that after carefully selecting the appropriate amplifier for the sensor, an additional amplifier is needed to maintain system precision and accuracy. So what are the design considerations when choosing an amplifier for sensor signal conditioning? The sensor impedance, sensor output voltage span, common mode signal levels and type of excitation required all influence proper amplifier selection. If a very-high-impedance sensor is being used, such as a pH sensor, then it is critical to choose an amplifier with extremely low input bias currents so that dc errors across large impedance changes are minimized. If the output voltage span is a few millivolts as with many bridge sensors, a low offset and most likely low-drift op amp or instrumentation amplifier will help to avoid losing the input signal in the midst of proportionally larger input errors of the amplifier. Microchip analog page Sample center microchipDIRECT Reference designs/ app notes Technical training 15 · PROjeCt ANAlOG · VOlUme 2 / NUmbeR 6 http://www.ti.com 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.